Pauladamsite, Cu4(SeO3)(SO4)(OH)4·2H2O, a new mineral from the Santa Rosa mine, Darwin district, California, USA

Abstract

AbstractPauladamsite (IMA2015-005), Cu4(SeO3)(SO4)(OH)4·2H2O, is a new mineral from the Santa Rosa mine, Darwin district, Inyo County, California, USA, where it occurs as a secondary oxidation-zone mineral in association with brochantite, chalcanthite, gypsum, ktenasite, mimetite, schulenbergite and smithsonite on limonitic gossan. Pauladamsite forms green, multiply twinned blades up to 0.5 mm long grouped in radial sprays. The streak is pale green. Crystals are transparent and have vitreous to silky lustre. The Mohs hardness is ∼2, the tenacity is brittle, the fracture is irregular and crystals exhibit one perfect cleavage on [001]. The calculated density is 3.535 g/cm3. Electron microprobe analyses provided: CuO 48.96, ZnO 3.56, SeO2 18.82, SO3 13.90, H2O 13.29 (calc.), total 98.53 wt.%. The empirical formula (based on 13 O apfu) is: (Cu3.55Zn0.25)∑3.80Se0.98SO13H8.50. Pauladamsite is triclinic, P1, a = 6.0742(7), b = 8.4147(11), c = 10.7798 (15) Å, α = 103.665(7), β = 95.224(7), γ = 90.004(6)°, V = 533.03(12) Å3 and Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [dobs in Å(I)(hkl)]: 10.5(46)(011); 3.245(100)(001); 5.81(50)(011); 2.743(49)(112); 3.994(67)(012); 3.431(23)(1̄12,1̄2̄1,1̄20); 2.692(57)(03̄2,1̄22,2̄1̄2); and 2.485(39)(21̄2,1̄3̄2,02̄4). The structure of pauladamsite (R1 = 10.6% for 2086 Fo > 4σF) contains Cu2+O6 octahedra, SO4 tetrahedra and Se4+O3 pyramids. There are four different CuO6 octahedra, each of which exhibits typical Jahn-Teller distortion, with four short equatorial Cu–O bonds and two much longer apical Cu–O bonds. The CuO6 octahedra share edges to form five-octahedra-wide bands extending along [100]. Adjacent bands are connected in the [011̄] direction by bridging SO4 tetrahedra and in the [011] direction by bridging Se4+O3 pyramids, thereby forming a framework.