Linking Mossbauer and structural parameters in elbaite-schorl-dravite tourmalines

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 Fe 2+ and Fe 3+ , it is not always useful in determining their site distribution. In particular, both for Fe 3+ and for (Fe 2+ -Fe 3+ ) interactions structural information is still essential. Fe 3+ MS doublets were identified and assigned to iron in Y and Z sites on the basis of structural information. In a few spectra, Fe 3+ 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 Fe 3+ in octahedral coordination. Electron delocalization between Fe 2+ and Fe 3+ was observed and related to three different interactions (Y-Y, Y-Z, and Z-Z). Notably, MS hyperfine parameters of Fe 2+ were self-consistent and particularly reliable in determining Fe 2+ site partitioning. Fe 2+ at Y was modeled by three doublets (ΔE Q = 2.45, 2.19, and 1.72 mm/s). The sum of their absorption areas perfectly matches the Y Fe 2+ populations derived from structural data (r 2 = 0.97). The fourth doublet observed (ΔE Q = 1.38 mm/s) is consistent with Fe 2+ at Z, which is an octahedron smaller and less distorted than Y (λ Z = 1.014, λ Y = 1.024). The absorption area of this doublet is highly correlated with the amount of Z Fe 2+ obtained from site-occupancy refinement (r 2 = 0.95). For Y Fe 3+ a link between the quadrupole splitting parameter ΔE Q and variations in the chemical/ structural environment surrounding Fe nucleus was observed. The ΔE Q of Y Fe 3+ increases with ZO 6 volume (r 2 = 0.84) and is linked to the variation of electrical field gradient generated by the Z R 2+ → Z Al substitution. Since the Z skeleton completely surrounds Y islands, ΔE Q of Y Fe 3+ shows much more susceptibility to inductive effects from the second rather than the first coordination sphere.