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

Abstract

Four macroscopically visible B–E (rim) zones of the natural grossular-andradite garnet with Grs_58–64Adr_36–42Sps₂ composition were crystallographically studied. The applied procedure includes the Rietveld refinements of the crystal structures in a series of six space groups, which were followed by the comparative analysis of the R-values, site occupancy factors, and the bond lengths and angles. It was established that it crystallized in the rhombohedral R‾3c space group. Various polyhedral distortions and structural order-disorder variations between the studied zones were also observed, determined and discussed. Therefore, rhombohedral symmetry of the studied grandite could be treated as primary and residual strain as the second cause for its slight optical anisotropy.