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

Abstract

The structure of tourmaline-supergroup minerals includes two types of octahedral sites: the ZO 6 octahedron is smaller and more distorted than the YO 6 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 Fe 2+ 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 ZO 6 and YO 6 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 Fe 2+ can strongly prefer YO 6 octahedron whose prolonged tetragonal dipyramidal geometry is more favourable for Fe 2+ in (t 2g ) 4 (e g ) 2 configuration. Received: 6 October 2017; accepted: 31 May 2018; handling editor: J. Cempírek The online version of this article (doi: 10.3190/jgeosci.260) contains supplementary electronic material. Keywords: tourmaline supergroup, octahedral metrics, bond-valence theory, Jahn-Teller effectvacancies (Henry et al. 2011), although Ti structural position is not clear yet.Although the site occupancy usually follows Goldschmidt rules (ionic radii of cations occupying the Z site are lower and therefore the ZO 6 octahedron is smaller), local structural and bond-valence requirements may result in various cation disorder. Along with the Al-Mg disorder (Hawthorne ...

Cation partitioning among crystallographic sites based on bond-length constraints in tourmaline-supergroup minerals

“…These variations mainly occur at the shared edge of the ZO 6 and YO 6 octahedra -not only at the O3, but also at O6 site (Bačík 2018). Moreover, the Z-and Y-site occupancy influences the distortion at the neighboring sites.…”

“…However, both the Z-O3 and Z-O7' bonds are longer. The Z-O3 bond length is expanded due to OH group dominance at the O3 site, which also causes the elongation of the Z-O7' bond (Bačík 2018). The O-H bond valence, which is slightly lower than 1 vu due to the hydrogen bond, reduces in the Z-O3 bond valence to 0.36 vu (Bačík 2018).…”

“…The Z-O3 bond length is expanded due to OH group dominance at the O3 site, which also causes the elongation of the Z-O7' bond (Bačík 2018). The O-H bond valence, which is slightly lower than 1 vu due to the hydrogen bond, reduces in the Z-O3 bond valence to 0.36 vu (Bačík 2018). However, the Z-O6 bond length in the most Al-rich tourmalines is distinctly lower, since it balances the bond valences in a complex relationship between YO 6 and ZO 6 octahedra (Bačík 2018).…”

“…DOI: https://doi.org/10.2138/am-2021-7804. http://www.minsocam.org/ cordierite > biotite > muscovite > tourmaline, garnet, chloritoid; F: tourmaline > biotite > muscovite; Cr: chromite > tourmaline > margarite > muscovite > corundum; Ni: talc > tourmaline > staurolite > muscovite > gahnite; Co: tourmaline > staurolite > muscovite > gahnite; Zn: gahnite > staurolite > tourmaline > muscovite; LREE: average continental crust > tourmaline (Henry and Dutrow 1996, 2018.…”

Considerations About Bi and Pb in the Crystal Structure of Cu-Bearing Tourmaline

“…Bond valences attached to the graph edges were derived from [46,47] for V-and W-site bonds and [48,49] for Z-and Y-site bonds. X-site valences were calculated from the structure of fluor-schorl [50] and olenite [51] which have the highest Na content at the X site from the whole dataset of [49]. The calculation revealed that the X-O2 bond valence is 0.14 vu (valence units) in both samples, while X-O4 and X-O5 bonds have valences between 0.07 and 0.09 vu.…”