EFFECTS OF THE ANTIMONY-BISMUTH RATIO IN THE WITTICHENITE-SKINNERITE JOIN

The minerals wittichenite(Cu₃Bi S₃) and skinnerite(Cu₃SbS₃) display little solid solution within their respective ternary systems up to 400° C, and are isostructural both at elevated and low temperatures (Pnma -high: P2₁2₁2₁ -low). At intermediate temperatures, ordering of copper atoms produces lower symmetry (P2₁/c) and/or structural modulations. These unquenchable structural changes are well studied for the end-member minerals, but there is little research on the effects of changing the Bi/Sb ratio.

Synthesis experiments at Bi/Sb ratios of 0.75 and 0.5 show no evidence of a miscibililty gap between the two minerals at 400° C and indicate considerable quaternary solid solution as well, producing room-temperature stable P2₁/c phases. Reitveld structural refinements of the x-ray powder diffraction data result in structures that are more consisiten with skinnerite than wittichenite. Neither modulations of the structure nor the lower symmetry (P2₁/c) is yet apparent. Possible explanations for this include little relation of structural changes to Bi/Sb ratio; the expansion and contraction of the sulfate sheets to accommodate the interchangeable atoms or that the change is between a ratio of 0.75 and 1.0 for Bi/Sb.