OSAKAITE, Zn_4SO_4(OH)_6 centre dot 5H_2O, A-NEW MINERAL SPECIES FROM THE HIRAO MINE, OSAKA, JAPAN

摘要

Osakaite, ideally Zn_4SO_4(OH)_6 centre dot 5H_2O, 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 P1 with alpha 8.358(5), b 8.337(4), c 11.027(2) A, alpha 94.79(2), beta 83.16(2), gamma 119.61 (4) deg, V 663.0(4) A~3 and Z velence_2. The strongest lines in the X-ray powder-diffraction pattern [d in A(I)(hkl)] are: 10.96( 100) (001), 2.717(21)(31-(bar)1), 1.574(18)(53-(bar) 3,24-(bar) 5, 52-(bar)3), 3.642(17)(003), and 5.47(16)(002). Inductively coupled plasma - atomic emis-sion spectrometry and thermogravimetric analysis gave ZnO 55.30, CuO 3.44, SO_3 14.66, H_2O 26.01, for a total of 99.41 wt percent. The empirical formula is (Zn_(3.75)Cu_(0..24))sum 3.99(S0_4)_(1.01)(OH)_(5.96) centre dot 4.99H_2O on the basis of O = 15. The mineral is optically biaxial negative with alpha 1.532(2), beta 1.565(2), gamma 1.567(2), and 2V_(calc) = 27.2 deg. 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 deg C.