Structural studies of mineral silicates and synthetic sulfosalts characterised by mixed occupancies

Abstract

In the first part of this study are presented the synthesis and results of structural investigations of mixed metal sulfosalts. Included are the structures of five new lead, bismuth and antimony sulfosalts; phase GZ1 (Bi4.39 Pb1.4 Sb3.21 S13), phase GZ2 (Bi1.74 Pb2.62 Sb2.15 S8), phase GZ3 (Bi0.87 Pb1.22 Sb0.42 S3), phase GZ4 (Sb4.66 Pb3.97 Bi1.37 S13) and phase GZ5 (Sb6 Pb14 S23), and the revaluation of the known phases zinckenite (Pb1.49 Sb3.64 S7), stibnite (Sb2 S3), lillianite (Pb3 Bi2 S6) and galenobismutite (Pb Bi2 S4). With the exception of stibnite and galenobismutite these phases are characterised by mixed occupancies, and the site populations were assigned via a combination of microprobe analyses, polyhedral distortion and bond valence analysis results. To this end two FORTRAN programs were written, OCCGEN96 and VOLUME96, the former to calculate the occupancy of a site based on the bond valence contributions of its constituents, and the latter to calculate its polyhedral volume.
Phase GZ1 exhibits a monoclinic structure with a central region reminiscent of stibnite, and its distribution of occupancies indicate the presence of a supercell produced by ordering. Phases GZ2 and GZ3 are hexagonal with features in common with zinckenite. GZ3 possesses the smallest hexagonal cell yet found for a sulfosalt and its structure contains three-fold metallic columns, in addition it displays a metal coordination environment not previously seen in this type of structure. The positional disorder, and partial occupancies (manifest as poorly defined hexagonal channels) exhibited by zinckenite and the other hexagonal sulfosalts are here explained for the first time in terms of alternately occupied configurations arranged on a superlattice. Phase GZ5 resulted from an attempt to crystallise the mineral semseyite whose structure is unknown, but possibly closely related to that of GZ5. The poorly defined phases from the literature have all been refined with better discrepancy indices and in the case of lillianite the results differ significantly from those previously reported.
Additional investigations of phase GZ1 were carried out using transmission electron microscopy, and these lead to the identification of a number of related phases and also revealed the presence of disordered and composite structures.
In the second part of the study diamond inclusions of lower mantle origin are investigated. In common with the sulfosalts they are characterised by mixed occupancies, however the distribution of metal species within mineral silicates is better understood than in sulfosalts, and they provided a good base from which to study the latter. The presented results include the crystal structure of the new mineral TAPP (Mg0.76 Fe0.15) (Al1.76 Cr0.16 Mn0.06) (Mg1.88 Fe0.08) Si1 (Si1.92 Al0.08) O12, which is of great significance in lower mantle genesis theory; the structures of two novel pyroxenes (Mg0.46 Al0.43 Fe0.06 Cr0.05) (Mg0.39 Na0.31 Ca0.16 Fe0.06 Mn0.04) (Si1.91 Al0.09) O6 and (Mg0.73 Al0.23 Cr0.03) (Mg0.70 Na0.06 Ca0.03 Fe0.16 Mn0.04) (Si0.83 Al0.17) SiO6 and the only sapphire (Al1.98Fe0.01Si0.01O3) to be found as a diamond inclusion. The structure of TAPP is particularly interesting as it has a garnet composition but is tetragonal not cubic. In the past the only non cubic garnets have been so only by a few angstroms at the most, while the cell of TAPP has a c parameter nearly three times that of a.

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