Rice (Oryza sativa) is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione-complexed As, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. The roles of phloem and xylem transport were investigated by applying a +/- stem-girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron x-ray fluorescence mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite-challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.

The length scale of the local chemical anisotropy responsible for the growth-temperature-induced perpendicular magnetic anisotropy of face-centered cubic CoPt3 alloy films was investigated using polarized extended x-ray absorption fine structure (EXAFS). These x-ray measurements were performed on a series of four (111) CoPt3 films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preference for Co-Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, and random chemical order otherwise. Furthermore, atomic pair correlation anisotropy was evidenced only in the EXAFS signal from the next neighbors to the absorbing Co atoms and from multiple scattering paths focused through the next neighbors. This suggests that the Co clusters are no more than a few atoms in extent in the plane and one monolayer in extent out of the plane. Our EXAFS results confirm the correlation between perpendicular magnetic anisotropy and two-dimensional Co segregation in CoPt3 alloy films, and establish a length scale on the order of 10 A for the Co clusters.

Crystalline nucleation is investigated during the dynamical heat treatment of a ZrO2-MgO-Al2O3-SiO2 glass by high-temperature Zr K-edge X-ray absorption near-edge spectroscopy. In the parent glass, the Zr coordination number is higher than in most glasses, explaining the structural instability of Zr during thermal treatment. The Zr local structure is modified at 30 degrees C above the nucleation onset. Crystalline nucleation is characterized by the formation of peculiar nano-ZrO2 particles, which are stable until completion of the thermal treatment. These nano-ZrO2 particles are stable during the elaboration of glass-ceramics, without any Zr4+ remaining in the glassy matrix of the final material.

We have determined D-Eu between augite and melt in samples that crystallized froin a highly spiked martian basalt composition at four f(o2) conditions. D-Eu shows a steady increase with f(o2) from 0.086 at IW- 1 to 0.274 at IW+3.5. This increase is because Eu3+ is more compatible than Eu2+ ill the pyroxene structure thus increasing f(o2) leads to ureater Eu3+/Eu2+ in the melt and more Eu (total) call partition into the crystallizing pyroxene. This Interpretation IS supported by direct determinations Of Eu valence state by XANES, which show a stead), increase of Eu3+/Eu2+ with increasing.fi, in both pyroxene (0.38 to 14.6) and glass (0.20 to 12.6) in the samples. Also, pyroxene Eu3+/Eu2+ is higher than that of adjacent glass in all the samples, which verifies that Eu3+ is more compatible than Eu2+ ill the pyroxene structure. Combining partitioning data with XANES data allows for the calculation of specific valence state D-values for augite/melt where DEu3+ = 0.28 and DEu2+ = 0.07.

Arsenopyrite is the most common sulfide host of invisible gold. Yet, despite many studies, the position of such gold in the structure of arsenopyrite has not been resolved conclusively. Here we report a multitechnique study of arsenopyrite samples from the Pezinok deposits (Slovakia) with moderate gold concentrations of 7-10 mu g/g. Secondary ion mass spectrometry showed that the invisible gold occurs as either (1) almost uniform, low-concentration of ``dispersed'' gold, or as (2) hot spots along fractures. X-ray absorption spectra at the Au L-III edge were collected from such hot spots. The spectra document metallic character of gold although no discrete gold particles were seen even after careful re-examining in back-scattered electron images. We conclude that such occurrences are most readily explained by the presence of gold nanoparticles. We suggest that the dispersed gold is the chemically-bound gold previously detected in these deposits by Au-197 Mossbauer spectroscopy. The concentration of the dispersed gold is too low for X-ray absorption spectroscopy.

Shooting ranges represent sites heavily polluted by Pb, Sb, Cu, Ni, and Zn, which are released during the weathering of bullets. The pristine bullets are made of a Pb-Sb core, Fe mantle, and minor amounts of Cu, Ni, and Zn in an interlayer between the core and mantle. At two selected sampling sites (Losone and Lucerne, both in Switzerland), corroding bullets were collected to determine the sinks of Sb within the weathering crust of the bullets. Bulk Sb concentrations in the crust were found to be as high as 1.3 wt.%. The oxalate-extractable fraction of Fe showed that the amorphous Fe oxides (e.g., ferrihydrite) prevail over goethite and lepidocrocite, which were identified by bulk X-ray diffraction experiments. Crystalline Pb phases are litharge (only found by X-ray diffraction) and cerussite, which result from weathering of the Pb core. No distinct Sb minerals were identified by X-ray diffraction. Investigations with electron microprobe (EMP) showed that Sb is mostly accumulated in those regions in the weathering crust where there is also a high concentration of Fe. in the weathering crust from Losone, such Fe-rich regions with Sb are represented by material that cements or rims silicate mineral grains. The cement was identified as lepidocrocite by micro-Raman analysis. At Lucerne, Sb is found in Fe-oxide aggregates, in sawdust particles where it may be bound to organic matter, or in aggregates enriched in Pb and depleted in Fe. Bulk EXAFS experiments suggested that the Fe oxides are the most important sink for Sb. Our modelling of Sb next-nearest neighbours suggests two types of inner-sphere complexes on the surfaces of Fe oxides. These are edge- and corner-sharing adsorption complexes. Hence, the predominant sink of Sb in the weathering crust of the bullets at the selected shooting ranges is Fe oxides, amorphous or crystalline. (C) 2008 Elsevier B.V. All rights reserved.

Copper K-edge X-ray absorption spectra were recorded from natural polyphase brine fluid inclusions in miarolitic quartz from the Omsukchan Granite, Russia, as a function of temperature up to 700 degrees C. The inclusions studied represent two distinct fluids, which contain either 0.02 or 0.94 wt.% Cu homogeneously distributed in solution at temperatures above salt dissolution (350-550 degrees C). X-ray absorption near edge structure (XANES) spectra of the solution phase in the Cu-poor inclusions exhibit an intense edge feature near 8983 eV that is characteristic of the linear [CuCl2](-) complex. This spectrum was obtained at all temperatures between 200 and 700 degrees C and from around 15 inclusions. Extended X-ray absorption fine structure (EXAFS) spectra recorded at 550 degrees C were modelled to give a Cu-Cl bond length of 2.11(2) angstrom, also consistent with [CuCl2](-). XANES spectra of the Cu-rich inclusions appear similar to that of [CuCl2](-) but are shifted to higher energy. This spectrum was obtained between 350 and 700 degrees C and from all three inclusions studied. EXAFS recorded at 700 degrees C gave a Cu-ligand bond length of 2.19(1) angstrom. This complex is yet to be identified. The results for the Cu-poor inclusions indicate that [CuCl2](-) is stable at magmatic temperatures, extending the known range of this complex by 200 degrees C. This is the first time that Cu speciation has been determined at these temperatures and provides an example of how inclusions can be used as sample cells for the spectroscopic study of fluids at extreme conditions. (C) 2008 Elsevier B.V. All rights reserved.

Differential sorption and transport characteristics of the leaf mesophyll layer of the Prayon and Ganges ecotypes of the hyperaccumulator Thlaspi caerulescens were examined. Cd-109 influx and efflux experiments were conducted with leaf sections, and X-ray absorption near edge structure (XANES) data were collected from leaves as a general comparison of in vivo cadmium (Cd) coordination. There were modest differences in cell wall sorption of Cd between ecotypes. There were obvious differences in time- and concentration-dependent Cd influx, including a greater V-MAX for Prayon but a lower K-M for Ganges for concentration-dependent Cd uptake and a notably greater Cd uptake by Ganges leaf sections at 1000 mu m Cd. Leaf sections of Prayon had a greater Cd efflux than Ganges. The XANES spectra from the two ecotypes suggested differences in Cd coordination. The fundamental differences observed between the two ecotypes may reflect differential activity and/or expression of plasma membrane and tonoplast transporters. More detailed study of these transporters and the in vivo coordination of Cd are needed to determine the contribution of these processes to metal homeostasis and tolerance. New Phytologist (2009) 181: 626-636 10.1111/j.1469-8137.2008.02693.x.

We report analyses of aerogel tracks using (1) synchrotron X-ray computed microtomography (XRCMT), (2) laser confocal scanning microscopy (LCSM), and (3) synchrotron radiation X-ray fluorescence (SRXRF) of particles and their paths resulting from simulated hypervelocity impacts (1-2), and a single similar to 1 mm aerogel track from the Stardust cometary sample collector (1-3). Large aerogel pieces can be imaged sequentially, resulting in high spatial resolution images spanning many tomographic fields of view ('lambda-tomography'). We report calculations of energy deposited, and tests on aromatic hydrocarbons showing no alteration in tomography experiments. Imaging at resolutions from similar to 17 to similar to 1 micron/pixel edge (XRCMT) and to <100 nm/pixel edge (LCSM) illustrates track geometry and interaction of particles with aerogel, including rifling, particle fragmentation, and final particle location. We present a 3-D deconvolution method using an estimated point-spread function for aerogel, allowing basic corrections of LCSM data for axial distortion. LCSM allows rapid, comprehensive, non-destructive, high information return analysis of tracks in aerogel keystones, prior to destructive grain extraction. SRXRF with LCSM allows spatial correlation of grain size, chemical, and mineralogical data. If optical methods are precluded in future aerogel capture missions, XRCMT is a viable 3D imaging technique. Combinations of these methods allow for complete, nondestructive, quantitative 3-D analysis of captured materials at high spatial resolution. This data is fundamental to understanding the hypervelocity particle-aerogel interaction histories of Stardust grains.

We describe an approach for acquiring high quality x-ray absorption fine structure (XAFS) spectroscopy spectra with wide energy range at high pressure using diamond anvil cell (DAC). Overcoming the serious interference of diamond Bragg peaks is essential for combining XAFS and DAC techniques in high pressure research, yet an effective method to obtain accurate XAFS spectrum free from DAC induced glitches has been lacking. It was found that these glitches, whose energy positions are very sensitive to the relative orientation between DAC and incident x-ray beam, can be effectively eliminated using an iterative algorithm based on repeated measurements over a small angular range of DAC orientation, e.g., within +/- 3 degrees relative to the x-ray beam direction. Demonstration XAFS spectra are reported for rutile-type GeO2 recorded by traditional ambient pressure and high pressure DAC methods, showing similar quality at 440 eV above the absorption edge. Accurate XAFS spectra of GeO2 glass were obtained at high pressure up to 53 GPa, providing important insight into the structural polymorphism of GeO2 glass at high pressure. This method is expected be applicable for in situ XAFS measurements using a diamond anvil cell up to ultrahigh pressures.

The EXAFS spectra of Cu and Pd foil from many different beamlines and synchrotrons are compared to address the dependence of the amplitude reduction factor (S-0(2)) on beamline specific parameters. Even though S-0(2) is the same parameter as the EXAFS coordination number, the value for S0 2 is given little attention, and is often unreported. The S0 2 often differs for the same material due to beamline and sample attributes, such that no importance is given to S-0(2)-values within a general range of 0.7 to 1.1. EXAFS beamlines have evolved such that it should now be feasible to use standard S0 2 values for all EXAFS measurements of a specific elemental environment. This would allow for the determination of the imaginary energy (Ei) to account for broadening of the EXAFS signal rather than folding these errors into an effective S-0(2)-value. To test this concept, we model 11 Cu-foil and 6 Pd-foil EXAFS spectra from around the world to compare the difference in S-0(2)- and Ei-values.

