Symmetry and disorder in garnets on the pyrope (Mg3Al2Si3O12)-majorite (Mg4Si4O12) join from Cr3+ luminescence spectroscopy

Abstract

Cr3 +-bearing garnets with compositions of majorite, Mj72, Mj23, Mj21 and pyrope were studied by luminescence spectroscopy. The tetragonal distortion of the pure majorite sample is marked by the occurrence of two 2E doublets corresponding to Cr3+ replacing Si and Mg in the Oc1 and Oc2 octahedral sites. The other garnets are cubic and a single 2E doublet is observed. The width of the luminescence lines in the disordered cubic majorites is proportional to the number of Mg, Si, Al random configurations around the Cr3+ sites. In the tetragonal majorite, the linewidth is only slightly higher than that in pyrope, indicating ordering of Mg and Si on the octahedral sites. These results are consistent with previous X-ray diffraction and NMR spectroscopic studies, and show that Cr3+ luminescence spectroscopy is a potential tool for studying short range order in high-pressure silicate solid solutions relevant to the Earth's mantle.