Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Peter Bačík

The crystal-chemical autopsy of octahedral sites in Na-dominant tourmalines: octahedral metrics model unconstrained by the Y,Z-site disorder assignment

Journal of Geosciences, volume 63 (2018), issue 2, 137 - 154

DOI: http://doi.org/10.3190/jgeosci.260

The structure of tourmaline-supergroup minerals includes two types of octahedral sites: the ZO₆ octahedron is smaller and more distorted than the YO₆ octahedron. The octahedral sites metrics were studied and their dependency on the chemical composition unconstrained by Y,Z-site disorder assignment. Published chemical and structural data were collected from American Mineralogist Crystal Structure Database for tourmaline samples belonging to dravite-schorl, schorl-elbaite (including tsilaisites) and schorl (±dravite)-olenite series. Correlation analysis of this dataset provided the evidence of cation distribution between sites - Al and Mg are disordered between Z and Y sites, while Fe (mostly ferrous), Li and Mn strongly prefer Y site. Irregular cation distribution results in the variable metrics of both octahedra in tourmalines. It is the function of well-balanced relations between cations at octahedral and neighbouring sites based on bond-valence variations due to different ionic charges. Considering Z and Y cations, there is a dependence of the cation charge difference and the octahedral metrics. The most pronounced irregularity of both octahedra was observed in elbaite samples with the largest charge difference between Li and Al. In contrast, “buergerite” samples with trivalent Fe and Al at both octahedral sites have both octahedra almost isometric. Schorl and dravite samples display an increasing metric irregularity related to the Al and Mg content; increase in Mg reduces irregularity because Mg is distributed between both octahedral sites balancing charge difference. In contrast, Fe-rich and Al-rich schorl samples display larger irregularity which may result from selective incorporation of Fe²⁺ to the Y site. In olenite samples, the irregularity of both octahedra decreases with an increasing Al content. These variations are related to the shared edge of ZO₆ and YO₆ octahedra including both O3 and O6 site where bonds of both anions are balancing bond-valence requirements of the stable electroneutral structure. In addition to the bond-valence relations, effects of the internal geometry of atomic shells should be also considered, i.e. Jahn-Teller distortion that can be decisive factor in cation occupancy. Especially Fe²⁺ can strongly prefer YO₆ octahedron whose prolonged tetragonal dipyramidal geometry is more favourable for Fe²⁺ in (t_2g)⁴(e_g)² configuration.