In zircon U-Pb systematics, extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic Pb at low temperatures, often without any metamorphic event being in evidence. Here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic Pb in zircon is tetravalent. We review data and arguments in favour of this hypothesis. Diffusion profiles calculated for Pb2+ in a 25 mu m radius zircon xenocryst in a melt at 1000 degrees C, combined with the incompatibility of Pb2+, or for a zircon core inside a younger zircon rim at this temperature, show age effects that should have been observed in SIMS dating. Further, in zircon evaporation as well as in leaching experiments, common Pb is generally released preferentially to radiogenic Pb. After removal of less radiogenic Pb, the evaporation record generally shows pure radiogenic Pb during the final evaporation steps. The distribution of residual Pb in a leached titanite, revealed by PIXE, is similar to that of Ti. Lastly, XANES spectra of a 1 Ga old titanite (predominantly radiogenic Pb) and an Alpine one (predominantly common Pb) are significantly different, although the former does not resemble that of PbO2. The arguments why radiogenic Pb should be tetravalent are based on analogies with studies relating to the tetravalent state of Th-234 and the hexavalent state of U-234, which show that alpha-recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest Further, a zircon grain, being small, should remain highly oxidizing in its interior by the constant loss of beta-particles, maintaining the 4+ state of radiogenic Pb. From its effective ion radius, similar to that of Zr4+, and its charge, Pb4+ has to be compatible in the zircon lattice. Also, by analogy with U4+, Th4+ and Hf4+, its diffusivity should be several orders of magnitude lower than that measured for Pb2+ in zircon. These factors can account for the extreme retentivity of U-Pb clocks even at high temperatures. On the other hand, radiogenic Pb situated in a-recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state, making it both incompatible and soluble. Thus, discordia can be generated in weathering. The curious observation that discordant Archaean zircon suites generally define trends to lower intercepts at up to 800 Ma instead of zero, appears to reflect a c. 14 to 28% better average accessibility to leaching of radiogenic Pb-207 compared to Pb-206, in accord with the somewhat higher recoil energy of the last (branched) alpha-decays in the U-235 chain. (C) 2008 Elsevier B.V. All rights reserved.

While theoretical standards such as those from FEFF have many advantages for EXAFS analysis, the quality of fit statistics and estimation of parameter uncertainties has always been somewhat problematic using them. Eventhough the calculations reproduce EXAFS features sufficiently well to give bond distances accurate to 0.01 angstrom or so, fits to high-quality experimental spectra will have fit residuals far larger than the experimental uncertainties. One of the possible causes for this mis-fit is from approximations made in the phenomenological calculations of the inelastic losses, though this has been difficult to test conclusively in the past. Recent improvements in calculations of inelastic losses using a many-pole self-energy (MPSE) using a real-space calculation of the dielectric function have been shown to give significant improvements in XANES calculations. Here, we discuss the use of MPSE calculations for EXAFS analysis, and show that these calculations from FEFF 8.5 can quantitatively improve fit quality for EXAFS, reducing mis-fits by a factor of 2 or more compared to FEFF 6. This approach may also allow the semi-physical fit parameter S-0(2) to be avoided. The implications for improving precision for EXAFS analysis and the nature of the remaining fit residual are discussed.

Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle(1-3) or hydrous melting at temperatures only modestly greater than those found today(4,5). Here we determine the original Fe3+/Sigma Fe ratio of 2.7- Gyr- old komatiitic magma from Belingwe, Zimbabwe(6), preserved as melt inclusions in olivine, to be 0.10+/-0.02, using iron K- edge X- ray absorption near- edge structure spectroscopy. This value is consistent with near- anhydrous melting of a source with a similar oxidation state to the source of present- day mid- ocean- ridge basalt. Furthermore, this low Fe3+/Sigma Fe value, together with a water content of only 0.2-0.3 wt% ( ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H-2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron ( that is, the Fe3+/Sigma Fe ratio would be similar to 1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (similar to 1,700 degrees C), rather than melting under hydrous conditions ( 3-5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

The benefits of Mn and Zn fluid fertilizers over conventional granular products in calcareous sandy loam soils have been agronomically demonstrated. We hypothesized that the differences in the effectiveness between granular and fluid Mn and Zn fertilizers is due to different Mn and Zn reaction processes in and around fertilizer granules and fluid fertilizer bands. We used a combination of several synchrotron-based x-ray techniques, namely, spatially resolved micro-x-ray fluorescence (mu-XRF), micro-x-ray absorption near edge structure spectroscopy (mu-XANES), and bulk-XANES and -extended x-ray absorption fine structure (EXAFS) spectroscopy, along with several laboratory-based x-ray techniques to speciate different fertilizer-derived Mn and Zn species in highly calcareous soils to understand the chemistry underlying the observed differential behavior of fluid and granular micronutrient forms. Micro-XRF mapping of soil-fertilizer reactions zones indicated that the mobility of Mn and Zn from liquid fertilizer was greater than that observed for equivalent granular sources of these micronutrients in soil. After application of these micronutrient fertilizers to soil, Mn and Zn from liquid fertilizers were found to remain in comparatively more soluble solid forms, such as hydrated Mn phosphate-like, Mn calcite-like, adsorbed Zn-like, and Zn silicate-like phases, whereas Mn and Zn from equivalent granular sources tended to transform into comparatively less soluble solid forms such as Mn oxide-like, Mn carbonate-fike, and Zn phosphate-like phases.

We report Ni K-edge fluorescence x-ray absorption fine structure spectra (XAFS) for Fe0.75Ni0.05S0.20 and Fe0.75Ni0.05Si0.20 ternary alloys from room temperature up to 1600 K. A high-temperature furnace designed for these studies incorporates two x-ray transparent windows and enables both a vertical orientation of the molten sample and a wide opening angle, so that XAFS can be measured in the fluorescence mode with a detector at 90 degrees with respect to the incident x-ray beam. An analysis of the Ni XAFS data for these two alloys indicates different local structural environments for Ni in Fe0.75Ni0.05S0.20 and Fe0.75Ni0.05Si0.20 melts, with more Ni-Si coordination than Ni-S coordination persisting from room temperature through melting. These results suggest that light elements such as S and Si may impact the structural and chemical properties of Fe-Ni alloys with a composition similar to the earth's core.

Methods that rapidly detect, characterize, quantify, and decontaminate surfaces are essential following chemical or biological incidents. Our work focused on developing a ``smart'' surface, one that monitors itself and the overlying atmosphere and triggers a decontamination step when surface contamination is detected. Titanium dioxide was used to coat a ceramic surface containing skeletal impregnated platinum electrodes. The electrical resistance of the surface became altered by the introduction of a contaminant into the overlying atmosphere and its chemisorption to the surface. This change in resistance in turn initiated illumination by an external ultraviolet lamp to induce photocatalytic oxidation of the sorbed contaminant. Film resistance was concentration-dependent, allowing self-decontamination to be triggered at set pollutant levels. Preliminary results suggest that advanced surfaces and films can be developed to identify contaminant type and concentration and to initiate photocatalytic decontamination to meet regulatory or safety standards. Copyright (C) 2008 Laura R. Skubal et al.

The oxidation status of uranium in sediments is important because the solubility of this toxic and radioactive element is much greater for U(VI) than for U(IV) species. Thus, redox manipulation to promote precipitation of UO2 is receiving interest as a method to remediate U-contaminated sediments. Presence of Fe and Mn oxides in sediments at much higher concentrations than U requires an understanding of their redox status as well. This study was conducted to determine changes in oxidation states of U, Fe, and Mn in U-contaminated sediments from Oak Ridge National Laboratory. Oxidation states of these elements were measured in real-time and nondestructively using X-ray absorption spectroscopy on sediment columns supplied with synthetic groundwater containing organic carbon (OC, 0, 3, 10, 30, and 100 mM OC as lactate) for over 400 days. In sediments supplied with OC >= 30 mM, 80% of the U was reduced to U(IV), with transient reoxidation at about 150 days. Mn(III,IV) oxides were completely reduced to Mn(II) in sediments infused with OC >= 3 mM. However, Fe remained largely unreduced in all sediment columns, showing that Fe(III) can persist as an electron acceptor in reducing sediments over long times, This result in combination with the complete reduction of all other potential electron acceptors supports the hypothesis that the reactive Fe(III) fraction was responsible for reoxidizing U(IV).

The limited penetration of cytotoxic drugs into tumors is a significant contributing factor to the limited effectiveness of cancer chemotherapy. Here we have shown that X-ray fluorescence microtomography is a suitable technique for imaging the distribution of Pt drugs within multicellular tumor spheroids, models of solid tumors. This has revealed that there is no significant difference between the distributions of the range of Pt(II) and Pt(IV) complexes investigated, suggesting that complexes that are more resistant to cellular uptake may be needed to ensure more complete distribution throughout a tumor.

CO2-rich solutions are common in geological environments. An XANES (X-ray absorption near-edge structure) study of Br in CO2-bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre-edge feature at temperatures from 298 to 423 K. Br in CO2-free solutions does not show such a feature. The feature becomes smaller and disappears as temperature increases, but reappears when temperature is reduced. The size of the feature increases with increasing X(CO2) in the fluid inclusion, where X(CO2) is the mole fraction of CO2 in the solution [n(CO2)/(n(CO2) + n(H2O) + n(RbBr)); n indicates the number of moles]. The pre-edge feature is similar to that shown by covalently bonded Br, but observed and calculated concentrations of plausible Br-bearing covalent compounds (Br-2, CH3Br and HBr) are vanishingly small. An alternative possibility is that CO2 affects the hydration of Br sufficiently that the charge density changes to favour the 1s-p level transitions that are thought to cause the pre-edge peak. The distance between the first two post-edge maxima in the XANES also decreases with increasing X(CO2). This is attributed to a CO2-related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO2-bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore-formation and metamorphism.

Density-driven polyamorphism of GeO2 glass under high pressure has been studied by density, x-ray scattering, and optical Raman measurements. Density data obtained by an x-ray absorption method display distinct compression behavior in different pressure regions, with rapid density increases at 5 and 10 GPa and a plateau at 6-9 GPa. Simultaneous x-ray diffraction reveals that the position of the first sharp diffraction peak (FSDP) increases nearly linearly towards higher scattering vector with pressure up to 10 GPa. Both the width of the FSDP and the Raman stretching band of Ge-O-Ge (419 cm(-1)) increase with pressure but exhibit changes in behavior at 2.5 and 7.5 GPa, indicating intermediate states exist in the glass before the collapse of local tetrahedral and pentahedral structural units, respectively. At pressures above 15 GPa, post-octahedral compression with progressive enhancement in network correlation is observed. The results indicate not only the discrete but also rotating intermediate states exist in GeO2 glass under pressures up to 35 GPa.

