Kinoite from Fuka, Okayama Prefecture, Japan

Kinoite was found in a dump at the Fuka mine, Okayama Prefecture, Japan. It occurs as aggregates of flaky crystals, and rarely as subhedral platy crystals up to 1 mm wide in a vein cutting into crystalline limestone. Associated minerals are stringhamite, calcite and an unidentified mineral. The empirical formula of the mineral is (Ca_2.00Mga_0.02)∑_2.02(Cu _1.92Fe_0.04Co_0.04)∑_2.00Si _2.98O₁₀ · 2.25H₂O on the basis of O=10 (anhydrous). The unit cell parameters are a=6.989 (1), b=12.904 (2), c=5.659 (1) Å, and β=96.15 (2)°. The mineral is optically biaxial negative with refractive indices α= 1.642 (2), β= 1.662 (2), and γ= 1.675 (2). The Vickers microhardness is 536 kg mm^-2 (50 g load) and the density is 3.14 g cm^-3. It is likely that kinoite at the Fuka mine was formed as a primary mineral by a reaction of Cu- and Si-bearing fluids with limestone.