Chukhrovite-(Ca), Ca4.5Al2(SO4)F13·12H2O, a new mineral species from the Val Cavallizza Pb-Zn-(Ag) mine, Cuasso al Monte, Varese province, Italy

Abstract

Chukhrovite-(Ca), Ca 4.5 Al 2 (SO 4 )F 13 ·12H 2 O, is the Ca-dominant species of the chukhrovite mineral group. It occurs at the Val Cavallizza Pb–Zn–Ag mine, Cuasso al Monte, Varese province, Italy. Chukhrovite-(Ca) is found as low temperature hydrothermal crystallizations covering the surfaces of brittle fractures crosscutting a marcasite and REE-bearing fluorite vein. Associated minerals include marcasite, gypsum, and hydrated Fe oxides. This new calcium aluminofluoride forms sharp octahedra up to 150 μm in diameter. Crystals are translucent to transparent, colourless to white, with a white streak and a vitreous lustre. Chukhrovite-(Ca) is isotropic with n = 1.432(1), and is non-fluorescent either under short-wave (254 nm) or long-wave (366 nm) ultraviolet light. The mineral is brittle with a Mohs hardness of 3.5(5), and without a distinct cleavage or fracture. Its density, calculated from the single-crystal unit-cell parameters and assuming 12 H 2 O molecules per formula unit, is 2.23 g/cm 3 . The empirical formula, based on 3 (Al + S) atoms per formula unit and calculated from an average of five selected point analyses, is: (Ca 4.33 Na 0.11 Fe 0.03 ) ∑4.47 Al 2.10 (S 0.90 O 3.72 ) F 13.10 ·5.98H 2 O, with the water content calculated by difference to 100 %. Chukhrovite-(Ca) is cubic with space group Fd 3; its single-crystal unit-cell parameters are a = 16.749(1) A and V = 4698.6(1) A 3 , for Z = 8. The eight strongest lines in the X-ray powder diffraction pattern are [ d in A ( I / I 0 ) hkl ]: 9.665 (100) 111, 5.921 (31) 022, 5.053 (16) 113, 4.190 (10) 004, 3.226 (15) 333 and 115, 2.556 (10) 533, 2.182 (12) 355 and 137, 1.915 (17) 626. The mineral, which has been approved by the CNMNC under number IMA 2010-081, is named chukhrovite-(Ca), since it corresponds to the Ca-rich equivalent of chukhrovite-(REE) (REE = Ce, Nd, Y), in which REE are replaced by Ca.