Modest perturbations induced by an externally-applied electric field can generate significant variations in effective oxygen fugacity in high temperature silicates. This result has at least two-fold importance: first, it is a new petrologic technique to examine the behavior of a single sample under a large range of effective oxygen fugacity; and second, it is a mechanism for planetary electric fields to generate potentially significant chemical heterogeneities within planetary interiors. The redox state of Fe and V within a partially melted basaltic andesite was manipulated in situ in a piston-cylinder experiment with a DC power supply providing a source and sink of electrons to the sample. A 1 V electrical potential difference was applied across vanadium-doped synthetic basalt samples for 24 h. at 20 kbar and 1400 degrees C in a specially-designed piston cylinder sample assembly. Three experiments were performed: a control sample with no applied voltage, one with bottom ground and top anode (+1 V), and a third with top ground and bottom anode (+1 V). Synchrotron-based X-ray absorption near edge structure spectroscopy (XANES) was used to provide maps of iron and vanadium oxidation states with 5 mu m x 5 mu m spatial resolution throughout the recovered samples. Systematic increasing oxidation states of V and Fe were observed approaching the anode. Oxidation states were mapped to corresponding local oxygen fugacities by comparison with a series of samples synthesized under known oxygen fugacity conditions from previous studies. Both Fe and V markers indicate that the 1 V potential drop across the sample induces effective oxygen fugacity perturbations of 10 orders of magnitude. Therefore, it is possible that the presence of modest poloidal electric fields (similar to 10(-6) V/m) within the Earth's outer core may provide an electrochemical driving force for localized charge transfer reactions in certain overlying mantle areas, generating local order-of-magnitude differences in effective oxygen fugacity. (C) 2007 Elsevier B.V. All rights reserved.

X-ray fluorescence computed tomography (XFCT) is a synchrotron-based imaging modality employed for mapping the distribution of elements within slices or volumes of intact specimens. A pencil beam of external radiation is used to stimulate emission of characteristic X-rays from within a sample, which is scanned and rotated through the pencil beam in a first-generation tomographic geometry. The measurements so obtained can be represented as generalizations of the attenuated Radon Transform. The range of angular scanning employed is the subject of some variability in the XFCT imaging community, as some groups rotate the object through a full 360 degrees, while others employ only a 180 degrees rotation. In both cases, the entire object is scanned through the beam at each projection view. The use of a 180 degrees rotation is sometimes justified by implicit reference to a well-known symmetry property of the Radon transform, but that symmetry does not hold for the attenuated Radon transform. In this work, we demonstrate that a full scan of the object at each view coupled with a 360 degrees rotation does contain a two-fold data redundancy. While the redundancy does not give rise to a simple symmetry condition as in the case of the Radon transform, we will show that it does indeed license the use of 180 degrees scheme. However, we will also demonstrate that when there is a single external fluorescence detector, the redundancy also licenses a potentially more attractive reduced-scan scheme, in which the object is rotated through a full 360 degrees, but in which only the half of the object closest to the fluorescence detector is scanned at each projection view. This new scheme may permit both reduced imaging times and improved image quality.

X-ray fluorescence computed tomography (XFCT) is a synchrotron-based imaging modality employed for mapping the distribution of elements within slices or volumes of intact specimens. A pencil beam of external radiation is used to stimulate emission of characteristic X-rays from within a sample, which is scanned and rotated through the pencil beam in a first-generation tomographic geometry. The measurements so obtained can be represented as generalizations of the attenuated Radon Transform. The range of angular scanning employed is the subject of some variability in the XFCT imaging community, as some groups rotate the object through a full 360 degrees, while others employ only a 180 degrees rotation. In both cases, the entire object is scanned through the beam at each projection view. The use of a 180 degrees rotation is sometimes justified by implicit reference to a well-known symmetry property of the Radon transform, but that symmetry does not hold for the attenuated Radon transform. In this work, we demonstrate that a full scan of the object at each view coupled with a 360 degrees rotation does contain a two-fold data redundancy. While the redundancy does not give rise to a simple symmetry condition as in the case of the Radon transform, we will show that it does indeed license the use of the 180 degrees scheme. However, we will also demonstrate that when there is a single external fluorescence detector, the redundancy also licenses a potentially more attractive reduced-scan scheme, in which the object is rotated through a full 360 degrees, but in which only the half of the object closest to the fluorescence detector is scanned at each projection view. This new scheme may permit both reduced imaging times and improved image quality.

Iron-nickel (Fe-Ni) alloy is regarded as the most abundant constituent of Earths core, with an amount of 5.5 wt% Ni in the core based on geochemical and cosmochemical models. The structural role of nickel in liquid Fe-Ni alloys with light elements such as S or Si is poorly understood, largely because of the experimental difficulties of high-temperature melts. Recently, we have succeeded in acquiring Ni K-edge fluorescence x-ray absorption fine structure (XAFS) spectra of Fe-Ni-S and Fe-Ni-Si melts and alloys. Different structural environment of Ni atoms in Fe-Ni-S and Fe-Ni-Si melts is observed, supporting the effect of light elements in Fe-Ni melts.

In addition to affecting biogeochemical transformations, the speciation of As also influences its transport from tailings at inoperative mines. The speciation of As in tailings from the Sulfur Bank Mercury Mine site in Clear Lake, California (USA) (a hot-spring Hg deposit) and particles mobilized from these tailings have. been examined during laboratory-column experiments. Solutions containing two common, plant-derived organic acids (oxalic and citric acid) were pumped at 13 pore volumes d(-1) through 25 by 500 mm columns of calcined Hg ore, analogous to the pedogenesis of tailings. Chemical analysis of column effluent indicated that all of the As mobilized was particulate (1.5 mg, or 6% of the total As in the column through 255 pore volumes of leaching). Arsenic speciation was evaluated using X-ray absorption spectroscopy (XAS), indicating the dominance of arsenate [As(V)] sorbed to poorly crystalline Fe(III)-(hydr)oxides and coprecipitated with jarosite [KFe3(SO4, ASO(4))(2)(OH)(6)] with no detectable primary or secondary minerals in the tailings and mobilized particles. Sequential chemical extractions (SCE) of <45 mu m mine tailings fractions also suggest that As occurs adsorbed to Fe (hydr)oxides (35%) and coprecipitated within poorly crystalline phases (45%). In addition, SCEs suggest that As is associated with 1 N acid-soluble phases such as carbonate minerals (20%) and within crystalline Fe-(hydr)oxides (10%). The finding that As is transported from these mine tailings dominantly as As(V) adsorbed to Fe (hydr)oxides or coprecipitated within hydroxysulfates such as jarosite suggests that As release from soils and sediments contaminated with tailings will be controlled by either organic acid-promoted dissolution or reductive dissolution of host phases. (C) 2007 Elsevier Ltd. All rights reserved.

In situ reduction of toxic Cr(VI) to less hazardous Cr(III) is becoming a popular strategy for remediating contaminated soils. However, the long-term stability of reduced Cr remains to be understood, especially given the common presence of Mn(III,IV) oxides that re-oxidize Cr(III). This 4.6 year laboratory study tracked Cr and Mn redox transformations in soils contaminated with Cr(VI), which were then treated with different amounts of organic carbon (OC). Changes in Cr and Mn oxidation states within soils were directly and nondestructively measured using micro-X-ray absorption near-edge structure spectroscopy. Chromate reduction was roughly first-order, and the extent of reduction was enhanced with higher OC additions. However, significant Cr(III) re-oxidation occurred in soils exposed to the highest Cr(VI) concentrations (2560 mg kg(-1)). Transient Cr(III) re-oxidation up to 420 mg kg(-1) was measured at 1.1 years after OC treatment, followed by further reduction. Chromate concentrations increased by 220 mg kg(-1) at the end of the study (4.6 years) in one soil. The causal role that the Mn oxidation state had in re-oxidizing Cr was supported by trends in Mn K-edge energies. These results provide strong evidence for long-term dependence of soil Cr oxidation states on balances between OC availability and Mn redox status.

The arsenic (As) solid-state speciation (i.e., oxidation state, precipitates, and adsorption complexes) is one of the most important factors controlling dissolved As concentrations at As contaminated sites. In this case study, two representative subsurface samples (i.e., oxidized and semi-reduced sites) from former lead arsenate contaminated soils in the northeastern United States were chosen to investigate the effects of aging on As retention mechanisms using multiscale spectroscopic techniques. X-ray powder diffraction (XRD), synchrotron based microfocused (mu) XRD, in situ mu-synchrotron based X-ray fluorescence spectroscopy (SXRF), and mu-X-ray absorption near edge structure (XANES) spectroscopy were used to compliment the final bulk X-ray absorption spectroscopy (XAS) analyses. In the sample from an oxic area, As is predominantly (similar to 71%) present as As(V) adsorbed onto amorphous iron oxyhydroxides with a residue (similar to 29%) of an original contaminant, schultenite (PbHAsO4). Contrarily, there is no trace of schultenite in the sample from a semi-reduced area. Approximately 25% of the total As is present as adsorbed phases on amorphous iron oxyhydroxide and amorphous orpiment (AS(2)S(3)). The rest of the fractions (similar to 46%) were identified as As(V)-Ca coprecipitates. This study shows that aging effects can significantly alter the original chemical constituent (schultenite) in soils, resulting in multi and site-specific As solid-state speciation. The variability in spatial and temporal scale may be important in assessing the environmental risk and in developing in situ remediation technologies.

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 degrees C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu+ and Cu2+ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu+ and Cu2+ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu2+ is the stable oxidation state in solution at room temperature and Cu' at high temperatures. The change in oxidation state with temperature was completely reversible. Cu+ was found to occur exclusively as the linear species [CuCl2](-) in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K+, there is evidence for higher order coordination of Cu', in particular the tetrahedral complex [CuCl4](3-). The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (similar to 180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Micro-X-ray fluorescence (mu-XRF) microprobe analysis and micro-X-ray absorption near-edge structure (mu-XANES) spectroscopy were employed to identify Fe and Mn phases and their association with selected metals in two biosolids (limed composted [LC] and Nu-Earth) before and after treatment to remove organic carbon (OC). Spatial correlations derived from elemental mapping of XRF images showed strong correlations between Fe and Cd, Cr, Pb, or Zn (r(2) = 0.65-4.92) before and after removal of most of the OC. The strong correlation between Fe and Cu that was present in intact samples disappeared after OC removal, suggesting that Cu was associated with OC coatings that may have been present on Fe compounds. Except for Fe and Cr, the spatial correlations of metals with Mn were improved after treatment to remove OC, indicating that the treatment may have altered more than the OC in the system. The Fe mu-XANES spectra of the intact biosolids sample showed that every point had varying mixtures of Fe(II and III) species and no two points were identical. The lack of uniformity in Fe species in the biosolids sample illustrates the complexity of the materials and the difficulty of studying biosolids using conventional analytical tools or chemical extraction techniques. Still, these microscopic observations provide independent information supporting the previous laboratory and field hypothesis that Fe compounds play a major role in retention of environmentally important trace elements in biosolids. This could be due to co-precipitation of the metals with Fe, adsorption of metals by Fe compounds, or a combination of both mechanisms.

