OSAKAITE, Zn4SO4(OH)6{middle dot}5H2O, A NEW MINERAL SPECIES FROM THE HIRAO MINE, OSAKA, JAPAN

Osakaite, ideally Zn 4 SO 4 (OH) 6 ·5H 2 O, is a new mineral species that occurs on the gallery wall and in cracks in altered shale at the Hirao mine, Minoo City, Osaka Prefecture, Japan. The mineral is found as stalactitic aggregates of hexagonal platy crystals up to 0.5 mm across and 0.01 mm in thickness. The associated minerals are hydrozincite, smithsonite, “chlorite” and “limonite”. Osakaite is either pale blue to colorless or white, and is transparent with a pearly luster. It has a perfect {001} cleavage. The mineral is triclinic, and the most probable space-group is P 1, with a 8.358(5), b 8.337(4), c 11.027(2) A, α 94.79(2), β 83.16(2), γ 119.61(4)°, V 663.0(4) A 3 and Z = 2. The strongest lines in the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 10.96(100) (001), 2.717(21)(311), 1.574(18)(533,245,523), 3.642(17)(003), and 5.47(16)(002). Inductively coupled plasma − atomic emission spectrometry and thermogravimetric analysis gave ZnO 55.30, CuO 3.44, SO 3 14.66, H 2 O 26.01, for a total of 99.41 wt%. The empirical formula is (Zn 3.75 Cu 0.24 )∑ 3.99 (SO 4 ) 1.01 (OH) 5.96 ·4.99H 2 O on the basis of O = 15. The mineral is optically biaxial negative with α 1.532(2), β 1.565(2), γ 1.567(2), and 2 V calc = 27.2°. The density is 2.70(2) g/cm 3 (meas.) and 2.75 g/cm 3 (calc.). The Vickers microhardness is 20.3 (16.5–22.8) kg/mm 2 (10 g load), corresponding to a Mohs hardness of 1. It is likely that at the Hirao mine, osakaite was formed in Zn-, Cu- and SO 4 -bearing groundwater derived from sphalerite and chalcopyrite in the host rock at around 20°C.