A NEW BARIUM VANADIUM "ORTHOSILICATE" MINERAL FROM BRITISH COLUMBIA

A previously undescribed barium-silicon-vanadium oxysalt mineral has been found at a locality on the eastern edge of the Shuswap metamorphic complex of British Columbia, Canada. This mineral, referred to as VBS1a, is part of an equilibrium-textured assemblage that includes, quartz, celsian, apatite, galena, sphalerite, and pyrrhotite. The regional grade of metamorphism has been characterized as “chlorite zone greenschist facies” and the sulfide mineralization has been interpreted to be “pre- to syn-tectonic”. Therefore VBS1a is interpreted to be a metamorphic mineral produced during this moderate grade of metamorphism. A combination of microprobe data and structural refinement data imply an empirical formula of Ba_1.05V_11.75Ti_1.31Fe_0.49Cr_0.34Mg_0.03Al_0.02Si_2.06O₂₇. The ideal end-member formula is proposed to be Ba(V⁴⁺)₂(V³⁺)₁₂Si₂O₂₇. The mixed-valence vanadium is required for charge balance and the relative amounts of V⁴⁺ versus V³⁺ is firmly supported by the distribution of bond valences and mean-octahedral bond lengths for the 3 distinct octahedral sites in the structure. VBS1a is trigonal (space group Pbar3) with cell parameters a=b=7.6014Å, c=9.2195Å, V=461.4ų and Z=1.  VBS1a is grey to black and optically opaque with a D_calc of 4.83g/cm³. The structure of VBS1a is based on the ABCB closest packing of oxygens parallel to (001) and contains four layers of two types per unit cell.  The two types of layers are: X – an octahedral and tetrahedral sheet, and Y – an octahedral and trigonal pyramidal sheet; they are topologically identical to those found in hematolite. Both layers are doubled through inversion centers, resulting in a stacking sequence ++--. In layer X symmetrically equivalent V1 octahedra form V₃O₁₃ groups (doubled to V₆O₁₈) centered on the triad and linked to each other through corner-sharing SiO₄ tetrahedra. In layer Y the V2 octahedra form V₃O₁₃ groups, also on the triad, that are linked by edge sharing with the V3 octahedra and form a large trigonal-pyramidal cavity for Ba. Layers are connected primarily by octahedral-edge sharing and other corner sharing, respectively, between equivalent and nonequivalent layers. As silicon and barium play interstitial roles in the layers of octahedra, although potentially an orthosilicate, the mineral is more akin to an oxide.