Dissymmetrization in tourmaline: the atomic arrangement of sectorally zoned triclinic Ni-bearing dravite

Abstract

Although putatively possessing hexagonal R3m symmetry, reports of optically anomalous tourmaline are common, and recently an occurrence of triclinic tourmaline was reported with dissymmetrization that resulted from non-equivalency of the occupants of the Y sites. We report the atomic arrangement of Ni-bearing dravite from the Berezovskoe gold deposit, Middle Urals, Russia, in the non-conventional triclinic space-group R1 (R = 4.41%) to facilitate comparison with the conventional tourmaline R3m cell. The dissymmetrization occurs as a result of inequalities among both the hexagonally equivalent Y and hexagonally equivalent Z tourmaline sites. The atomic arrangement of this triclinic dravite demonstrates that the atomic arrangement of tourmaline is robust, and is capable of incorporating various substituents by modifying the putative hexagonal structure in lower symmetries, suggesting that further exploration of tourmaline's role in trace-element variation is warranted. Optical studies demonstrate the heterogeneous biaxial character of the crystals. Domains of different optical orientation and 2V correspond directly to trigonal prism |100|, |010| and pedion |001| sectors, indicating optical sectoral zoning. Compositional sectoral and concentric zoning are also observed within the crystals. Spectroscopic studies show the optical absorption spectrum of the Berezovskoe tourmaline has strong absorptions in the 400, 600–700, and 1100 nm regions, in addition to OH features near 1450, 2300, and 2700 nm. We conclude that the color in the E ⊥ c polarization comes dominantly from Fe mixed-oxidation-state couples on the Y sites, and from Cr^(3+). Contributions to the color from the nickel are believed to be minor and will fall in the regions of strong Cr and Fe absorption. The ordered arrangement of cations on the Y and Z sites and the correlation of optical orientation with specific sectors indicate that dissymmetrization occurs during growth by differential incorporation at structurally different atomic sites at the surface of the crystal, which in the bulk are symmetrically equivalent.

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