Static Compression Measurements of Hydrous Magnesium Silicate Phases. Daniel J. Frost and Yingwei Fei, Geophysical Laboratory and Centre for High Pressure Research, 5251 Broad Branch Road, NW, Washington, DC 20015
A number of dense hydrous magnesium silicates have been identified from experiments at high pressures and temperatures. These hydrous phases are potential hosts of water in the Earth's mantle. In order to understand the role of water in the mantle, we need to characterize these hydrous phases and to determine their compression behavior at high pressure and temperature. Here we present experimental results of static compression measurements performed on samples of phase D and hydrated ß-Mg2 SiO4 , which are two of the most important hydrous phases potentially stable within the earth's mantle.
Phase D is a Si-rich hydrous magnesium silicate containing over 15 weight % water. It has trigonal symmetry. We measured the compression behavior of this phase under hydrostatic (neon as a pressure medium) and non-hydrostatic (no pressure medium) conditions, using energy-dispersive synchrotron x-ray diffraction technique. This phase was found to posses a high bulk modulus which reflects the occurrence of Si exclusively in octahedral coordination.
Nominally anhydrous phase ß-Mg2 SiO4 is a very important mineral in the Earth's transition zone. The hydrated form of ß-Mg2 SiO4 has a monoclinic unit cell, that can contain up to 3 wt% water. The static compression data for this phase were obtained by using a monochromatic synchrotron x-ray beam and the image plate technique. The experiment was conducted in a diamond-anvil cell up to 30 GPa, using neon as a pressure medium. The pressure was calibrated using the lattice parameters of gold. The bulk modulus measured for hydrated ß-Mg2 SiO4 is approximately 17% lower than for the dry mineral. This has significant implications for the stability field of ß-Mg2 SiO4 within the mantle, which may be extended in the presence of water at the expense of higher pressure polymorphs.