THE COPPER/ARSENIC SYSTEM AND THE COPPER ARSENIDE MINERALS

Abstract

In addition to the α solid solution of arsenic in copper, three intermediate phases exist in the copper/arsenic system. The ε phase, stable only below ca. 340 °C, has the hexagonal A3 [Formula: see text] structure with a = 2.588, c = 4.226 Å, and the approximate formula Cu8As. The compound Cu3−xAs with 0 ≤ x ≤0.3, is trigonal, [Formula: see text]with a = 7.132, c = 7.304 Å at x = 0. This compound has no polymorphic forms between 200 °C and the melting point. The arsenic-rich compound Cu5−uAs2, with 0 ≤ u ≤ 0.1, melts incongruently at about 700 °C, and decomposes at 300 °C to arsenic and Cu3As. The structure has not been determined, but the powder diffraction pattern is recorded. The solubility of copper in arsenic appears to be negligible.It is suggested that the mineral α domeykite is a high-pressure phase, and that the mineral algodonite is a high-pressure modification of the ε phase. The mineral β domeykite is isostructural with Cu3As.