1999 Volume 21 Issue 1 Pages 9-14
Hexahydroborite was found in a vein consisting of borate minerals which developed along the boundary between crystalline limestone and skarns at Fuka, Okayama Prefecture, Japan. Hexahydroborite occurred as aggregates of pyramidal crystals up to 2mm wide on the cavity wall, in association with olshanskyite and calcite. Wet analyses and ICP-MS gave the empirical formula Ca1.001B2.102O4.154·5.846H2O on the basis of O=10, which was consistent with the ideal formula Ca[B(OH)4]2·2H2O. The reflections of X-ray powder data for hexahydroborite from Fuka were indexed on the monoclinic cell, a=16.011(2), b=6.688(1), c=7.954(2)Å, β=103.81(1)°, determined by single crystal method. The mineral was optically biaxial positive with refractive indices α=1.502(2), β=1.505(2), γ=1.509(2) and had a Mohs hardness of 2.5 and a density of 1.84gcm−3. It is likely that hexahydroborite at Fuka was formed by a reaction of ground water with sibirskite and/or parasibirskite at a low temperature around 25°C.
