Frolovite from Fuka, Okayama Prefecture, Japan

Abstract

Frolovite was found in a vein consisting of borate minerals which developed along the boundary between crystalline limestone and skarns at Fuka, Okayama Prefecture. Frolovite occurred as aggregates of granular crystals up to 0.15 mm long and 0.05 mm wide, in association with olshanskyite and calcite. Wet analyses and EPMA gave the empirical formula C a_0.998B_2.008O₈H_7.981 on the basis of O=8, which was consistent with the formula Ca[B(OH)₄]₂ reported by Simonov et al. (1976). X-ray powder data for frolovite from Fuka were determined with accuracy. The reflections were indexed on the triclinic cell reported by Simonov et al. (1976). The unit cell parameters are a=7.764(4), b=5.679(4), c=8.126(5)Å, α=113.15(1), β=101.604(7) and γ=107.86(1)°. It was optically biaxial negative with refractive indices α=1.561, β=1.573, γ=1.584; VHN²⁵ =168 kg/mm² and a measured density of 2.22 g/cm³.
It is likely that frolovite at Fuka was formed by an alteration of olshanskyite, where the Ca ion had been removed and water had been added at a low temperature of around 70°C.