PUTZITE, (Cu_(4.7)Ag_(3.3))_(SIGMA8)GeS_6, A NEW MINERAL SPECIES FROM CAPILLITAS, CATAMARCA, ARGENTINA: DESCRIPTION AND CRYSTAL STRUCTURE

摘要

Pulzite, chemically (Cu_(4.7)Ag_(3.3)GeS_6, was discovered in old dumps near the Rosario vein, Capillitas raining district. Department of Andalgala, Catamarca Province, Argentina. The Capillitas diatreme is located within the Farallon Negro volcanic complex, which is host to large porphyry Cu-Au deposits (Bajo de la Alumbrera, Agua Rica) and genetically related epithermal vein-npe deposits. The mineral assemblage that contains putzite is composed of bornite, chalcocite. sphalerite, tennantite, wittichenite, thalcusite, catamarcaite (IMA 2003-020), an undefined Cu-Fe-Zn-Ge sulfide possibly representing the Ge-domi-nant analogue of stannoidite, chalcopyrite, galena, luzonite and quartz. Putzite occurs as aggregates of anhedral grains, up to 3 X 1 nun. embedded in a matrix of predominantly chalcocite with relics of bornite. The mineral is opaque, iron-black with a weak violet tint, has a metallic luster and a black streak. It is brittle, the fracture is irregular to subconchoidal (rarely splintery), and the cleavage, whose orientation is unknown, is distinct in polished sections. VHN_(50) ranges between 183 and 201 (mean 188) kg/mm~2, which corresponds to a Mohs hardness of about 3-31/2. The density, 5.788 g/cm~3, was calculated using the empirical formula and Z= 4. In plane-polarized reflected light in air, the mineral is pale rose to pale violet (in contrast with chalcocite), lacks internal reflections and is isotropic. The reflectance spectra in air and in oil are tabulated. The chemical composition, which was determined with an electron microprobe, shows weak variation from grain to grain. The average composition of one sample (four grains. 29 analyses) is: Cu 33.04, Ag 39.33, Ge 7.79, S 20.55, total 100.71 wt. percent; another specimen yielded Cu 32.71, Ag 39.83, Ge 7.62, S 20.59, total 100.75 wt. percent. This leads to empirical formulae (based on total atoms = 15) of (Cu_(4.78)Ag_(3.35))(SIGMA(8.13)Ge_(0.99)S_(5.89) and (Cu_(4.73)Ag_(3.40))(SIGMA(8.13)Ge)_(0.97)S_(5.91), respectively. The ideal formula, (Cu_(4.7)Ag_(3.3))(SIGMA8GeS_6, requires: Cu 32.48, Ag 38.71, Ge 7.89, S 20.92, total 100.00 wt. percent. Putzite is cubic, with a unit-cell parameter a refined from X-ray powder-diffraction data equal to 10.201(3) A, V = 1061.5(6) A~3, space group F43m (216). The strongest seven X-ray powder-diffraction lines [d in A(I)(hkl)] are: 5.896(30)(111), 3.074(60)(311), 2.943(100)(222), 2.083(30)(422), 1.962(50)(333), 1.805(70)(440) and 1.725(25X531). The crystal structure of putzite was solved by direct methods to an R index of 5.97 percent for 259 observed reflections measured with MoKalpha X-radialion. Close metal-metal contacts indicate Cu,Ag disorder. The structure description is based on regular polyhedra such as GeS_4, SCu_6, SAg_(12) and SCu_(12), instead of the conventional discussion of various two-, three-, or four-fold coordinations around (disordered) Ag and Cu. Putzite belongs to the argyrodite group and is an Ag-rich variety of the synthetic inorganic compound 7-Cu_8GeS_6,. The mineral name honors Hubert Putz (b. 1973), who discovered the new species during field work in Catamarca, for his significant contribution to the mineralogy of germanium in the Capillitas deposit. The mineral and the mineral name have been approved by the Commission of New Minerals and Mineral Names, IMA (2002-024).