The redox states of volcanic and impact melts from the Earth, Moon, and Mars have been estimated from the valence state of V in basaltic glasses (Sutton et al. 2005). The V valence has been determined using synchrotron micro X-ray absorption near-edge structure spectroscopy (XANES) (Sutton et al. 2005), which allows for in situ measurements on samples with a micrometer spatial resolution and similar to 100 ppm elemental sensitivity. Here, we interpret those results for the natural samples and compare them to the literature. The results show that terrestrial melts are dominated by V4+, lunar samples by V3+, with Martian melts a mixture of both V3+ and V4+. The f(O2) estimates derived from the V valence are consistent with those determined by other proven methods, whereby terrestrial basalts experience f(O2) conditions within 1 or 2 log units of the QFM buffer, lunar basalts equilibrate at 1 to 2 log units below the IW buffer, and Martian basalts fall somewhere between the QFM and IW buffer. The results illustrate the usefulness of this technique; i.e., a robust oxybarometer covering over six orders of magnitude, applicable to samples that record f(O2) conditions from reduced extraterrestrial bodies to the oxidized Earth.

Arsenic is a contaminant in the groundwater of Bangladesh and West Bengal, India, where an estimated 57 million people may be drinking water with unsafe arsenic levels. The source of arsenic appears to be natural, solid-phase arsenic in the sediments, and various theories have been put forth regarding the modes of arsenic release, ranging from the oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Currently, however, reductive dissolution of Fe(III) (hydr) oxides and concomitant arsenic release is the most widely accepted explanation of high arsenic groundwater concentrations. Using micro-X-ray fluorescence elemental mapping and micro-X-ray absorption near-edge structure spectroscopy, we detect arsenic-bearing sulfides in the aquifer sediments from our field site in Munshiganj, Bangladesh. Reduction of Fe and As in surface soil layers is apparent, but Fe(III) (hydr)oxides are not detected in the Holocene aquifer materials. Rapid abiotic desorption of arsenic from sediments is observed in batch experiments, and positive controls with ferrihydrite dismiss the role of ferric (hydr)oxides in arsenic retention within the aquifer sediments. Based on our laboratory results, we do not see evidence for ferric (hydr)oxide reductive dissolution at well-depths in the Holocene aquifer. In contrast, our data suggest that arsenic is only released via redox cycling in surface soils/sediments and thus must then be transported to well-depth through the sandy aquifer. (c) 2006 Elsevier B.V. All rights reserved.

Plutonium oxidation state distribution on Yucca Mountain tuff and synthetic pyrolusite (beta-MnO2) suspensions was measured using synchrotron X-ray micro-spectroscopy and microimaging techniques as well as ultrafiltration/solvent-extraction techniques. Plutonium sorbed to the tuff was preferentially associated with manganese oxides. For both Yucca Mountain tuff and synthetic pyrolusite, Pu(IV) or Pu(V) was initially oxidized to more mobile Pu(V/VI), but over time, the less mobile Pu(IV) became the predominant oxidation state of the sorbed Pu. The observed stability of Pu(IV) on oxidizing surfaces (e.g., pyrolusite), is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. These findings have important implications in estimating the risk associated with the geological burial of radiological waste in areas containing Mn-bearing minerals, such as at the Yucca Mountain or the Hanford Sites, because plutonium will be predominantly in a much less mobile oxidation state (i.e., Pu(IV)) than previously suggested (i.e., Pu(V/VI)).

The distribution of V in magmatic rocks is controlled primarily by spinel stability. Extensive previous experimental work at oxidized conditions on doped (V-rich) compositions has led to the recognition of the importance of temperature, oxygen fugacity, and spinet composition, but also left ambiguity with respect to the relative importance of these variables in controlling D-v(spinel/melt). One major uncertainty has been the valence of V in the spinet and glass. Spinel-melt pairs were equilibrated at low and variable oxygen fugacities, with a range of V and Ti contents. XANES spectra were measured on the spinet and glass products, and pre-edge peaks measured and calibrated against valence with the use of glass and oxide standards. The valence of V is always greater in the glass than in the spinels. In spinet, V is dominantly 3+ at oxygen fugacities near the FMQ (fayalite magnetite quartz) buffer, but we find evidence for mixed 3+, 4+, and 5+ at oxidized conditions (FMQ to air), and 2+ and 3+ at very reduced conditions [FMQ to IW-1 (1 log f(O2) unit below the iron wustite buffer)]. Increased V contents in spinels are correlated with increased D-v(spinel-melt), at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in D-v(spinel-melt) and a shift to more reduced V (smaller pre-edge peak), which may be related to Fe-V exchange equilibria. Using the new partition coefficients, together with published results and valence information, expressions have been derived to predict D-v(spinel/melt) for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of D-v(spinel/melt) values relevant to natural systems. Calculation of V depletions in planetary mantles from basalt suites must take silicate, oxide, and metal fractionation into account, as is demonstrated using terrestrial, lunar, martian, and eucritic samples.

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 X 3 Am beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated. Copyright (c) 2005 Elsevier Ltd.

The Sr/Ca of coral skeletons demonstrates potential as an indicator of sea surface temperatures (SSTs). However, the glacial-interglacial SST ranges predicted from Sr/Ca of fossil corals are usually higher than from other marine proxies. We observed infilling of secondary aragonite, characterised by high Sr/Ca ratios, along intraskeletal pores of a fossil coral from Papua New Guinea that grew during the penultimate deglaciation (130 +/- 2 ka). Selective microanalysis of unaltered areas of the fossil coral indicates that SSTs at similar to 130 ka were <= 1 degrees C cooler than at present in contrast with bulk measurements ( combining infilled and unaltered areas) which indicate a difference of 6-7 degrees C. The analysis of unaltered areas of fossil skeletons by microprobe techniques may offer a route to more accurate reconstruction of past SSTs.

Full understanding of atomic arrangement in amorphous oxides both at ambient and high pressure is an ongoing fundamental puzzle. Whereas the structures of archetypal oxide glasses such as v-B2O3 at high pressure are essential to elucidate origins of anomalous macroscopic properties of more complex melts, knowledge of the high-pressure structure and pressure-induced coordination changes of these glasses has remained elusive due to lack of suitable in situ experimental probes. Here, we report synchrotron inelastic X-ray scattering results for v-B2O3 at pressures up to 22.5 GPa, revealing the nature of pressure-induced bonding changes and the structure. Direct in situ measurements show a continuous transformation from tri-coordinated to tetra-coordinated boron beginning at 4-7 GPa with most of the boron tetra-coordinated above 20 GPa, forming dense tetrahedral v-B2O3. After decompression from high pressure the bonding reverts back to tri-coordinated boron but with the data suggesting a permanent densification.

This paper shows that synchrotron-based fluorescence and absorption-edge computed microtomographies (CMT) are well-suited for determining the compartmentalization and concentration of metals in hyperaccumulating plant tissues. Fluorescence CMT of intact leaf, stem, and root samples revealed that Ni concentrated in stem and leaf dermal tissues and, together with Mn, in distinct regions associated with the Ca-rich trichomes on the leaf surface of the nickel hyperaccumulator Alyssum murale ``Kotodesh''. Metal enrichment was also observed within the vascular system of the finer roots, stem, and leaves but absent from the coarser root, which had a well-correlated metal coating. Absorption-edge CMT showed the three-dimensional distribution of the highest metal concentrations and verified that epidermal localization and Ni and Mn colocalization at the trichome base are phenomena that occurred throughout the entire leaf and may contribute significantly to metal detoxification and storage. Ni was also observed in the leaf tips, possibly resulting from release of excess Ni with guttation fluids. These results are consistent with a transport model where Ni is removed from the soil by the finer roots, carried to the leaves through the stem xylem, and distributed throughout the leaf by the veins to the dermal tissues, trichome bases, and in some cases the leaf tips.

The Bond Valence model gives a simple, accurate relationship between the bond distance, co-ordination number, and formal valence of ions in many systems. This empirical model thus gives complementary information to EXAFS and XANES, and has been used as an independent check of XAFS results by many workers. More generally, the Bond Valence represents prior knowledge of the system that can be used to constrain the XAFS analysis itself. Since Bond Valence sums are not perfectly satisfied, their use as exact constraints in XAFS analysis may not be warranted and they are probably best suited as restraints - inexact conditions placed on the analysis that act as additional data or prior knowledge of the system. The practical use of Bond Valence sums as constraints and restraints in EXAFS analysis is demonstrated for iron oxides, and the use of Bond Valence sums as prior knowledge that can be applied to a wide-range of XAFS analyses is discussed.

We introduce a new software package for analysis of XAS data. This package is based on the IFEFFIT library of analytical and numerical algorithms and uses the perl/Tk graphics toolkit. The two main components are ATHENA, a program for XAS data processing, and artemis, a program for EXAFS data analysis using theoretical standards from FEFF. These programs provide high quality analytical capabilities in a manner that is accessible to novices yet powerful enough to meet the demands of an expert. The programs run on all major platforms and are freely available under the terms of a free software license.

A software package for the analysis of X-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data analysis using theoretical standards from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of atomic absorption data. These programs enable high-quality data analysis that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.

Chemical analyses of U.S. stockpiled mustard chemical warfare agent show some agent destined for destruction contains mercury [L. Ember, Chem. Eng. News 82 (2004) 8]. Because of its toxicity, mercury must be removed from agent prior to incineration or be scrubbed from incineration exhaust to prevent release into the atmosphere. Understanding mercury/agent interactions is critical if either atmospheric or aqueous treatment processes are used. We investigate and compare the state of mercury in water to that in thiodiglycol, a mustard simulant, as co-contaminants are introduced. The effects of sodium hypochlorite and sodium hydroxide, common neutralization chemicals, on mercury in water and simulant with and without co-contaminants present are examined using X-ray absorption fine spectroscopy (XAFS). (c) 2005 Elsevier B.V. All rights reserved.

A new microscale oxybarometer for solar system basaltic glasses, based on vanadium K-edge X-ray absorption near edge structure (XANES) spectroscopy, is described. Vanadium is unique among abundant elements in siliceous materials in that it can potentially occur in nature in four valence states: V2+, V3+, V4+ and V5+. Consequently, the vanadium redox system is a robust oxybarometer covering at least six orders of magnitude in buffer-relative oxygen fugacity. The method was calibrated using synthetic glass standards produced under known fO(2) and temperature conditions. Correction for temperature differences among standards and unknowns was quantified using microXANES data for isobaric synthetic glass couples. Application of the method to lunar, martian, and terrestrial glasses yielded fO(2) estimates from 1.6 log units more reduced than the iron-wustite (IW) buffer (IW-1.6) for lunar glasses, to IW + 4.0 for terrestrial glass inclusions. The martian and terrestrial results are in good agreement with previous estimates by other methods. The inferred fO(2) values for lunar pyroclastic glasses are similar to 0.5 log unit more reduced than previous estimates, but the differences are comparable to analytical uncertainties. Micro-extended X-ray absorption fine structure spectra were consistent with the valence states determined by microXANES and provided additional constraints on vanadium site geometry. These results demonstrate the value of this new oxybarometer, which can be applied nondestructively to individual grains in conventional thin sections with similar to micrometer resolution and similar to 100 ppm elemental sensitivity. Copyright (c) 2005 Elsevier Ltd.

