Fedorovskite from the Fuka mine, Okayama Prefecture, Japan

Abstract

Fedorovskite was found as aggregates in crystalline limestone associated with gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Fedorovskite occurs as gray to dark gray aggregates of anhedral translucent crystals up to 0.8 mm across in association with shimazakiite, uralborite, vimsite, cuspidine, fluorite, and calcite. An electron microprobe analysis of fedorovskite gave an empirical formula Ca_2.013(Mg_1.901Fe_0.072Mn_0.023Zn_0.002Co_0.001Ni_0.001)_Σ2.000(B_3.852Si_0.104)
_Σ3.956O_7.000(OH_5.421F_0.579)_Σ6.000 based on O = 7 and OH + F = 6. The mineral is orthorhombic, and the unit cell parameters refined from X–ray diffraction are a = 8.915(7), b = 13.086(16), c = 8.295(9) Å, and V = 967.7(18) ų. The calculated density is 2.692 g cm⁻³. The fedorovskite from the Fuka mine was probably formed as a secondary mineral from calcium borates such as shimazakiite in a reaction with magnesium–bearing late hydrothermal solution.