Pararaisaite, the Dimorph of Raisaite, from the North Star Mine, Tintic, Utah, USA

Abstract

Pararaisaite, Cu^(2+)Mg[Te^(6+)O_4(OH)_2] •6H_2O, is a new mineral from the North Star mine, Tintic district, Juab County, Utah, USA. It is an oxidation-zone mineral occuring in vugs in a matrix of massive quartz with embedded crystals of baryte and goldfieldite. Crystals are deep blue, striated prisms up to 0.4 mm in length, elongated on [010] and exhibiting the forms {100}, {001}, {102}, {102}, and {114}. The mineral is transparent with vitreous luster, white streak, Mohs hardness 2½, brittle tenacity, splintery fracture, and two cleavages: perfect on {001} and good on {100}. The measured density is 2.85(2) g/cm^3. Pararaisaite is biaxial (+) with α = 1.600(2), β = 1.616(2), γ = 1.713(3) (white light); 2V = 47(1)°; slight r > v dispersion; orientation Z = b, X ≈ a, Y ≈ c; and pleochroism X very pale purple, Y purple, Z blue green (X << Z < Y). The Raman spectrum is consistent with the presence of tellurate, OH, and H_2O. Electron-microprobe analyses gave the empirical formula (Mg_(1.10)Cu_(0.93)Te_(0.96)Sb_(0.01)) _(Σ3)O_(12)H_(14.12). The mineral is monoclinic, space group P2_1/c, with a 9.6838(5), b 5.75175(19), c 17.6339(12) Å, β 90.553(6) °, V 982.14(9) Å^3, and Z = 4. The five strongest X-ray powder diffraction lines are [d_(obs) Å(I)(hkl)]: 8.77(100)(002), 4.824(71)(200,111), 4:248(85)(202; 202), 2.419(50)(400,024), and 1.8929(48)(226; 226). Pararaisaite is dimorphous with raisaite. The structure contains straight edge-sharing chains of alternating Cu^(2+)O_4(OH)_2 and Te^(6+)O_4(OH)_2 octahedra. The chains link to one another via shared octahedral corners to form [Cu^(2+)Te^(6+)O_4(OH)_2]^(2–) sheets. Interlayer Mg(H_2O)_6 octahedra link the sheets via hydrogen bonds.

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