| Abstract | Putzite, chemically (Cu4.7Ag3.3)?8GeS6, was discovered in old dumps near the Rosario vein, Capillitas mining district, Department of Andalgalá, Catamarca Province, Argentina. The Capillitas diatreme is
located within the Farallo?n Negro volcanic complex, which is host to large porphyry Cu-Au deposits (Bajo de la Alumbrera, Agua Rica) and genetically related epithermal vein-type 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-dominant analogue of stannoidite, chalcopyrite, galena, luzonite and quartz. Putzite
occurs as aggregates of anhedral grains, up to 3 × 1 mm, 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. VHN50 ranges between 183 and 201 (mean 188) kg/mm2, which corresponds to a Mohs hardness of about 3-3½. The
density, 5.788 g/cm3, 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.%; another specimen yielded Cu 32.71, Ag 39.83, Ge 7.62, S 20.59, total 100.75 wt.%. This leads to empirical formulae (based on total atoms = 15) of (Cu4.78Ag3.35)?8.13Ge0.99S5.89 and (Cu4.73Ag3.40) ?8.13Ge0.97S5.91,
respectively. The ideal formula, (Cu4.7Ag3.3) ?8GeS6, requires: Cu 32.48, Ag 38.71, Ge 7.89, S 20.92, total 100.00 wt.%. Putzite is cubic, with a unit-cell parameter a refined from X-ray powder-diffraction data equal to 10.201(3) Å, V = 1061.5(6) Å3,
space group F43m (216). The strongest seven X-ray powder-diffraction lines [d in Å(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(25)(531). The crystal structure of putzite was
solved by direct methods to an R index of 5.97% for 259 observed reflections measured with MoKa X-radiation. Close metal-metal contacts indicate Cu,Ag disorder. The structure description is based on regular polyhedra such as GeS4, SCu6, SAg12 and
SCu12, 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 y-Cu8GeS6. 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). |