Kurchatovite from the Fuka mine, Okayama Prefecture, Japan

Abstract

Kurchatovite occurs as colorless granular crystals up to 1 mm at the Fuka mine, Okayama Prefecture, Japan. The mineral is associated with shimazakiite, calcite and johnbaumite. The Vickers microhardness is 441 kg mm⁻² (50 g load), corresponding to 4½ on the Mohs’ scale. The calculated density is 3.23 g cm⁻³. Electron microprobe analyses of kurchatovite gave empirical formulae ranging from Ca_0.987(Mg_1.004Fe_0.020Mn_0.001)_Σ1.025B_1.992O₅ to Ca_0.992(Mg_0.466Fe_0.523)_Σ0.989B_2.013O₅ based on O = 5, and kurchatovite forms a continuous solid solution in this range. The mineral is orthorhombic, Pbca, and the unit cell parameters refined from XRD data measured by a Gandolfi camera are a = 36.33(14), b = 11.204(3), c = 5.502(17) Å, V = 2239(12) ų. It is formed by a layer of MgO₆–octahedra, a layer of CaO₇–polyhedral and B₂O₅ consisting of two BO₃–triangles. The kurchatovite from the Fuka mine was probably formed by supplying Mg and Fe to shimazakiite from hydrothermal solution at a temperature between 250 to 400 °C.