Sulfarsenide minerals are a subgroup of the sulfide minerals in which arsenic replaces sulfur as an anion in the formula. Antimony and bismuth may occur with or in place of arsenic, as in ullmannite. The chemical formula of a sulfarsenide resembles that of a sulfosalt, but the structures are distinctly different: in sulfosalts arsenic replaces a metal ion, whereas in sulfarsenides arsenic occupies the sulfur anion site.
The sulfarsenides are grouped with the sulfides in both the Dana and Strunz classification systems. Their crystal structures are commonly isotypic with arsenopyrite or cobaltite, with arsenic in octahedral or tetrahedral coordination with metals and sulfur linking the metal-arsenic framework.
The sulfarsenides include two main groups:
Arsenopyrite is the most common sulfarsenide. It is silver-white to steel-grey, metallic, with a Mohs hardness of 5.5 to 6 and a specific gravity near 6.0, and commonly occurs with gold, pyrite, chalcopyrite and quartz. Arsenopyrite is a common gold-bearing mineral in gold deposits, where gold may occur as micro-inclusions or lattice defects.
Cobaltite is silver-white to pale red, metallic, with a hardness of 5 and a specific gravity near 6.3, and is one of the cobalt ore minerals. Gersdorffite is isotypic with cobaltite and is a nickel sulfarsenide. Ullmannite contains antimony, arsenic and bismuth with variable composition and occurs in nickel-cobalt deposits.
Sulfarsenides form mainly in medium- to low-temperature hydrothermal deposits, contact-metasomatic deposits and metamorphic deposits. Arsenopyrite is common in gold, tin and tungsten deposits and occurs with quartz, tourmaline, pyrite and pyrrhotite. Cobaltite and gersdorffite occur in cobalt-nickel arsenide deposits with nickeline, skutterudite and native bismuth.
Sulfarsenide formation is controlled by temperature, sulfur fugacity, arsenic fugacity and pH. Under reducing conditions arsenic combines with metals and sulfur as As³⁻ or As₂²⁻ species, whereas under oxidising conditions sulfarsenides are unstable and decompose to arsenates, sulfates and oxides. Arsenopyrite oxidation releases arsenic and acid and is an important pollution source in mine environments.
Arsenopyrite is an important gold-bearing mineral in gold deposits, where gold occurs as invisible gold or micro-inclusions in the arsenopyrite lattice. Such ores require fine grinding and oxidative pre-treatment, such as roasting, bio-oxidation or pressure oxidation, to break down the arsenopyrite and liberate gold. Arsenopyrite is also a major source of arsenic, but arsenic toxicity limits its direct use.
Cobaltite and gersdorffite are cobalt and nickel ore minerals, especially in cobalt-nickel deposits in Morocco, Canada and northern Europe. Smelting sulfarsenides requires control of arsenic emissions to avoid air and water pollution. Arsenopyrite and arsenic-bearing minerals in tailings can produce arsenic-rich acid drainage if exposed to oxidising conditions and must be controlled by covers, neutralisation or stabilisation.
Sulfarsenides are easily confused with sulfosalts because their formulae are similar. The difference is structural: in sulfarsenides arsenic is an anion forming an anionic framework with sulfur, whereas in sulfosalts arsenic or antimony is a cation replacing metals. Sulfarsenides also differ from arsenides, which contain little or no sulfur, such as nickeline (NiAs) and skutterudite (CoAs₃).
Identification usually combines metallic lustre, hardness, specific gravity, streak and associated minerals, and requires electron microprobe or X-ray diffraction to confirm the coordination of arsenic, sulfur and metals. Hand-specimen identification of arsenopyrite, cobaltite and gersdorffite is difficult, and chemical analysis is often necessary.