Zoltaiite, a new barium-vanadium nesosubsilicate mineral from British Columbia: Description and crystal structure

摘要

Zoltaiite, ideal formula BaV4+₂V3+₁₂Si₂O₂₇, space group P, a = 7.601(1), c = 9.219(1) Å, V = 461.34(1) Å3, Z = 1, is a new mineral found on the eastern edge of the Shuswap metamorphic complex of British Columbia, Canada. It is a metamorphic mineral formed under greenschist-facies P-T conditions as part of an assemblage that includes quartz, celsian, apatite, sphalerite, pyrrhotite, galena, and pyrite. Zoltaiite has a Mohs hardness of 6–7, no cleavage, an anhedral to semi-prismatic habit, and a calculated density of 4.83 g/cm3. It is opaque with reflectance and color similar to those of sphalerite. The strongest eight lines of the X-ray powder diffraction pattern [d in Å (I) (hkl)] are 3.103(78)(021), 2.934(89)(22), 2.785(67)(013), 2.679(48)(022), 2.403(50)(211), 2.190(100)(212), 1.934(53)(213), and 1.438(63)(140). The empirical formula, derived from electron-microprobe analysis and the crystal structure, is Ba_1.05(Ti_1.31V4+_0.69)_2.00(V3+_11.06Fe3+_0.49Cr_0.34)_11.89Si_2.06O₂₇ based on O = 27. The crystal structure was solved by direct methods and refined on the basis of F2₀ using all unique reflections measured with MoK X-radiation on a CCD-equipped diffractometer. The final R factor was 3.2%, calculated using 659 unique observed reflections. The unit cell contains four layers of two types parallel to (001): X, an octahedral and tetrahedral sheet, and Y, an octahedral plus barium sheet; both layers are doubled through inversion centers resulting in the sequence XXYY... Two consecutive equivalent layers are interconnected through shared octahedral edges, whereas consecutive non-equivalent layers are linked through shared corners. The high calculated density is consistent with the dense packing of the structure.