Linking Mössbauer and structural parameters in elbaite-schorl-dravite tourmalines
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Giovanni B. Andreozzi
,
Ferdinando Bosi
Abstract
Accurate crystal-chemical analysis of complex minerals such as tourmalines belonging to the elbaite-schorl-dravite series was obtained by combining Mössbauer spectroscopy (MS) and structural information. Well-defined relationships were established between the two approaches, leading to a close match of results obtained and a strong link between observed parameters.
Although MS information is a powerful tool for quantifying the amount of Fe2+ and Fe3+, it is not always useful in determining their site distribution. In particular, both for Fe3+ and for (Fe2+-Fe3+) interactions structural information is still essential. Fe3+ MS doublets were identified and assigned to iron in Y and Z sites on the basis of structural information. In a few spectra, Fe3+ doublets with very low parameters (δ ~ 0.2 mm/s) were observed and, in contrast with the T-site assignment of previous works, were assigned to Fe3+ in octahedral coordination. Electron delocalization between Fe2+ and Fe3+ was observed and related to three different interactions (Y-Y, Y-Z, and Z-Z). Notably, MS hyperfine parameters of Fe2+ were self-consistent and particularly reliable in determining Fe2+ site partitioning. Fe2+ at Y was modeled by three doublets (ΔEQ = 2.45, 2.19, and 1.72 mm/s). The sum of their absorption areas perfectly matches the YFe2+ populations derived from structural data (r2 = 0.97). The fourth doublet observed (ΔEQ = 1.38 mm/s) is consistent with Fe2+ at Z, which is an octahedron smaller and less distorted than Y (λZ = 1.014, λY = 1.024). The absorption area of this doublet is highly correlated with the amount of ZFe2+ obtained from site-occupancy refinement (r2 = 0.95).
For YFe3+ a link between the quadrupole splitting parameter ΔEQ and variations in the chemical/ structural environment surrounding Fe nucleus was observed. The ΔEQ of YFe3+ increases with ZO6 volume (r2 = 0.84) and is linked to the variation of electrical field gradient generated by the ZR2+ → ZAl substitution. Since the Z skeleton completely surrounds Y islands, ΔEQ of YFe3+ shows much more susceptibility to inductive effects from the second rather than the first coordination sphere.
© 2015 by Walter de Gruyter Berlin/Boston
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