Comparative compressibilities of majorite-type garnets

Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope (Mg_2.84Fe_0.10Ca_0,06) Al₂Si₃O₁₂], and four synthetic specimens with octahedrally-coordinated silicon: majorite Mg₃(MgSi)Si₃O₁₂], calcium-bearing majorite (Ca_0.49Mg_2.51)(MgSi)Si₃0₁₂], sodium majorite (Na_1.88Mgp_0.12)(Mg_0.06Si_1.94)Si₃O₁₂], and an intermediate composition (Na_0.37Mg_2.48)(Mg_0.13Al_1.07 Si₀₈₀) Si₃O₁₂]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.