Inyoite from Fuka, Okayama Prefecture, Japan

Abstract

Inyoite was found as fissure fillings in calcium borate minerals, which occur as an irregularly shaped body in the crystalline limestone near the gehlenite-spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Inyoite occurs as aggregates of tabular crystals up to 1 mm wide, and rarely as euhedral crystals up to 0.5 mm wide in fissures of calcium borate minerals such as nifontovite, pentahydroborite, sibirskite and parasibirskite. The fissure fillings are composed only of inyoite. This is the first finding of inyoite in Japan. The type of occurrence is also different from those in many other localities in the world. Electron microprobe and CHNS / O analyses gave the empirical formula Ca_1.99B_5.96O_5.92(OH)₁₀·8.08H₂O on the basis of O = 24. The unit cell parameters are a = 10.616(2), b = 12.068(1), c = 8.404(1) Å and β = 114.01(1)°. The mineral is optically biaxial negative with refractive indices α = 1.492, β = 1.506 and γ = 1.517, giving a calculated 2V = 82°. The Vickers microhardness is 91 kg mm⁻² (10 g load) and the Mohs hardness number is 2.5. The measured density is 1.875 g cm⁻³. It is likely that the inyoite at the Fuka mine was formed by a reaction of ground water with calcium borate minerals at a temperature of around 20°C.