Anisotropic garnet from the Yamansu ore deposit, Xinjiang, China

Abstract

An anisotropic garnet was found in skarn ores occurring in the Yamansu deposit, east part of Xinjiang, China. Optical birefringence of the anisotropic garnet varies from 0.001 to 0.004. The chemical composition of the garnet essentially belongs to the grossular (Grs)-andradite (And) solid solution ranging from Grs_35.3And_60.1 to Grs_55.8And_38.5, with minor (<8%) uvarovite, pyrope, almandine and spessartine constituent. Negative correlation was slightly observed between Al and Fe in the garnet. The absence of OH stretching vibration in IR spectrum suggests no substitution of [SiO₄] with tetrahedral [(OH)₄] in the garnet.
A refinement of the crystal structure, I1 (R = 0.0435), showed no significant drifts of atoms from the ideal positions corresponding to the atomic positions in the Ia3d cubic symmetry (R = 0.0544). The Al-Fe ordering was observed in the octahedral M site, whereas the Ca-Fe ordering in the dodecahedral site was negligible. The Al-Fe ordering showed no symmetric distribution among the 8 octahedral M sites, indicating the true symmetry of this garnet to be triclinic I1.