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 Cu 8 As. The compound Cu 3−x As 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 Cu 5−u As 2 , with 0 ≤ u ≤ 0.1, melts incongruently at about 700 °C, and decomposes at 300 °C to arsenic and Cu 3 As. 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 Cu 3 As.