Natural Arsenides

a group of minerals representing compounds of the metals Fe, Ni, and Co—as well as Pt and Cu—with arsenic. The natural arsenides crystallize in various systems—hexagonal (modderite CoAs, niccolite NiAs, and others), cubic (domeykite Cu₃As, sperrylite PtAs₂ and skut-terudite CoAS₃), rhombic (loellingite FeAs₂, safflorite [Co, Fe]As₂, rammelsbergite NiAs₂, and others), and tetragonal (maucherite Ni₃As₂). Crystallochemically, natural arsenides are related to the coordinated and islandlike structures. Simple arsenides, as well as diarsenides with paired anions of the As₂⁴⁻ type in their structure, are formed. The isomorphism of Ni, Co, and Fe is characteristic for the natural arsenides. The density of the natural arsenides ranges from 5,500–6,000 to 10,500 kg/m³. Their hardness on the mineralogical scale is 4.0–6.5. They are nontransparent and have a metallic luster. Natural arsenides are found occasionally in magmatic deposits in connection with basic and ultrabasic rock. They are most frequently found in hydrothermal deposits of nickel-cobalt and silver-nickel-cobalt formations. Loellingite is observed in very significant quantities in arsenopyrite and tin-arsenic deposits. Occasional mineralogical finds of natural arsenides have been observed in iron ore, gold ore, and lead-zinc deposits. Natural arsenides play an essential role in the composition of numerous metallic mineral deposits and serve as a source for obtaining nickel, cobalt, platinum, and arsenic. Upon oxidation they are transformed into arsenates.