CRYSTALLOGRAPHY OF EARTH AND PLANETARY INTERIORS. Russell J. Hemley, Geophysical Laboratory and Center for High-Pressure Research, Carnegie Institution of Washington, Washington, D.C. 20015, USA

Recent developments in diamond-anvil cell x-ray diffraction methods have ushered in a new era for structural study of Earth and planetary materials at the high pressure-temperature conditions that prevail deep within the planets. These methods include single-crystal and polycrystalline diffraction with both polychromatic and monochromatic synchrotron radiation for in situ determination of crystal structures, phase transitions, equations of state, and elasticity of materials. Structural variations with pressure, temperature, and composition of silicates and oxides form the key for understanding the composition and global properties of the Earth's mantle (to 135 GPa or 1.35 Mbar). These studies have recently been complemented by x-ray crystallography of natural mineral inclusions in diamonds brought up from the deep mantle. Experiments on iron and iron alloys to pressures above 300 GPa elucidate the nature of the Earth's core. Recently, these studies have been combined with new laser heating techniques for x-ray diffraction at simultaneous high pressures and temperatures above 100 GPa, and with new methods to determine directly the effects of hydrostatic and uniaxial stress on crystal structures at these pressures. Finally, these methods have been applied to the structural properties of the solidified gases and ices relevant to bodies of the outer solar system. This includes x-ray diffraction of solid hydrogen above 100 GPa, new high-density H₂-bearing compounds, and H₂O-ice at very high pressures.