Costly disposal of uranium (U) contaminated sediments is motivating research on in situ U(VI) reduction to insoluble U(IV) via directly or indirectly microbially mediated pathways. Delivery of organic carbon (OC) into sediments for stimulating U bioreduction is diffusion-limited in less permeable regions of the subsurface. To study OC-based U reduction in diffusion-limited regions, one slightly acidic and another calcareous sediment were treated with uranyl nitrate, packed into columns, then hydrostatically contacted with tryptic soy broth solutions. Redox potentials, U oxidation state, and microbial communities were well correlated. At average supply rates of 0.9 mu mol OC (g sediment)(-1) day(-1), the U reduction zone extended to only about 35-45 mm into sediments. The underlying unreduced U(VI) zone persisted over 600 days because the supply of OC was diffusion-limited and metabolized within a short distance. These results also suggest that low U concentrations in groundwater samples from OC-treated sediments are not necessarily indicative of pervasive U reduction because interior and exterior regions of such sediment blocks can contain primarily U(VI) and U(IV) respectively.

Typha latifolia (cattail) sequesters arsenic within predominantly ferric iron root coatings, thus decreasing mobility of this toxic element in wetland sediments. Element-specific XRF microtomographic imaging illustrated a high spatial correlation between iron and arsenic in root plaques, with little arsenic in the interior of the roots. XANES analyses demonstrated that the plaque was predominantly ferric iron and contained approximately 20 As(III) and 80% As(V), which is significant because the two oxidation states form species that differ in toxicity and mobility. For the first time, spatial distribution maps of As oxidation states were developed, indicating that As(III) and As(V) are both fairly heterogeneous throughout the plaque. Chemical extractions showed that As was strongly adsorbed in the plaque rather than coprecipitated. Iron and arsenic concentrations ranged from 0.03 to 0.8 g Fe g(-1) wet plaque and 30 to 1200 mug As g(-1) wet plaque, consistent with a mechanism of As adsorption onto Fe(III) oxyhydroxide plaque. Because this mechanism decreases the concentrations of both As(III) and As(V) in groundwater, we propose that disruption of vegetation could increase the concentrations of mobile arsenic.

Individual 1-3 mm biotite and muscovite clasts from Hanford sediment were contacted with 0.08 M CsNO3. They were examined using electron or X-ray microprobe methods, as intact specimens or sectioned perpendicular to their basal planes. Cs+ was observed to preferentially sorb to mica edges, steps on mica surfaces, or fractured regions. The localization of Cs conformed to hypothesized strong binding to frayed edge sites in preference to sites on basal planes. In section, Cs+ was found to penetrate the mica interior, forming discrete zones of concentration, particularly in muscovite. In biotite, Cs was more abundant, permeating the clasts, but also forming discrete zones of higher concentration. Concentrated Cs on both clast edges and within clast interiors corresponded to microscopic but relatively extensive zones where K was depleted. The localization of sorbed Cs in areas where K was depleted suggested that weathering reactions had caused the formation of frayed edge sites within the micas. Cs+ accessed crystal interiors by diffusion along channels following crystal defects, cracks, or partings where pore fluids had previously migrated to form the interior alteration zones. On the nanometer scale, areas with localized Cs were disrupted, confirming that frayed edge sites were developed in clast interiors.

Attributed to their specific atomic bonding, the soft, graphite-like, hexagonal boron nitride (h-BN) and its superhard, diamond-like, cubic polymorph (c-BN) are important technological materials with a wide range of applications(1). At high pressure and temperature, h-BN can directly transform to a hexagonal close-packed polymorph (w-BN)(2) that can be partially quenched after releasing pressure. Previous theoretical calculations(3-5) and experimental measurements (primarily on quenched samples)(6-9) provided substantial information on the transition, but left unsettled questions due to the lack of in situ characterization at high pressures. Using inelastic X-ray scattering to probe the boron and nitrogen near K-edge spectroscopy, here we report the first observation of the conversion process of boron and nitrogen sp(2)- and p-bonding to sp(3) and the directional nature of the sp(3) bonding. In combination with in situ X-ray diffraction probe, we have further clarified the structure transformation mechanism. The present archetypal example opens two enormous, element-specific, research areas on high-pressure bonding evolutions of boron and nitrogen; each of the two elements and their respective compounds have displayed a wealth of intriguing pressure-induced phenomena(10) that result from bonding changes, including metallization(11,12), superconductivity(13,14), semiconductivity(15), polymerization(16) and superhardness(2,17,18).

Uranium contamination of soils and sediments often originates from acidic or alkaline waste sources, with diffusion being a major transport mechanism. Measurements of U(VI) diffusion from initially pH 2 and pH 11 solutions into a slightly alkaline Altamont soil and a neutral Oak Ridge soil were obtained through monitoring uptake from boundary reservoirs and from U concentratien profiles within soil columns. The soils provided pH buffering, resulting in diffusion at nearly constant pH. Micro X-ray absorption near edge structure spectra confirmed that U remained in U(VI) forms in all soils. Time trends of U(VI) depletion from reservoirs and U(VI) concentration profiles within soil columns yielded K-d values consistent with those determined in batch tests at similar concentrations (approximate to1 mM) and much lower than values for sorption at much lower concentrations (nM to muM). These results show that U(VI) transport at high concentrations can be relatively fast at non-neutral pH, with negligible surface diffusion, because of weak sorption.

The structure of the hydroxylated hematite (0 0 0 1) surface was investigated using crystal truncation rod diffraction and density functional theory. The combined experimental and theoretical results suggest that the surface is dominated by two hydroxyl moieties-hydroxyls that are singly coordinated and doubly coordinated with Fe. The results are consistent with the formation of distinct domains of these surface species; one corresponding to the hydroxylation of the surface Fe-cation predicted to be most stable under UHV conditions, and the second a complete removal of this surface Fe species leaving the hydroxylated oxygen layer. Furthermore, our results indicate that the hydroxylated hematite surface structures are significantly more stable than their dehydroxylated counterparts at high water partial pressures, and this transition in stability occurs at water pressures orders of magnitude below the same transition for alpha-alumina. These results explain the observed differences in reactivity of hematite and alumina (0 0 0 1) surfaces with respect to water and binding of aqueous metal cations. (C) 2004 Elsevier B.V. All rights reserved.

Oxidative dissolution of a primary Mn-silicate phase (rhodonite) was studied via synchrotron X-ray techniques. The study was designed to combine the element-specific chemical technique of Glancing Incidence X-ray Absorption Spectroscopy (GIXAS) with the surface structural technique of X-ray scattering in order to produce the first depth resolved study of Mn-silicate low-temperature reactivity. A chemo-mechanically polished polycrystalline rhodonite sample was characterized and then reacted with pH 3.5 nitric acid. The surface originally had a mosaic structure and 15.5 (+/-1) Angstrom r.m.s. roughness. Surface composition was not measurably different from bulk rhodonite before reaction, indicating that the surface preparation regimen had not produced an altered surface. After 1 h of reaction, the roughness of the mineral surface decreased and reflectivity oscillations developed, resulting from the formation of a leached layer. This layer was 74.7 (+/-2) Angstrom thick with an electron density equal to 72 of that of bulk rhodonite (equal to the loss of similar to1 in 2. Mn atoms). Both the primary and the buried interfaces had similar roughnesses; 4.9 and 4.5 (+/-1.0) Angstrom, respectively. Diffuse scatter indicated that the correlation length between surface features also decreased. The GIXAS analysis showed that the Mn remaining in the surface had become oxidized, with the degree of oxidation decreasing as a function of depth. Oxidation penetrated at least 140 Angstrom into the structure. A further 2.5 h of reaction at pH 3.5 caused dissolution of the leached layer and reduced the thickness of this altered region to 16.0 (+/-2) Angstrom, while surface roughness increased slightly to 6.2 ( +/-1.0) Angstrom. Depletion of Mn in this region increased only slightly relative to the first reaction step; the electron density was 67% that of bulk rhodonite, equivalent to the loss of 2 in 3 Mn atoms. The thickness of the oxidized region however, persisted. Analysis by XPS on the same specimen corroborates the X-ray results.

Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (similar to400 mum(3)) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought. Copyright (C) 2003 Elsevier Science Ltd.

The local structure of an amorphous a-GdxSi1-x (x=0.04,0.07,0.12,0.18) magnetic semiconductor is determined from combined x-ray absorption fine structure (XAFS) measurements at the Gd L-3 and Si K absorption edges. XAFS data indicate that the structure is amorphous, with Gd atoms surrounded predominantly by Si atoms, consistent with a random substitution of Gd in the Si network. Gd substitution induces a large local distortion resulting in a Gd-Si distance of 2.98(3) Angstrom compared to a Si-Si distance of 2.39(1) Angstrom. The lack of x dependence in the Gd local environment indicates that the distortions around the Gd do not interact with one another, even at x=0.18. In addition, a small but systematic increase in the mean squared disorder of the Si-Si interatomic distance is observed with x. These results suggest that the amorphous Si network is very effective in relaxing the local strain through random displacements of Si atoms within a few Angstrom of the perturbing Gd atoms.

The occurrence of trace amounts of uranyl in natural calcite has posed a long-standing problem in crystal chemistry because of speculation that the size and shape of the uranyl ion may preclude its incorporation in a stable lattice position in calcite. This also defines an important environmental problem because of its bearing on the transport and sequestration of uranyl released from nuclear facilities and uranium mining operations. Calcite is a nearly ubiquitous mineral in soils and groundwater aquifers. X-ray absorption spectroscopy and X-ray fluorescence microprobe studies of uranium in relatively U-rich similar to13 700-year-old calcite from a speleothem in northernmost Italy indicate substitution of uranyl for a calcium and two adjacent carbonate ions in calcite. These new data imply that uranyl has a stable lattice position in natural calcite, indicating that it may be reliably sequestered in calcite over long time scales.

Compressed under ambient temperature, graphite undergoes a transition at similar to17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

Using synchrotron X-ray fluorescence (XRF) microanalysis, including XRF tomography, and Xray absorption near-edge structure (XANES) analyses, the distribution, and nature of incorporation of Fe and Ni impurities in as-grown diamond crystals, synthesized under high-pressure and high-temperature (HPHT) conditions, have been characterized. We find significantly different behavior for Fe and Ni as impurities in diamond. Nickel is dispersed and sector-zoned, with concentrations in {111} growth sectors at least 3 times those in {100} sectors, whereas Fe exists in the form of micro-aggregates or clusters with no observable sector correlation. Fe K-edge XANES shows that Fe is oxidized in diamond and has a valence of 2+. Comparison of XANES spectra from numerous standard compounds indicates that Fe is very likely bonded with oxygen as FeO.

