The crystallography of sartorite

Extract

Sartorite is one of four minerals once thought to be isomorphous. Of these only two are truly so—chalcostibite, copper antimony sulphide, CuSbS₂; and emplectite, copper bismuth sulphide, CuBiS₂—both orthorhombic minerals. The third, zinckenite, has been shown by two of the authors to be hexagonal and to approach more closely the formula Pb₆Sb₁₄S₂₇ ═ 6PbS.7Sb₂S₃ than PbSb₂S₄ ═ PbS.Sb₂S₃. It seemed appropriate to continue a study of the series by an X-ray investigation of sartorite, a lead arsenic sulphide formerly supposed isomorphous with zinckenite. More than usual interest was attached to such a task, since discrepant interpretations of the symmetry and setting of sartorite had been obtained by well-known crystallographers.

Indeed, the difficulties of indexing different crystals of sartorite on the same lattice proved insuperable to G. F. H. Smith and R. H. Solly, so that they devised three incongruent lattices—one monoclinie and two anorthic. Also they used slightly different elements for each crystal. Their device, however, was not uniformly successful with certain crystals of sartorite previously measured by C. O. Trechmann. Two crystals in particular eluded their efforts. These, they decided, could not belong to the same species, but must be another mineral, perhaps a dimorphous form of sartorite, which they then named sartorite-α.