Sillente from fuka, okayama prefecture, Japan

Abstract

Sillenite from Fuka occurs as cubes up to 5 mm across in a calcite vein cutting gehlenite-spurrite skarn, probably as a primary mineral in a new mode of occurrence to this mineral as well as the first description of its idiomorph. The wet and microprobe analyses gave SiO₂ 2.22, 1.85; Bi₂O₃ 97.74, 96.48; totals 99.96%, 98.33%, respectively. The average yields the empirical formula Bi_24.06Si_1.95O₄₀ (basis O = 40), proving the validity of the ideal formulaBi₂₄Si₂O₄₀ advocated after the reference to many synthetic works on Bi₂₄X₂O_39–40 where X = Fe, Al, Ge, Ti, P, As, and V. The single crystal and X-ray powder works verify the symmetry to be cubic, space group I23, a=10.110(2)Å, Z = 1. It is optically isotropic, refractive index >2.50; VHN₅₀=345∼386 kg/mm², density 9.16 g/cm³ (meas.), 9.18 g/cm³ (calc.).