Uralborite from Fuka, Okayama Prefecture, Japan

Uralborite, CaB₂O₂(OH)₄, was found in a vein consisting of borate minerals that cut into crystalline limestone at Fuka, Okayama Prefecture, Japan. It occurs as aggregates of fibrous crystals up to 0.2 mm long and as euhedral crystals up to 7 mm long and 3 mm wide, in association with sibirskite, borcarite, fluorite and calcite. Electron microprobe analyses and ICP gave the empirical formula Ca_1.006B_2.019O_2.069(OH)_3.931 on the basis of O=6. X-ray powder diffraction were indexed on the monoclinic cell, a=6.923(1), b=12.326(1), c=9.831(1)Å, β=97.09(1)°, determined by a single crystal method. The mineral was optically biaxial positive with refractive indices α=1.605(2), β=1.611(2), γ=1.618(2). The Vickers microhardness was 372 kg mm^-2 and the Moh's scale of hardness was 4.5. The density was 2.58(2) g cm^-1. It is likely that uralborite at Fuka was formed as a secondary mineral by a late-hydrothermal alteration of sibirskite.