X-ray spectroscopy measurements have been combined with macroscopic uptake data and transmission electron microscopy (TEM) results to show that Pb(II) uptake by Burkholderia cepacia is due to simultaneous sorption and biomineralization processes. X-ray microprobe mapping of B. cepacia biofilms formed on alpha-Al2O3 surfaces shows that Pb(II) is distributed heterogeneously throughout the biofilms because of the formation of Pb ``hot spots''. EXAFS data and TEM observations show that the enhanced Pb accumulation is due to the formation of nanoscale crystals of pyromorphite (Pb-5(PO4)(3)(OH)) adjacent to the outermembrane of a fraction of the total population of B. cepacia cells. In contrast, B. cepacia cell suspensions or biofilms that were heat-killed or pretreated with X-rays do not form pyromorphite, which suggests that metabolic activity is required. Precipitation of pyromorphite occurs over several orders of magnitude in [H+] and [Pb] and accounts for approximately 90% of the total Pb uptake below pH 4.5 but only 45-60% at near-neutral pH because of the formation of additional Pb(II) adsorption complexes. Structural fits of Pb L-III EXAFS data collected for heat-treated cells at near-neutral pH suggest that Pb(II) forms inner-sphere adsorption complexes with carboxyl functional groups in the biofilms.

Biogeochemical transformations of redox-sensitive chemicals in soils can be strongly transport-controlled and localized. This was tested through experiments on chromium diffusion and reduction in soil aggregates that were exposed to chromate solutions. Reduction of soluble Cr(VI) to insoluble Cr(III) occurred only within the surface layer of aggregates with higher available organic carbon and higher microbial respiration. Sharply terminated Cr diffusion fronts develop when the reduction rate increases rapidly with depth. The final state of such aggregates consists of a Cr-contaminated exterior, and an uncontaminated core, each having different microbial community compositions and activity. Microbial activity was significantly higher in the more reducing soils, while total microbial biomass was similar in all of the soils. The small fraction of Cr(VI) remaining unreduced resides along external surfaces of aggregates, leaving it potentially available to future transport down the soil profile. Using the Thiele modulus, Cr(VI) reduction in soil aggregates is shown to be diffusion rate- and reaction rate-limited in anaerobic and aerobic aggregates, respectively. Thus, spatially resolved chemical and microbiological measurements are necessary within anaerobic soil aggregates to characterize and predict the fate of Cr contamination. Typical methods of soil sampling and analyses that average over redox gradients within aggregates can erase important biogeochemical spatial relations necessary for understanding these environments.

X-ray fluorescence elemental maps and Cu K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for individual vapor and brine phase fluid inclusions from the Mole Granite, NSW, Australia. The maps indicate that Cu is concentrated in the vapor inclusions and, at room temperature, distributed uniformly in the condensed liquid. Opaque precipitates in these inclusions do not contain Cu. The absorption spectra identify the stable complexes as [Cu(H2O)(6)](2+) at 25 degreesC, [CuCl2](1-) at 200 degreesC, and either [CuCl](1-) or [CuCl(H2O)] at the homogenization temperature of around 400 degreesC. These changes in Cu coordination and oxidation state are fully reversible. We suggest that the vapor phase partitioning of Cu as a chloride complex from a high-density brine may occur under acidic conditions. Estimates of fluid acidity at the time of boiling may potentially predict the metal distribution between epithermal and porphyry-type environments in hydrothermal systems.

Molar volumes of molten indium have been measured in an isothermal compression up to 8.5 GPa at 710(3) K in an externally heated diamond anvil cell. The measurement is based on the x-ray diffraction and x-ray absorption of materials using a synchrotron monochromatic x-ray microbeam. The fit to the results with the Birch-Murnaghan equation of state gives parameters of V-0=16.80 cm(3), K-0=23.9(6) GPa, assuming that K'=4. This method should be applicable for measuring molar volumes of liquids and other amorphous materials in the diamond anvil cell. (C) 2002 American Institute of Physics.

The melting behaviour, structure, and density of molten indium at high pressures have been studied in an externally heated diamond anvil cell (DAC) using x-ray diffraction/scattering measurements. Melting at high pressure was identified by the appearance of diffuse scattering from the melt. Analysis of the diffuse scattering shows that at 710(3) K the coordination number at the nearest neighbour increases from 10.1(4) at 1.0 GPa to 12.1(5) at 6.3 GPa. A method for measuring the density of amorphous materials is introduced for DAC studies and the first result on molten indium is presented. (Some figures in this article are in colour only in the electronic version).

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO3 (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 mum scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment. Copyright (C) 2001 Elsevier Science Ltd.

We report the results of polarized Co K EXAFS experiments on a series of CoPt3 films grown by molecular beam epitaxy on (100) MgO single crystal substrates over a range of temperatures from 473 K to 1073 K. These samples exhibit substantial perpendicular magnetic anisotropy that is strongly dependent on the substrate growth temperature T-g. We measure a preference for in-plane Co-Co pairs that is correlated with the magnetic properties.

Using EXAFS, we investigated the changes in the local structure of Cd(II) complexes formed with a series of low-molecular-weight thiols at different thiol/cadmium stoichiometries. For dilute solutions, a comparison of theoretical and experimental standards proved the latter approach superior. The number of bound oxygen atoms decreased while the number of sulfur atoms increased as the thiol/cadmium ratios increased, indicating that up to four thiol compounds can bind to cadmium in aqueous solutions. The computer code FEFFIT was modified to employ experimental and theoretical standards with equal facility.

Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.

Some of the advanced EXAFS analysis features of FEFF and FEFFIT are described. The scattering path formalism from FEFF and cumulant expansion are used as the basic building blocks of EXAFS analysis, giving a flexible and robust parameterization of most EXAFS problems. The ability to model EXAFS data in terms of generalized physical variables is shown, including the simultaneous refinement of two different polarizations for Co K EXAFS data of CoPt3.

IFEFFIT, an interactive program and scriptable library of XAFS algorithms is presented. The core algorithms of AUTOBK and FEFFIT have been combined with general data manipulation and interactive graphics into a single package. IFEFFIT comes with a command-line program that can be run either interactively or in batch-mode. It also provides a library of functions that can be used easily from C or Fortran, as well as high level scripting languages such as Tcl, Perl and Python. Using this library, a Graphical User Interface for rapid `online' data analysis is demonstrated. IFEFFIT is freely available with an Open Source license. Outside use, development, and contributions are encouraged.

Eu L, X-ray absorption near edge structure (XANES) spectroscopy was conducted with a wavelength dispersive (WDS) spectrometer to determine the valence state of europium in a natural, Mn-and REE-rich apatite from Llallagua, Bolivia. Europium exists in both the divalent and trivalent states in the Llallagua apatitie with a Eu2+/Eu3+ ratio between 0.12 and 0.22. With the enhanced energy resolution of WDS the EuL alpha (1) fluorescence line can be resolved from the MnK alpha line, allowing for significant reduction of background in the EuL3 absorption edge region and resolution of the Eu XAS, which is difficult by conventional methods because of fluorescence peak interference. The anomalous partitioning behavior of Eu in these samples (Rakovan and Reeder 1996) can be explained by the observed presence of Eu2+ and Eu3+ and is consistent with the suggested size effect on intrasectoral zoning of REEs in apatite.

The distribution of metal contaminants such as chromium in soils can be strongly localized by transport limitations and redox gradients within soil aggregates. Measurements of COO diffusion and reduction to Cr(III) were. obtained in soil columns representing transacts into soil aggregates in order to quantify influences of organic carbon (OC) and redox potentials on Cr transport distances and microbial community composition. Shifts in characteristic redox potentials, and the extent of Cr(VI) reduction to Cr(III) were related to OC availability. Depth profiles of Cr(VI, III) obtained with micro X-ray absorption near edge structure (micro-XANES) spectroscopy reflected interdependent effects of diffusion and spatially dependent redox potentials on reduction kinetics and microbial community composition. Shallow diffusion depths (2-10 mm) and very sharply terminated diffusion fronts in columns amended with OC (80 and 800 ppm) reflected rapid increases in Cr reduction kinetics over very short (mm) distances. These results suggest that Cr contamination in soils can be restricted to the outsides of soil aggregates due to localized transport and rapid reduction and that bulk sample characterization is inadequate for understanding the controlling biogeochemical processes.

The speciation of Au in gold-bearing arsenopyrite (FeAsS) from four gold deposits (Olympiada, Sentachan, Sao Bento and Sheba) was determined by micro-X-ray absorption near-edge structure (XANES) on grains well characterized microscopically and by electron-microprobe and secondary-ion mass spectrometry analyses and images. ``Invisible'' gold in arsenopyrite occurs in two apparently mutually exclusive chemical forms: chemically bound and elemental. Arsenopyrite from the Sentachan, Sao Bento and Sheba deposits contains chemically bound gold. With comparable constituent electronegativities and a white-line feature in the XANES indicating unoccupied An 5d-states, but absorption-edge positions comparable to Au1+ species, the bonding is interpreted as being covalent rather than ionic. The invisible gold in arsenopyrite from the Olympiada deposit, on the other hand, occurs as very small particles of Au-0, probably less than a few nanometers in diameter. Micro-XANES data for the Olympiada and Sentachan arsenopyrite support earlier results obtained by Au-197 Mossbauer spectroscopy on arsenopyrite concentrates. In some arsenopyrite crystals, the gold concentration is closely related to growth zoning. This feature represents conditions during crystallization and does not correlate with the chemical form of the gold. Similarly, selenium, where present, correlates with gold in some deposits and not in others, irrespective of the gold speciation. The finding of two types of invisible gold in arsenopyrite from different deposits has beneficial implications for extractive metallurgy.

The physical and chemical properties of the hydrated alpha-Al2O3 (0001) surface are important for understanding the reactivity of natural and synthetic aluminum-containing oxides. The structure of this surface was determined in the presence of water vapor at 300 kelvin by crystal truncation rod diffraction at a third-generation synchrotron x-ray source. The fully hydrated surface is oxygen terminated, with a 53% contracted double Al Layer directly below. The structure is an intermediate between alpha-Al2O3 and gamma-Al(OH)(3), a fully hydroxylated form of alumina. A semiordered oxygen Layer about 2.3 angstroms above the terminal oxygen Layer is interpreted as adsorbed water. The clean alpha-Al2O3 (0001) surface, in contrast, is AL terminated and significantly relaxed relative to the bulk structure. These differences explain the different reactivities of the clean and hydroxylated surfaces.

