IMA 2017-110 = pararaisaite

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Kampf; A.R., Housley, R.M., Rossman, G.R. (2018): Pararaisaite, the Dimorph of Raisaite, from the North Star Mine, Tintic, Utah, Usa. Canadian Mineralogist, 56, (in press).

Abstract:
Pararaisaite, Cu2+Mg[Te6+O4(OH)2]·6H2O, 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}, ...⁠, 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/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, 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 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) Å, β 90.553(6)°, V 982.14(9) Å3, and Z = 4. The five strongest X-ray powder diffraction lines are [dobs Å(I)(hkl)]: 8.77(100)(002), 4.824(71)(200,111), ...⁠, 2.419(50)(400,024), and .... 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 [Cu2+Te6+O4(OH)2]2– sheets. Interlayer Mg(H2O)6 octahedra link the sheets via hydrogen bonds.