Stabilities and compositions of alpha -domeykite and algodonite

alpha -domeykite, the cubic and low temperature form of Cu 3 As, decomposes at 90 + or - 10 degrees C and approximately 1 atmosphere pressure to beta -domeykite, composition Cu (sub 2.7) As and algodonite, composition Cu (sub 5.2) As. Algodonite has a composition range from Cu 8 As to Cu (sub 5.2) As, and a maximum temperature of stability of 300 + or - 20 degrees C at the composition Cu 8 As.The presence of nearly pure masses of alpha -domeykite, such as those commonly observed in the Lake Superior copper district, is a clear indication of deposition below 90 degrees C.