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

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza," Piazzale Aldo Moro 5, 00185, Roma, Italy
² Department of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden
³ Bayerisches Geoinstitut, Universität Bayreuth, D95440 Bayreuth, Germany

Correspondence: ^* E-mail: gianni.andreozzi{at}uniroma1.it

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 ( ~ 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 (E_Q = 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 (E_Q = 1.38 mm/s) is consistent with Fe²⁺ at Z, which is an octahedron smaller and less distorted than Y (_Z = 1.014, _Y = 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 E_Q and variations in the chemical/structural environment surrounding Fe nucleus was observed. The E_Q of ^YFe³⁺ increases with ZO₆ volume (r² = 0.84) and is linked to the variation of electrical field gradient generated by the ^ZR^{2+ Z}Al substitution. Since the Z skeleton completely surrounds Y islands, E_Q of ^YFe³⁺ shows much more susceptibility to inductive effects from the second rather than the first coordination sphere.

Key Words: Tourmaline • Mössbauer spectroscopy • structural formula • Fe disorder

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