Detailed analyses of crystalline, hydrated, and precipitated strontium compounds and an aqueous strontium solution by synchrotron extended X-ray absorption fine structure (EXAFS) were used to quantify local thermal and static disorder and to characterize strontium coordination in a variety of oxygen-ligated bonding environments. Analysis of anharmonic vibrational disorder (i.e., significant contribution from a third cumulant term (C-3) in the EXAFS phase-shift function) in compounds with low and high static disorder around strontium showed that first-shell anharmonic contributions were generally not significant above experimental error in the EXAFS fits (R +/- 0.02 Angstrom with and without C-3) The only case in which a significant apparent decrease in Sr-O distance was observed with increasing temperature, and for which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye-Waller factor (sigma(2)) for strontium compounds as a function of backscatterer atomic number (Z), interatomic Sr-Z distance, and temperature of spectral data collection showed systematic increases in sigma(2) as a function of increasing temperature and Sr-Z bond length. At values of sigma(2) greater than approximate to 0.025 Angstrom(2) (for N < 12 and RSr-Z > 3 Angstrom), backscattering was generally not significant above noise levels in spectra of compounds of known crystal structure. Comparison of the EXAFS spectra of freshly precipitated SrCO3 (spectra collected wet) to that of dry, powdered strontianite (SrCO3(s)) indicated no significant differences in the local atomic structure around strontium. Analysis of partially hydrated strontium in natural Ca-zeolite (heulandite) showed that strontium is substituted only in the calcium (Ca2) site. Backscattering from aluminum and silicon atoms in the zeolite framework were apparent in the EXAFS spectra at low and room temperature at distances from central strontium of <4.2 Angstrom. Comparison of strontium structural coordination determined in this and previous studies suggests that previous EXAFS determinations of hydrated strontium may have underestimated first-shell interatomic distances and coordination numbers because minor contributions to the EXAFS phase-shift and amplitude functions were not accounted for, either theoretically or empirically. (C) 2000 Academic Press.

The bonding environment of sorbed species at rock-water interfaces provides information on their mobility in natural systems. To investigate Pu sorption at a natural surface on a microscopic scale, we studied the bonding environment of sorbed Pu on zeolitic tuff with micro-XAS techniques. Using a focused 4 by 7 mu m(2) beam, XAS spectra were taken at Mn-smectite regions where high levels of sorbed Pu were found. The Pu did not sorb to Fe-rich regions. XANES spectra show the presence of Pu(VI), based on edge energy. Preliminary analyses of the micro-EXAFS spectra indicate the absence of a first shell splitting of axial and equatorial Pu-O bonds-an environment unlike what is observed for most hexavalent actinides.

The estimate of measurement uncertainties in XAFS spectra is one of the most important and difficult aspects of error analysis for XAFS data. Since several different analysis methods are used to determine structural parameters and their uncertainties from XAFS data, and no universally agreed-upon standard exists, the ability to compare estimates of uncertainties from different methods is vital. We discuss numerical aspects of the propagation of measurement uncertainties from raw absorption coefficient to analyzed signal in k- and R-space. We focus our attention on the conversion of measurement uncertainties between k- and R-space representations of XAFS data, and on the testing of this conversion procedure for data dominated by random fluctuations.

An archive of XAFS data collected on standard model compounds of transition metals has been constructed and made available by the world wide web. The data in this library have all been taken in transmission on powder samples free of thickness effects. Data are stored in individual files in a standardized ASCII column format. Fields describing the contents of files can be searched easily. The data are intended to provide standards for comparison of EXAFS analysis procedures, for empirical analysis of XANES features, and to test theoretical XANES calculations. Emphasis has been placed on inorganic transition metal compounds to assist the analysis of environmentally relevant samples.

GeoSoilEnviroCARS, sector 13 of the Advanced Photon Source at Argonne National Laboratory, provides a micro-beam facility for XAFS, XANES, XRF, and x-ray diffraction studies. A Kirkpatrick-Baez mirror pair gives a focussed monochromatic undulator beam down to 1 x 1 mu m with sufficient intensity for x-ray fluorescence mapping and extended XAFS of dilute systems at energies above 4KeV. Special emphasis for these facilities has been given to environmental and earth science studies, including dilute contaminants in natural sediments. As an example, XRF maps and EXAFS data taken with a 4 mu m by 7 mu m spot size are shown for the Pu L-III edge of a natural tuff exposed to a dilute aqueous solution of Pu. In addition, x-ray diffraction and scattering capabilities allow the study of surfaces and surface adsorbates with micron-sized beams using xray reflectivity, x-ray standing-waves, and grazing-incidence XAFS.

The narrow band-gap semiconductor Pb1-xGexTe has a low-temperature ferroelectric rhombohedral phase whose average structure is a distorted rocksalt structure. We have measured the extended x-ray-absorption-fine-structure (EXAFS) spectra of Pb1-xGexTe with x approximate to 0.3 at the Ge and Te K edges and at the Pb L-III edge. Guided by first-principles calculations, we create a model for the local structure as a distortion from the ideal rocksalt structure. By corefining the spectra from these three edges, we demonstrate that the data are consistent with our fitting model and we directly measure several secondary structural distortions predicted by the theory. This work demonstrates a powerful approach to the determination of local structures in complex materials by using first-principles calculations in conjunction with EXAFS measurements. [S0163-1829(99)13041-8].

Improved calculations of Bragg peak intensities near atomic resonance are obtained by including the effect of the local environment around the resonant atoms on the resonant scattering amplitudes Delta f=f'+if'' Theoretical absorption cross sections calculated by the ab initio x-ray-absorption code FEFF are used to obtain the imaginary part f'' by extension of the optical theorem to nonforward scattering under the dipole approximation. The real part f' is obtained by a limited range Kramers-Kronig transform of the difference between f'' based on FEFF and existing theoretical calculations of f'' based on an isolated-atom model. The atomic part of Delta f calculated by FEFF for the resonant atom embedded in the local potential is assumed to have spherical symmetry; however, no restriction is placed on the spectral features due to multiple scattering of the intermediate-state virtual photoelectron. Bragg peak intensities calculated in the kinematic approximation using the FEFF-based Delta f are compared to intensities calculated using the isolated-atom Delta f and to experimental data for Cu metal and YBa2Cu3O6.8 at the Cu K absorption edge,and for UO2 at the U M-IV absorption edge. [S0163-1829(98)06541-2].

We present the optical designs, modeling, bender design and test results of x-ray micro-focusing optics used to micro-focus monochromatic undulator x-rays at the Advanced Photon Source (APS) The system uses two 100mm long, actively bent mirrors in a Kirkpatrick Baez arrangement. A detailed analytical model of the system's performance is described along with ray tracing results. A description of the integration of the benders into a complete micro-focusing system is provided. The system is easy to setup and use and is presently used in earth science research coupled to techniques such as micro-spectroscopy, fluorescence microprobe, and energy dispersive diffraction. The optics' performance is measured on the GeoSoilEnviroCARS microprobe experimental station at APS sector 13. Focusing tests using 10keV undulator x-rays resulted in a double focused beam with a horizontal and vertical full width at half maximum of 0.80 mu m x 0.85 mu m, and flux density gain greater than 10(5).

The separate complex resonant scattering amplitudes for the two inequivalent Cu sites in YBa2Cu3O7-delta have been determined using diffraction anomalous fine structure (DAFS). The combined amplitudes Delta f(Q,E) for eight specular (00l) reflections were isolated from the measured intensity using the iterative dispersion algorithm of Pickering, et al., modified to accommodate contributions to the imaginary part of the scattering amplitude from the heavy Ba and Y atoms. The individual site response functions f `'(E) were solved by singular value decomposition of the matrix of crystallographic coefficients and applying the inverse matrix to Delta f(Q,E) at each value of energy. For comparison, a second set of coefficients was obtained by simultaneously fitting the DAFS fine structure functions chi(Q, k) using the computer program FEFFIT under a set of constraints. The chi(Q, k) were modeled as linear combinations of the two Cu site chi(k) functions using a kinematic structure factor model with Q as the independent variable and theoretical chi(k) from FEFF7.

Diffraction Anomalous Fine-Structure (DAFS) combines the sensitivity to long-range-order of diffraction with the short-range-order sensitivity of XAFS. This makes it possible to use a set of DAFS measured intensities to simultaneously refine both long-and short-range structural parameters, while maintaining some constraints between them. This method combines a calculation of the structure factor based on the unit cell of the crystal with a calculation of the fine-structure chi(E) around each resonant site. Tabulated values of the scattering amplitude are used away from the resonant energies, while the near-edge anomalous scattering amplitude is calculated for the resonant sites using a differential Kramers-Kronig transform of an embedded atom absorption coefficient mu(0)(E) from FEFF. We discuss some of the subtleties of this approach to DAFS analysis.

XAFS measurements and analyses of Rb+ ions in 0.5 molal RbBr at ambient and several supercritical (P > P-c = 220 bar, T > T-c = 374 degrees C) conditions are presented. The first coordination shell of Rb+ ion is seen to be dominated by oxygen from nearby water molecules. Upon going from ambient to supercritical conditions, the Rb-O coordination number decreases significantly and the Rb-O distance decreases by similar to 0.10 Angstrom relative to the ambient Rb-O distance (similar to 2.93 Angstrom). Although ion pairing has been predicted for supercritical conditions (due in part to a substantial decrease of the dielectric constant at the critical point), and has been observed with XAFS in other aqueous solutions, we see no direct evidence of pairing of Br- with Rb+ in our measurements.

X-ray absorption fine structure (XAFS) measurements and molecular dynamics simulations(MD) are used to explore the extent of Br-ion hydration in supercritical water solutions. The structure of the first hydration shell under ambient conditions is compared to that in the supercritical region spanning a temperature range from 25 to 475 degrees C and pressures from 1 to 650 bar. The RbBr salt concentration was varied from 0.02 to 1.5 molal. The wide range of conditions studied allowed a detailed examination of the effect of temperature, density, and concentration on the extent of ion hydration under supercritical conditions. The present results provide important new insights into factors affecting ion hydration in supercritical water. Changing the density of the supercritical solution by a factor of 1.5 causes only minor changes in the extent of ion hydration at 425 degrees C, whereas a pronounced dehydration occurs as the temperature is increased from 25 to 475 degrees C, Specifically, the number of water molecules in the first hydration shell is reduced from 7.1(+/-1.5) under ambient conditions to 2.8(+/-0.4) under the supercritical conditions of 425 degrees C and 413 bar. Over a concentration range of almost two orders-of-magnitude, there is little change in the extent of hydration. MD simulations of this system are used to generate XAFS spectra that are directly compared to the experimental results. Analysis of the MD-simulated XAFS spectra verified the data reduction technique used for the high temperature conditions, There is qualitative agreement between the simulation and experiment with respect to the number of nearest neighbor waters, the nearest-neighbor distances, the degree of disorder in the first shell, and the trends of these parameters with increasing temperature. It is, however, evident that refinements in the water-bromine intermolecular potentials are required to fully capture the observed behavior under supercritical conditions.

X-ray absorption fine structure (XAFS) measurements and analyses are presented for Rb+ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 degrees C and pressures from 382 to 633 bar. For al reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well-defined hydration shell around the cation even at 424 degrees C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 Angstrom is also observed under supercritical conditions. The reduction in the number of water-ion bonds is analogous to the reduction in the amount of water-water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters-of-hydration under supercritical conditions may also be in part due to formation of contact-ion pairs. (C) 1996 American Institute of Physics.

