Proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3) form a complete series of solid solutions at temperatures at least as low as 300° C. Evidence from natural occurrences suggests a miscibility gap at lower temperatures, but demonstration of such a gap has not been made in the field or in the laboratory.
As would be expected from the crystal structures, the c-axis length of the solid solutions is nearly constant, while the o-axis length is a function of the composition, increasing with increasing Sb content. Because of the resulting change in axial ratio, diffraction-angle separation between certain pairs of peaks in the diffraction pattern may be used directly as an indication of composition.
Study of specimens of natural ruby silvers has not revealed any examples of equilibrium two-ruby-silver assemblages. A specimen of pyrargyrite from Casapalca, Peru, has unitcell dimensions corresponding to a composition of about 35 formula per cent Ag3AsS3, about twice the arsenic content of any pyrargyrite heretofore reported.