Crystal Structures of Dense Hydrous Magnesium Silicates. Charles T. Prewitt and Hexiong Yang, Geophysical Laboratory and Center for High Pressure Research, 5251 Broad Branch Road, NW, Washington, DC 20015

The presence of water was essential for the origin of life on Earth and for the volcanic, tectonic, and erosion processes that have shaped Earth's surface and determined much of the character of its deep interior. In order to understand the role of water (hydrogen) in Earth's evolution, it is necessary to know how hydrogen is bonded in mineral structures and what determines the stability of hydrous phases under varying environmental conditions.

Over the past 30 years a number of hydrous magnesium silicates that are potential constituents of the Earth's mantle were synthesized at high pressures and temperatures, and labeled as 10 Å, 3.65 Å, A, B, C, D, E, F, and superhydrous B (the "alphabet phases"). Because there has just not been enough coordinated work on synthesis and crystallography, questions still remain about their compositions and crystal structures. Nevertheless, considerable progress has been made over the past several years and this work is reviewed here.

Until recently, very little systematic work has been done to evaluate the role of hydrogen in these high-pressure phases, but because of growing interest in the role of hydrogen in deep-Earth processes, substantial new information about hydrogen bonding is becoming available. Diffraction and spectroscopic studies provide data for determining where hydrogen is located in quenched high-pressure phases as well as how hydrogen bonds change as a function of pressure.

Dense hydrous magnesium silicates (DHMS) such as phase B, superhydrous (shy) B, and phase D are shown experimentally to be stable at elevated pressures and temperatures, as high as 25 GPa and 1200deg.C. Recent work on phase D, which contains all of its Si in octahedral coordination, shows it to have a density of 3.50 gm/cm³ and a bulk modulus of 200 GPa, both of which are larger than for any other high-pressure hydrous magnesium silicate. Thus, it is a prime candidate for water storage in the transition zone and lower mantle.