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.

Lowering R3m Symmetry in Mg-Fe-Tourmalines: The Crystal Structures of Triclinic Schorl and Oxy-Dravite, and the Mineral luinaite-(OH) Discredited

“…The possible biaxial subsymmetries of space group R3m are the subgroups Cm (monoclinic) and P1 (triclinic). Thus, in accord with the studies of [12][13][14], we selected the space-group type P1 for the refinement in line with the significant departure of α and β from 90 • in unconstrained unit-cell refinements. To facilitate the comparison between triclinic and trigonal structures, the primitive triclinic cell used in this study [e.g., sample LUI-AUS, a = 7.20820 (10), b = 9.5000(2), c = 9.4947(2) Å, α = 113.7519(8) • , β = 104.5539(8) • , γ = 104.6823 (9) • and V = 527.835(18) Å 3 ] was recast in the non-standard space-group R1, defining a pseudohexagonal unit-cell similar to the typical hexagonal triple R cell but with unconstrained unit-cell parameters (Table 3).…”

Crystal structure and stability of Ni-rich synthetic tourmaline. Distribution of divalent transition-metal cations over octahedral positions

“…General criteria that regulate the patterns of distribution of transitionmetal cations over octahedrally coordinated positions are at present unavailable. Most authors agree that large divalent transition-metal cations (Mn 2+ , Fe 2+ , Cu 2+ , Co 2+ , Ni 2+ ) position themselves solely or predominantly in the Y octahedron (Burns et al, 1994;Bloodaxe et al, 1999;MacDonald and Hawthorne, 1995a;Hughes et al, 2011). Regularities of these cation occurrences in the Z octahedron are the subject of scientific debate.…”

“…There are several reports of Ni 2+ in natural tourmalines (Henry and Dutrow, 2001;Baksheev and Kudryavtseva, 2004;Hughes et al, 2011); the Ni content does not exceed 4 wt.% of NiO (Baksheev and Kudryavtseva, 2004). In these areas we have attempted to study crystal-chemical regularities of Ni-bearing tourmalines using synthetic crystals.…”

Nickel- and Fe3+-rich oxy-dravite from the Artana Mn prospect, Apuan Alps (Tuscany, Italy)

“…The mineralogical study confirmed the first identification, pointing out a high Ni content and suggesting the necessity of its full crystal-chemical investigation. Indeed, nickeloan tourmalines have been reported from a few localities worldwide (e.g., Henry and Dutrow 2001;Baksheev and Kudryavtseva 2004;Voudouris et al 2019) and structural data have been mainly collected on synthetic analogs (e.g., Rozhdestvenskaya et al 2012;Vereshchagin et al 2015); the only natural refined sample was reported as triclinic (Hughes et al 2011).…”