Rietveld crystal structure refinement of a natural rhombohedral grossular-andradite garnet from Serbia

Abstract

Four macroscopically visible B–E (rim) zones, within a large natural ferric grossular garnet of Grs58–64Adr36–42Sps2 composition, are described by means of Rietveld refinements of the crystal structures in a series of six space groups, followed by comparative analysis of the R-values, site occupancy factors, and bond lengths and angles. The garnet crystallized in the rhombohedral R-3c space group. Various polyhedral distortions and structural order-disorder variations between the zones studied are also described and discussed. The rhombohedral symmetry of the ferric grossular garnet analysed can be regarded as primary, residual strain being a secondary cause for its slight optical anisotropy.