9 research outputs found
XANES evidence for sulphur speciation in Mn-, Ni- and W-bearing silicate melts
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 6847-6867, doi:10.1016/j.gca.2009.08.013.S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses synthesised at 1400° C and 1 bar with compositions in the CaO-MgO-Al2O3-SiO2-S plus MnO,
NiO, or WO3 systems were used to investigate sulphur speciation in silicate glasses.
S K-edge spectra comprised a composite peak with an edge between 2470 and 2471.4 eV,
which was attributed to S2-, and a peak of variable height with an edge at 2480.2 to 2480.8
eV, which is consistent with the presence of S6+. The latter peak was attributed to sample
oxidation during sample storage. W-rich samples produced an additional lower energy peak
at 2469.8 eV that is tentatively attributed to the existence of S 3p orbitals hybridised with
the W 5d states.
Deconvolution of the composite peak reveals that the composite peak for Mn-bearing
samples fits well to a model that combines three Lorentzians at 2473.1, 2474.9 and 2476.2
eV with an arctan edge step. The composite peak for W-bearing samples fits well to the
same combination plus an additional Lorentzian at 2469.8 eV. The ratio of the proportions
of the signal accounted for by peaks at 2473.1eV and 2476.2eV correlates with Mn:Ca molar
ratios, but not with W:Ca ratios. Spectra from Ni-bearing samples were qualitatively similar
but S levels were too low to allow robust quantification of peak components. Some part of
the signal accounted for by the 2473.1 eV peak was therefore taken to record the formation
of Mn-S melt species, while the 2469.8 peak is interpreted to record the formation of W-S
melt species. The 2474.9 eV and 2476.2 eV peaks were taken to be dominated by Ca-S
and Mg-S interactions. However, a 1:1 relationship between peak components and specific
energy transitions is not proposed. This interpretation is consistent with known features of
the lower parts of the conduction band in monosulphide minerals and indicates a similarity
between sulphur species in the melts and the monosulphides. S XANES spectra cannot be
reproduced by a combination of the spectra of the component element monosulphides.
Mn-, W- and Ni- XANES and EXAFS for synthetic glasses without sulphide exsolution
did not show any sensitivity to the presence of sulphur, which is unsurprising as S:O ratios
were sufficiently low that metals would be mostly co-ordinated by O. Mn EXAFS spectra
were consistent with divalent Mn in 5 co-ordinated Mn-O melt species. W spectra were
consistent with tetrahedrally co-ordinated hexavalent W, most likely in scheelite-like melt
species, and Ni spectra were consistent with [4] co-ordinated divalent Ni. These results
indicate lower coordinations for bothWand Ni than those inferred by some previous workers.
Cation coordination may reflect the proportion of non-bridging oxygens, which is lower in
the Ca-rich and Al-poor samples investigated here than for previous studies.This work was performed with
814 support from the Australian Synchrotron Research Program (ASRP), which is funded by the
815 Commonwealth of Australia under the Major National Research Facilities Program