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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.