Phase Transitions in Selected Silicate Minerals at High Pressure and Room Temperature from Single-Crystal X-Ray Diffraction

Abstract

The high-pressure and temperature behavior of many silicates relevant to the deep Earth has been extensively studied with powder X-ray diffraction techniques. However single-crystal X-ray diffraction is needed for detailed crystallographic analysis of mineral structures. This dissertation represents the beginning of a program to apply single-crystal techniques to more complex structures at higher pressures than found in previous studies. These initial experiments focus on the high-pressure, room-temperature behavior of representative key mantle minerals (forsterite olivine, orthopyroxene) and important silicate mineral classes (cordierite).

In each of the three systems investigated, new high-pressure phases were observed and their structures obtained (forsterite II and III, α- and β-post-orthopyroxene, cordierite II and III), some of which were not detected or overlooked in powder experiments. The high-pressure, 300 K structures are significantly different from the high-pressure, high-temperature structures of similar compositions. Several of the structures have the lowest symmetry possible, space group P1 (forsterite II, cordierite II and III). Some have mixtures of coordination polyhedra (e.g. 4-, 5-, and 6-coordinated Si, 4- and 5-coordinated Al, 5- and 6-coordinated Mg) that have rarely been reported in the literature (α- and β-post-orthopyroxene, cordierite II and III). One (forsterite III) unexpectedly adopts an inverse variant of a structure (CaTi2O4) within the well-known post-spinel structure family, but without ever passing through the usual spinel precursor. All three systems show increased polymerization of Si polyhedra across high-pressure phase transitions. Many of these phases may be metastable, but additional work is necessary to determine the energetics of these phases compared with high P-T phases of the same compositions. Future directions will include further systematic study of silicate structures at room temperature to assess whether the trends described here apply more broadly, as well as technique development to facilitate the application of single-crystal X-ray diffraction to minerals at simultaneous high pressure and temperature.