A method is described to acquire x-ray absorption fine structure (XAFS) spectra of high-pressure liquid and supercritical fluid solutions. The technique employs a short length of fused-silica capillary tubing that has an inner diameter of 250 mu m and an outer diameter of 360 mu m. A hairpin bend is formed near the center of the capillary and the bend is then placed end-on directly in the focused x-ray beam. Fluorescence spectra were acquired in a 90 degrees geometry using a 13 element Ge detector. Demonstration XAFS spectra are reported for a Mn organometallic complex dissolved in subcritical and supercritical CO2. Although the maximum pressure of these studies was 160 bar, with slight modification, the method will be applicable to studies requiring pressures as high as 4 kbar. (C) 1996 American Institute of Physics.

Mixed salts AgBr0.6Cl0.4 and their pure components were investigated by XAFS. The concentration of the mixture was chosen to produce the homogeneous phase. Detailed analysis of the Ag and Br K edge data revealed an angular deviation of bonds in the mixture from collinearity. The corresponding RMS buckling angles (11 +/- 2 degrees) were determined using the strong angular dependence of the forward scattering amplitude in double- and triple-scattering focusing paths. The XAFS results were used to perform molecular-dynamics simulations which allowed to visualize the actual structure of the mixture.

A comparison between theory and experiment is made for the XAFS of the Zr and Ba K-edges in BaZrO3. BaZrO3 is chosen because of its simple and well-understood structure but combines metallic atoms with oxygens. It is found that adding phase corrections to the theoretical XAFS significantly improves the accuracy of the structural parameters. These corrections are changes in the core electron threshold energy E(o) for each single scattering path involving a different type of backscattering atom. This, together with the fact that no corrections were needed for pure metal Cu, suggests that the phase corrections needed are due to charge distribution effects which are neglected in the theory.

XAFS measurements over a temperature range of 200-900 K are presented for pure Ag and for the minority components in the alloys GaxAg1-x, and AuxAg1-x for x = 0.05. Detailed information about the temperature dependence of the radial distribution function for the near-neighbor distance is found by assuming a one-dimensional anharmonic potential of the form V(r) = 1/2ar(2) + br(3) + cr(4) for the interatomic band. The force constants a, b, and c of this potential are determined for each bond, and are shown to give unique thermodynamic information about the systems.

Theoretical standards for scattering amplitudes and phase shifts are often necessary for XAFS analysis, as for cases in which multiple scattering paths are important over the R-range of interest. Even when not necessary, they are often more convenient and reliable than experimental standards. We discuss several important considerations that must be taken into account to successfully compare ab initio theoretical calculations from FEFF to experimental XAFS spectra, and present a computer program, FEFFIT, to assist in using FEFF to get reliable information from experimental XAFS data.

The oscillatory structure in the diffraction anomalous fine structure (DAFS) spectrum contains at least as much local structural information as X-ray absorption fine structure (XAFS). We present three XAFS analysis programs, ATOMS, AUTOBK, and FEFFIT, which are easily extended to background removal and fine structure analysis in DAFS. The background subtraction and curve-fitting of DAFS data require the crystallographic structure factor, F, for the material. ATOMS calculates F for an arbitrary crystal from tables of anomalous scattering factors. AUTOBK, an automated background removal program, estimates the shape of the DAFS background and correctly normalizes the resulting chi(k) FEFFIT, an XAFS curve-fitting program using multiple scattering theory and sophisticated model-building capabilities, is easily extended to handle the DAFS oscillatory structure. We show the success of these programs in handling DAFS spectra.

Detailed analysis of XAFS spectra of the antiferrodistortive crystals K1-xNaxTaO3 show that the local rotation angles of the oxygen octahedra in crystals with x > 0.6 are non-zero and large, hundreds of degrees above the transition temperatures to the cubic phase.

The requirements for a reliable analysis program for XAFS spectra to obtain structural information are listed. Besides the more generally recognized items, it is emphasized that criteria to assess whether the model found is reasonable, and a fast and reliable error analysis which assures that the information content of the data is not exceeded, should be included. The University of Washington analysis program UWXAFS is described which satisfies the listed requirements and includes multiple scattering (MS) contributions in an efficient manner utilizing the FEFF program, and fits in R-space to allow limiting the MS paths.

The grain boundary regions of nanophase Cu metal are investigated using the x-ray absorption fine structure (XAFS) technique. Typical samples made by standard techniques need to be greatly thinned if measured in transmission in order to eliminate experimental artifacts which erroneously lower the apparent coordination number. To avoid this problem the samples were measured by the total electron yield technique. The results indicate a grain boundary structure which, on the average, is similar to that in conventional polycrystalline Cu, contrary to previous XAFS measurements made in transmission which indicated a lower coordination number.

A detailed x-ray-absorption fine-structure (XAFS) investigation of two mixed alkali halides Rb0.76K0.24Br and RbBr0.62Cl0.38 was performed. The concentrations of the mixtures had been chosen to produce a single homogeneous phase for each, and it was checked by XAFS that the salts were randomly mixed on the atomic level. Detailed analysis of the data including multiple-scattering contributions revealed an rms buckling angular deviation of both mixtures from the average NaCl collinear structure of 7-9-degrees. The angles are defined by three atomic positions determined through double- and triple-scattering paths. These angles are new parameters which should be added to characterize the buckled crystals. Adding to diffraction results the parameters determined from XAFS as input into a molecular-dynamics simulation the structures of the mixed salts with their fluctuations about the NaCl structure are solved and displayed.

Quantitative measurements of oxygen octahedra local rotation angles are reported for the antiferrodistortive type perovskites, Na0.82K0.18TaO3 and NaTaO3. The results were obtained using a detailed analysis of x-ray absorption fine structure spectra. These results show that the oxygen octahedra rotation angle at the peak of the rotation angle distribution function of an antiferrodistortive crystal is nonzero and large, hundreds of degrees above the transition temperature.

X-ray absorption fine structure (XAFS) measurements of mixed salts RbxK1-xBr and RbBryCl1-y at their congruent melting points compositions reveal that their actual structure buckles about the NaCl average structure with an rms angular deviation from collinearity of 7-degrees-9-degrees. XAFS measures the buckling directly through three-body correlations, and verifies the homogeneity and randomness of the mixtures. The characteristic ionic sizes are found to be dependent on concentration, causing changes to 0.1 angstrom in interatom bond distances. A computer simulation based on the information from XAFS and diffraction displays the actual structure of the salts.

X-ray-absorption fine-structure (XAFS) data of RbBr, RbCl, and KBr at 30 K and 125 K were measured and analyzed. An ionized-atom multiple-scattering calculation and a correlated Debye model were used for fitting the theory used in the FEFF5 computer code to data. The modifications of FEFF5 necessary to obtain good fits to the data are discussed. The results demonstrate the domination of single-scattering and focusing paths in XAFS and the determination of vibrational information through at least 10 angstrom around the center atom. Numerical calculations were performed to analyze the cause of the difference found between the forward-scattering amplitudes of Rb+ and Br- focusing atoms. The second and third cumulants were determined of the first and second neighbors and were used to calculate the temperature dependence of the thermal-expansion coefficient including quantum effects at low temperature. Agreement with macroscopic thermal-expansion measurements was found.

Sn impurities in Pb and Ag hosts have been investigated by Mossbauer effect and in Pb by x-ray-absorption fine-structure (XAFS) studies. The Sn atoms are dissolved up to at least 2 at. % in Pb and up to at least 8 at. % in Ag for the temperature ranges investigated. The concentration limit for Sn-Sn interactions is 1 at. % for Pb and 2 at. % for Ag as determined experimentally by lowering the Sn concentration until no appreciable change occurs in the Mossbauer effect. XAFS measurements verify that the Sn impurities in Pb are dissolved and predominantly at substitutional sites. For both hosts the temperature dependence of the spectral intensities of isolated Sn impurities below a temperature T0 is as expected for vibrating about a lattice site. Above T0 the Mossbauer spectral intensity exhibits a greatly increased rate of drop-off with temperature without appreciable broadening. This drop-off is too steep to be explained by ordinary anharmonic effects and can be explained by a liquidlike rapid hopping of the Sn, localized about a lattice site. Higher-entropy-density regions of radii somewhat more than an atomic spacing surround such impurities, and can act as nucleation sites for three-dimensional melting.

A new technique for separating the background, mu0, from the XAFS, chi, is presented and shown to be an improvement to standard background removal methods, especially in the near-edge region. The technique optimizes the low-R components of chi(R). For a calculated absorption spectrum for pure Ti, the mu0 found by the presented technique agrees well with the calculated background function. For pure Pb, the extracted mu0 is shown to be nearly the same as that measured from the temperature dependence of the full absorption spectrum starting at an energy in the absorption edge. The reliability of the background from the new method at low-k values allows more information from the XAFS spectra to be used and provides an opportunity for sensitive tests of theoretical calculations in the near-edge region.

Two independent techniques are used to obtain the background function for the x-ray-absorption fine structure (XAFS) of pure Pb at energies that are normally inaccessible because they are in the edge. The results of the two techniques are shown to give the same chi(k) above a threshold energy in the absorption edge, where the background is 70% of the edge step. The reliability of the XAFS starting at unprecedentedly low k values (1.5 angstrom-1 above the Fermi energy) allows sensitive tests of several details of theoretical XAFS calculations. Electron-hole-excitation losses in addition to the plasmon-pole loss term are shown to be important, and the Hedin-Lundqvist many-body self-energy is found to be superior to that of Dirac-Hara for pure Pb. One of the presented methods of background removal can be employed for general XAFS analysis, and is easily automated.

XAFS and Mossbauer effect measurements were made on impurities that lower the bulk melting temperature, T(m), of Pb and Ag. In addition XAFS measurements were made on Tl impurities that raise T(m) of Pb. The spectral intensity of the Mossbauer signal in Pb varies as expected from harmonic vibrations of the 1 at.% Sn Mossbauer atoms below a temperature of about 140K. Above this temperature the spectral intensity drops rapidly which is attributed to a localized hopping associated with a high entropy density. In Ag hosts a similar behavior occurs above 900K. The XAFS signal from Hg impurities in Pb, which like Sn lowers T(m), starts deviating from vibrational behavior above 400K where the apparent coordination number decreases. This is also interpreted as a localized hopping. However, Tl, which raises T(m), shows the normal vibrational behavior in XAFS measurements in a Pb host. XAFS measurements on Ga impurities in Ag, which also lower T(m) of bulk Ag, show only expected anharmonic behavior up to 900K. All of these results suggest a liquid-like high entropy region within a radius of somewhat more than an atomic spacing around impurities that lower T(m). These high entropy, or `'premelted'', regions could act as nucleation sites for three dimensional melting and should dominate over both grain boundaries and free surfaces in nucleating bulk melting in large single crystals.

The local structural distortions in PbTiO3 and NaTaO3 were measured showing that their temperature dependence is totally different from the temperature dependence of the order parameter. These results show that the transitions are not simply displacive.