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 belong to space group P43n. For Sn-rich colusite, CuS₄ and (As,Sn)S₄ tetrahedra are connected through corner sharing in a sphalerite structure with the composition Cu₂₆Sn₂As₄S₃₂, and two V atoms are stuffed into interstitial sites in a sulvanite-like arrangement. For Sn-poor colusite, CuS₄ and AsS₄ tetrahedra are connected in a similar arrangement through corner sharing in a sphalerite-like structure with the composition Cu₂₄As₆S₃₂, with the two V atoms located in the same arrangement as in Sn-rich colusite. The number of valence electrons contributed by metal atoms is 64 for Sn-rich and Sn-poor colusite, whereby the substitution of two Sn atoms with two As atoms is balanced by two vacancies in one of the Cu sites. Based on structural and compositional considerations, a general formula for colusite is Cu_24+xV₂(As,Sb)_6−x(Sn,Ge)_xS₃₂, where x = 0−2.

Selected-area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) imaging of Sn-poor colusite with a composition of Cu_24.3V_2.0(As_5.7Sb_0.6Fe_0.1)S₃₂ from the epithermal Gies gold-silver telluride deposit, Montana, revealed that it is composed of domains of ordered and disordered cations. The ordered domain has space group P43n and a = 1.068 nm and consists of Sn-poor colusite. The disordered domain, with similar composition, has space group F43m and a = 0.534 nm. Simulated images based on proposed structural models produced an acceptable match to observed HRTEM images. One-dimensional modulations, with a wavelength of 3.57 nm, 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 properties similar to colusite, has been introduced in the mineralogical literature. The results of the present study and a review of the literature on the chemical composition, X-ray crystallography, and crystal chemistry of colusite and arsenosulvanite strongly suggest the same identity for the two species.