Phase relations in the systems Ag2S-Cu2S-PbS and Ag2S-Cu2S-Bi2S3 and their mineral assemblages

Abstract

Phase relations in the ternary systems Ag2S-Cu2S-PbS and Ag2S-Cu2S-Bi2S3 were studied using the silica vacuum technique. In the system Ag2S-Cu2S-Bi2S3 the phase relations are dominated by join-lines from galena to f.c.c. (Agx Cu2−xS) and b.c.c. (Cux Ag2−xS) at 500°C. With decreasing temperature, galena can coexist with all the phases on the Ag2S-Cu2S join. There are six solid solutions, and one new phase, i.e., “C” whose composition is Ag1.1 Cu4.8Bi5.8S12 in the system Ag2S-Cu2S-Bi2S3 at 500°C. The pavonite (AgBi3S5) contains 14 mole% Cu2S in solid solution, but only 3.0 mole% Ag2S in CuBi3S5 solid solution. The Cu3Bi5S9 ss and wittichenite (Cu3BiS3) ss can form join-lines with pavonite as and have the maximum contents of 9.0 and 18 mole% Ag2S. The most striking feature is the presence of bejaminite as a stable phase with a chemical formula of Ag2Bi4S7 on the Ag2S-Bi2S3 join. AgBiS2 of the PbS type occupies a fairly large field with a maximum of 23 mole% Cu2S.