Matthew Newville

The effect of CO2 on the speciation of bromine in low-temperature geological solutions: an XANES study.

CO(2)-rich solutions are common in geological environments. An XANES (X-ray absorption near-edge structure) study of Br in CO(2)-bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre-edge feature at temperatures from 298 to 423 K. Br in CO(2)-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(CO(2)) in the fluid inclusion, where X(CO(2)) is the mole fraction of CO(2) in the solution [n(CO(2))/(n(CO(2)) + n(H(2)(O)) + 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), CH(3)Br and HBr) are vanishingly small. An alternative possibility is that CO(2) 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(CO(2)). This is attributed to a CO(2)-related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO(2)-bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore-formation and metamorphism.

High-Speed, Coupled Micro-Beam XRD/XRF/XAFS Mapping at GSECARS: APS Beamline 13-ID-E

The present study describes new instrumentation developments at the GSECARS 13ID-E hard X-ray microprobe beamline at the Advanced Photon Source that allows for high-speed, coupled micro-beam X-ray diffractions/X-ray fluorescence/X-ray absorption fine structure mapping. These new methodologies provide Earth and environmental scientists with unique coupled tools for evaluating microscale mineralogical and chemical heterogeneities in fine-grained sediments, soils, shales, and mine tailings and associated secondary precipitates. In particular, new technologies and approaches for integrating fast mXRD mapping into routine X-ray microprobe beamline operations are described and several real-world examples are given of how this approach provides unique insights with regards to micrometer-scale heterogeneities in mineralogy and chemistry that are difficult to obtain by other methods. Examples described here include waste streams associated with mine tailings from the McClean Lake mining facility located on the eastern edge of the Athabasca Basin in northern Saskatchewan, Canada, and from mine-drainage waters from the epithermal Au-Ag-Cu deposits of the Lagunas Norte mine in the Peruvian Andes.