First Occurrence of the Garnet-Ilmenite Transition in Silicates

Pyrope garnet (Mg_3Al_2Si_3O_(12)) has been found to transform to an ilmenite-type phase at a loading pressure between 240 and 250 kilobars and at about 1000° to 1400°C in a diamond-anvil press coupled with laser heating. The lattice parameters for the ilmenite-type phase of (Mg_(.75) Al_(.25))(Si_(.75) Al_(.25))O_3 are a_0 = 4.755 ± 0.002 and c_0 = 13.360 ± 0.005 angstroms. The zero-pressure volume change associated with the garnet-ilmenite transition is calculated to be –7.1 percent. This result verifies the prediction that pyrope garnet would transform to the ilmenite structure at high pressure first suggested in 1962 by Clark et al. and Ringwood