Crystal structures and compressibilities of pyrope and grossular to 60 kbar

Crystal structures and compressibilities of the end-member garnets pyrope (synthetic Mg₃Al₂Si₃O₁₂ at 1 bar and 16, 31, 43, and 56 kbar) and grossular (natural Ca₃Al₂Si₃O₁₂ at 1 bar and 19, 35, and 61 kbar) have been determined from three-dimensional, single-crystal X-ray data. Both garnets have cubic unit-cell dimensions within experimental error: for pyrope a = 11.456(1)A at 1 bar and 11.332(5)A at 50 kbar; for grossular a = 11.846(1) A at 1 bar and 11.720(5) A at 50 kbar. Bulk moduli of pyrope and grossular are both approximately 1350(50) kbar, which agrees with previous static compression data but is significantly lower than elastic constant data.

The large, eight-coordinated polyhedra compress more than the aluminum octahedra, which compress more than the silicon tetrahedra. Polyhedral bulk moduli are 1150(125), 1300(100), 2200(500), and 3000(1000) kbar, respectively, for ^VIIICa²⁺, ^VIIIMg²⁺, ^VIAl³⁺, and ^IVSi⁴⁺. The observed compression of the silicon tetrahedron is significantly greater than previously reported experimental or theoretical values. In the eight-coordinated site, linear compression of four longer M-О bonds is approximately twice that of four shorter bonds. Thus, this polyhedrtm becomes more regular at high pressure.