Legrandite and koettigite from the Ogibira mine, Okayama, Japan

Abstract

Legrandite and koettigite are found in an oxidized arsenopyrite-sphalerite ore from the Ogibira mine, Okayama Prefecture. They occur as secondary products in cavities and on fracture surfaces in compact limonite. Legrandite is commonly honey-yellow in color with a vitreous luster, and occurs as prismatic crystals or as radial aggregates. The crystals with length up to 3.5 mm show the forms a(100), m(110), c(001) and p(-111). It is monoclinic and the unit cell parameters are: a=12.789(5), b=7.916(2), c=10.218(3)Å and β=104.36 (3)°. Wet chemical analysis gave an empirical formula (Zn_1.90 Fe_0.06 Al_0.01 Ca_0.01 Na_0.02 K_0.01)_Σ2.01 (As_1.00 P_0.01)_Σ1.01 O_4.00 (OH)_1.00·1.00 H₂O. It is optically positive with refractive indices, α=1.700, β=1.708, γ=1.738 and 2V about 42°. Its physical properties are: Vichers microhardness 254–270kg/mm²(25g load), Mohs hardness 4.5, and density 3.98 g/cm³(meas) and 4.038 g/cm³ (calc).
Koettigite is pale blue to blue in color with a vitreous luster. The mineral mostly occurs as parallel aggregates with length up to 2 cm along [001]. It is monoclinic and the unit cell parameters are: a=10.249(4), b=13.449(4), c=4.759(2)Å and β=105.11(4)°. Wet chemical analysis gave an empirical formula (Zn_2.53 Fe_0.32 Al_0.01 Mg_0.01 Ca_0.01 Na_0.11 K_0.01)_Σ3.00 (AS_2.00 P_0.02)_Σ2.02 O_7.99·8.01 H₂O. It is optically positive with α=1.617, β=1.646, γ=1.680 and 2V=84°. Its physical properties are: Mohs hardness 2.5, density 3.23 g/cm³ (meas) and 3.240 g/cm³(calc). This mineral has perfect cleavage on {010}.