Tangdanite, ideally Ca2Cu9(AsO4)4(SO4)0.5(OH)9·9H2O and monoclinic, is a new mineral species (IMA No. 2011-096) occurring in the Tangdan and Nanniping mines, southeast Dongchuan copper mining district, Dongchuan County, Kunming City Prefecture, Yunnan Province, P. R. China (26°11′N 103°51′E). The mineral is found in the oxidized zone (gossan) of an As-bearing Cu sulfide deposit and is clearly of supergene origin. Associated minerals are chalcopyrite, bornite, chalcocite, covellite, tennantite, enargite, cuprite, malachite, azurite, copper and brochantite. Crystals form radiating or foliated aggregates of flaky crystals up to 3 mm, flattened parallel to (100) and elongated along [001]. It is emerald green with a light green streak, translucent and has a pearly to silky lustre. It is sectile having perfect cleavage on {100} although neither parting nor fracture was observed. No fluorescence in long- or short-wave ultraviolet radiation was observed. The hardness is VHN50 42.0–43.6, mean 42.8 kg mm−2 (2–2½ on the Mohs scale). The density measured by pycnometry is 3.22 g cm−3 (Ma et al., 1980). The calculated density from the empirical chemical formula is 3.32 g cm−3. The compatability index gives 1 − (Kp/Kc) = −0.041 (good). The empirical formula (based on 36 O a.p.f.u) of tangdanite is Ca2.05Cu9.08(As1.03O4)4(S0.63O4)0.5(OH)9·9H2.04O. The simplified formula is Ca2Cu9(AsO4)4(SO4)0.5(OH)9·9H2O. The strongest five reflections in the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are: 4.782(100) (3İ 1 1), 4.333(71) (6 0 2), 5.263(54) (2İ 0 2), 3.949(47) (8 0 2) and 2.976(46) (15İ 1 1). The unit-cell parameters are a = 54.490(9), b = 5.5685(9), c = 10.4690(17) Å, β = 96.294(3)°, V = 3157.4(9) Å3, Z = 4. Its structure was solved and refined in space group C2/c, with R = 0.110.

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