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OSAKAITE, Zn₄SO₄(OH)₆·5H₂O, A NEW MINERAL SPECIES FROM THE HIRAO MINE, OSAKA, JAPAN

Masayuki Ohnishi^1,, Isao Kusachi² and Shoichi Kobayashi³

¹ Masutomi Geology Museum, 394 Nakademizu-cho, Kamigyo, Kyoto 602–8012, Japan
² Department of Earth Sciences, Faculty of Education, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
³ Department of Applied Science, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan

E-mail address: zshhbs11a{at}ybb.ne.jp

Osakaite, ideally Zn₄SO₄(OH)₆·5H₂O, 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, with a 8.358(5), b 8.337(4), c 11.027(2) Å, 94.79(2), β 83.16(2), 119.61(4)°, V 663.0(4) ų and Z = 2. The strongest lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 10.96(100) (001), 2.717(21)(31), 1.574(18)(53,25,53), 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, SO₃ 14.66, H₂O 26.01, for a total of 99.41 wt%. The empirical formula is (Zn_3.75Cu_0.24) _3.99(SO₄)_1.01(OH)_5.96·4.99H₂O on the basis of O = 15. The mineral is optically biaxial negative with 1.532(2), β 1.565(2), 1.567(2), and 2V_calc = 27.2°. The density is 2.70(2) g/cm³ (meas.) and 2.75 g/cm³ (calc.). The Vickers microhardness is 20.3 (16.5–22.8) kg/mm² (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 SO₄-bearing groundwater derived from sphalerite and chalcopyrite in the host rock at around 20°C.

Keywords: osakaite, basic zinc sulfate, namuwite, Hirao mine, Osaka, Japan.