Sibirskite from Fuka, Okayama Prefecture, Japan

Abstract

Sibirskite was found in a vein consisting of borate minerals which developed along the boundary between crystalline limestone and skarns at Fuka, Okayama Prefecture, Japan. Sibirskite occurs as aggregates of prismatic crystals up to 0.1 mm long and 0.02mm wide, in association with takedaite, frolovite and calcite. Wet and microprobe analyses of sibirskite from Fuka give the empirical formula Ca_1.004H_1.071B_0.974O₃ on the basis of 0=3. The formula, optical properties and X-ray powder data are consistent with those from the type locality. The space group and cell dimensions determined are P2₁/a, a=8.643(6), b=9.523(2), c=3.567(3) Å, β=119.23(3)° and Z=4. The calculated density is 2.59 g cm⁻³, and agrees with the measured value of 2.58 g cm⁻³.
It is likely that sibirskite at Fuka was formed by late-hydrothermal alteration of takedaite.