NEW OCCURRENCES AND REFINED CRYSTAL-CHEMISTRY OF COLUSITE, WITH COMPARISONS TO ARSENOSULVANITE

Structural refinements of Sn-rich and Sn-poor colusite from the Lorano marble quarry, Italy, revealed that the crystals are isometric and be long to space group P43nBAR. For Sn-rich colusite, CuS4 and (As,Sn)S4 tetrahedra are connected through comer sharing in a sphalerite structu re with the composition Cu26Sn2As4S32, and two V atoms are stuffed int o interstitial sites in a sulvanite-like arrangement. For Sn-poor colu site, CuS4 and AsS4 tetrahedra are connected in a similar arrangement through comer sharing in a sphalerite-like structure with the composit ion Cu24As6S32, with the two V atoms located in the same arrangement a s in Sn-rich colusite. The number of valence electrons contributed by metal atoms is 64 for Sn-rich and Sn-poor colusite, whereby the substi tution of two Sn atoms with two As atoms is balanced by two vacancies in one of the Cu sites. Based on structural and compositional consider ations, a general formula for colusite is Cu24+xV2(As,Sb)6-x(Sn,Ge)xS3 2, where x = 0-2. Selected-area electron diffraction (SAED) patterns a nd high-resolution transmission electron microscopy (HRTEM) imaging of Sn-poor colusite with a composition of Cu24.3V2.0(As5.7Sb0.6Fe0.1)S32 from the epithermal Gies gold-silver telluride deposit, Montana, reve aled that it is composed of domains of ordered and disordered cations. The ordered domain has space group P43nBAR and a = 1.068 nm and consi sts of Sn-poor colusite. The disordered domain, with similar compositi on, has space group F43mBAR and a = 0.534 nm. Simulated images based o n proposed structural models produced an acceptable match to observed HRTEM images. One-dimensional modulations, with a wavelength of 3.57 n m, are possibly responsible for the optical anisotropism and possibly for the anomalously low Vickers hardness. Another isometric sulfosalt, arsenosulvanite [Cu3(As,V,Sb,Fe,Ge)S4], with X-ray and optical proper ties similar to colusite, has been introduced in the mineralogical lit erature. The results of the present study and a review of the literatu re on the chemical composition, X-ray crystallography, and crystal che mistry of colusite and arsenosulvanite strongly suggest the same ident ity for the two species.