CATAMARCAITE, CU6GEWS8, A NEW GERMANIUM SULFIDE MINERAL SPECIES FROM CAPILLITAS, CATAMARCA, ARGENTINA: DESCRIPTION, PARAGENESIS AND CRYSTAL STRUCTURE

Abstract

Catamarcaite, ideally Cu6GeWS8, was discovered in bornite-rich ore from old dumps near the La Rosario vein, Capillitas epithermal deposit, Catamarca Province, Argentina. The new mineral is associated with bornite, digenite, chalcocite, covellite, sphalerite, hübnerite, luzonite, wittichenite, several Ge-bearing sulfides (putzite, unnamed Cu8Fe2ZnGe2S12, and a briartite-type phase) and quartz. Catamarcaite occurs as a thin rim lining cavities and fractures in chalcocite-bornite ore, and as aggregates, up to 1 mm in size, composed of anhedral to subhedral, intensely twinned grains within digenite - chalcocite - sphalerite ore. The mineral is opaque, grey, has a metallic luster and a black streak. It is brittle, and the fracture is irregular to subconchoidal. Neither cleavage nor parting is evident in polished sections. VHN25 ranges between 193 and 264 (mean 227) kg/mm2, which corresponds to a Mohs hardness of 31/2. The density could not be measured because of the small grain-size. Using the ideal formula and the unit-cell parameters from powder data, the calculated density is 4.892 g/cm3. In plane-polarized reflected light, catamarcaite is greyish white with a distinct brownish tint. It shows no pleochroism, and the bireflectance is absent in air and very weak in oil. Between crossed polars, it is weakly anisotropic in shades of grey. In oil and between slightly uncrossed polars, the rotation tints are pinkish grey to bluish green. Red internal reflections are occasionally observed along fractures. The reflectance values (in air and in oil, interval 400-700 nm) are tabulated; the COM values in air are (R1 and R2 in %): 470 nm: 24.5, 25.2; 546 nm: 24.1, 24.5; 589 nm: 24.5, 25.1; 650 nm: 23.4, 23.7. The average result of 34 electron-microprobe analyses is: Cu 42.72, Ag 0.14, Fe 0.17, Ge 7.84, W 20.89, S 27.79, total 99.55 wt.%, corresponding to the empirical formula Cu6.09Ag0.01 Fe0.03Ge0.98 W1.03S7.86 (on the basis of a total of 16 atoms). The ideal formula of catamarcaite, Cu6GeWS8, requires Cu 42.64, Ge 8.12, W 20.56, S 28.68, total 100.00 wt.%. Catamarcaite is hexagonal, space group P63mc, with unit-cell parameter refined from single-crystal data: a 7.5238(8), c 12.390(3) Å, V = 607.4(2) Å3, Z = 2, and c:a = 1.6467. The strongest eight X-ray powder-diffraction lines [d in Å,(I)(hkl)] are: 5.767(100)(011), 3.215(25)(112), 3.151(35)(021), 2.884(28)(022), 2.416(26)(121), 1.972(24)(025), 1.881(48)(220) and 1.744(26)(026). The crystal structure was solved by direct methods to an R1 index of 4.86% for 616 unique reflections measured with MoKα X-radiation. Catamarcaite is a derivative of a tetrahedral MeS structure with a stacking, along the c axis, of the ABAC type, with a combined hchc.. sequence. Thus two-layer packets with a cubic layer stacking are invariably separated by a boundary based on the hexagonal stacking of layers of tetrahedra (i.e., it is alternation of the "sphalerite" and "wurtzite" structure-types). The new mineral is chemically related to kiddcreekite, Cu6SnWS8, and hemusite, Cu6SnMoS8. The structural relationships among the three species are unknown. Catamarcaite is named after the Province of Catamarca, where the new mineral species was found. The mineral and mineral name have been approved by the Commission of New Minerals and Mineral Names, IMA (2003-020).