CRYSTAL STRUCTURE OF AS-GOLDFIELDITE

Abstract

Goldfieldite has the general formula (Cu, Fe, Ag) 12-x (Te, Sb, As, Bi) 4S 13-y, where Te > Sb + As + Bi. It is a rare mineral that can be formed only in a narrow range of physical and chemical conditions, namely a combination of oxidative conditions under which tellurium is stable with an oxidation number of 4, and high activities of sulfur and tellurium in the ore-generating solution. Two varieties, Sb-goldfieldite and As-goldfieldite, are found in nature. There has hitherto been no structural study of monocrystalline As-goldfieldite. We studied a specimen from the Yelshishche deposit in Bulgaria. Its chemical composition, as determined with a Cameca-46 microprobe (Cu 43.38 percent, Sb 0.23 percent As 5.30 percent, Te 25.74 percent, S 25.56 percent by weight), was extremely conductive for accurate structural study owing to the high tellurium content and the absence of elements capable of isomorphously substituting for copper. It is shown that the structure of As-goldfieldite is a defect structure, primarily because of the occurrence of copper vacancies when tetravalent tellurium replaces trivalent antimony and arsenic in the tetrahedrite lattice of such minerals as tetrahedrite and tennantite.