Pararaisaite, the Dimorph of Raisaite, From the North Star Mine, Tintic, Utan, U.S.A.

Kampf AR1, Housley RM2, Rossman GR2

1 Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Abstract

Pararaisaite, Cu2+Mg[Te6+O4(OH)2]6H2O, is a new mineral from the North Star mine, Tintic district, Juab County, Utah, U.S.A. It is an oxidation-zone mineral occuring in vugs in a matrix of massive quartz with imbedded crystals of barite 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}, {10-2}, and {114}. The mineral is transparent with vitreous luster, white streak, Mohs hardness 2, brittle tenacity, splintery fracture, and two cleavages: perfect on {001} and good on {100}. The measured density is 2.85(2) g/cm3. 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, Xa, 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 H2O. Electron-microprobe analyses gave the empirical formula (Mg1.10Cu0.93Te0.96Sb0.01)Σ3O12H14.12. The mineral is monoclinic, space group P21/c, with a = 9.6838(5), b = 5.75175(19), c = 17.6339(12) , β = 90.553(6), V = 982.14(9) 3, and Z = 4. The five strongest X-ray powder diffraction lines are [dobs (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 Cu2+O4(OH)2 and Te6+O4(OH)2 octahedra. The chains link to one another via shared octahedral corners to form charge neutral [Cu2+Te6+O4(OH)2] sheets. Interlayer Mg(H2O)6 octahedra link the sheets via hydrogen bonds.