Published online by Cambridge University Press: 14 March 2018
These two minerals are analogous silver compounds with similar pseudo-hexagonal monoclinic form and similar compositions: polybasite, 8(Ag,Cu)2.(Sb,As)2S3, and pearceite, 8(Ag,Cu)2S.As2S3. Cu replaces Ag in considerable and varying amounts in both minerals, and in polybasite there may also be substantial replacement of Sb by As. However, the rather numerous existing analyses do not indicate continuous chemical variation from arsenian polybasite to pearceite, and therefore it is proper to recognize the two minerals as distinct species rather than end-members of a series.
For the preceding paper of this series see E. W. Nuffield (1946) [M.A. 10–15]. No. II by L. G. Berry on jamesonite appeared in this Magazine, 1940, vol. 25, pp. 597–608.