Ba XAFS in Ba-rich standard minerals and the potential for determining Ba structural state in calcium carbonate

We present Ba L-3-X-ray Absorption Fine Structure (XAFS) data from a suite of barium carbonates (witherite, alstonite, barytocalcite), hydroxides, sulfate(vi) (barite) and a Ba-bearing organic compound to explore whether Ba L-3-XAFS could be used to fingerprint structural states in biominerals such as celestite, aragonite and calcite. Although there is a general similarity between all X-ray Absorption Near Edge Structure (XANES), subtle differences are observed in detail, which allow almost all phases to be distinguished. The XANES are considered as composites of four components, termed 'A' (5255 eV), 'B' (5258 eV), 'C' (5268 eV) and V (5273 eV). 'A' is observed in barium hydroxides and most visible in the first derivatives of the XANES data. The minimum after the Ba L-3 white line lies at 5257 eV for most materials but higher (5261 eV) for barium hydroxides due to the influence of the 'A' component. V is present in aragonite-group minerals (witherite and alstonite) and may be a fingerprint of that structural state. 'C' and V overlap and form a board hump at similar to 5270 eV, but the relative proportions of 'C' and V are variable between phases and are to some degree diagnostic. Refinement of Extended X-ray Absorption Fine Structure (EXAFS) allows estimates of first shell (Ba-O) bond distances in all materials, which are within 4% of average distances estimated from diffraction studies. Subsequent shells (Ba-S for barite; Ba-metal in witherite, alstonite and barytocalcite) can be resolved. The state of Ba:Ca order in alstonite and barytocalcite is successfully modelled and both are found to be fully ordered. The significant static disorder in Ba-bearing minerals is accommodated successfully by large Debye-Waller values in the refinements. Combinations of XANES and EXAFS allow all phases to be identified, with the exception that the two hydrated barium hydroxides cannot be distinguished from each other. The XANES of a celestite (SrSO4 containing similar to 100 ppm Ba) is comparable to the barite spectra after only seven cycles (collected over <5 h), showing that XANES can be resolved in samples with low Ba concentrations. However we were unable to analyse successfully an aragonitic Porites coral skeleton (containing similar to 3-4 ppm Ba) using the current instrumentation due to the proximity in energy of Ca K alpha secondary X-radiation to the Ba Lot energy and which overloaded the X-ray detector. The use of multilayer crystal detectors will be required to resolve the Ba Lot energy in calcium carbonate samples containing low Ba concentrations. Alternatively Ba EXAFS may be accessible through the Ba K edge. (C) 2009 Elsevier B.V. All rights reserved.