ARSENIDE

any member of a rare mineral group consisting of compounds of one or more metals with arsenic (As). The coordination of the metal is almost always octahedral or tetrahedral. In the former case, each metal ion occupies a position within an octahedron composed of six oppositely charged arsenic ions, whereas in the latter each of the metal ions is surrounded by six oppositely charged neighbours arranged tetrahedrally. Structurally the arsenides resemble the sulfides (e.g., galena, sphalerite, and argentite) and are frequently included in that mineral group (see sulfide). Two common arsenides are niccolite (NiAs) and skutterudite (CoAs3). Niccolite is a low-temperature hydrothermal mineral with hexagonal symmetry that is usually associated with nickel, cobalt, and silver sulfides. Skutterudite, on the other hand, is an intermediate- to high-temperature hydrothermal mineral with cubic or octahedral symmetry associated with arsenopyrite, native silver, and bismuth. Other arsenides include: maucherite Ni11As8 rammelsbergite NiAs2 safflorite (Co,Fe)As2 lllingite FeAs2 arsenopalladinite Pd3As dienerite Ni3As oregonite Ni2FeAs2 algodonite Cu6As sperrylite PtAs2 All arsenides have a metallic lustre, are opaque, and have high specific gravity and intermediate to low hardness. The succession of arsenide minerals maucherite, niccolite, rammelsbergite, skutterudite, safflorite, and lllingite corresponds to the transition from a reducing to an oxidizing environment.