2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 7
Presentation Time: 2:30 PM


MAO, Ho-kwang, DERA, Przemek and PREWITT, Charlie, Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, washington, DC 20015, mao@gl.ciw.edu

Single-crystal x-ray diffraction (SXD) at high pressures provide unique, important sources of structural information that is crucial for understanding microscopic mechanisms of minerals in the Earth’s deep interior. Sophisticated software and hardware have been developed and optimized for several varieties of diamond anvil cells (DAC) and integrated with in-house and synchrotron x-ray sources for accurate lattice parameter determinations and robust structure refinements from kilobars to megabars. We improved the commonly-used polar diamond cells (x-ray passing through the cone around the polar axis of the diamond cell) to reach 80 GPa, and the equatorial diamond cells (x-rays passing through the equatorial belt) to reach 120 GPa with unstrained samples of sufficient size and with wider access to reciprocal space. Helium pressure medium is used to preserve an unstrained single-crystal sample at high pressures. Macromolecular SXD up to 1 GPa have been carried out with monochromatic synchrotron x-ray beam through the DAC axis and diffraction peaks collected by a 2-dimensional detector. SXD of minerals to moderate pressures (30 GPa) are routinely conducted with x-ray beam through the single-crystal diamond anvils and polycrystalline beryllium seats. At higher pressures when the sample diffraction becomes diminishingly weak relative to that from the anvils, seat and gasket, it is necessary to eliminate the background by using energy dispersive x-ray diffraction with a pointed solid-state detector, or to reduce the background by constraining the incident beam path in the single-crystal diamonds (i.e., avoiding the beryllium seats and gasket) for 2-dimensional detectors. The developments provide definite long-sought answers to important scientific questions in Earth, planetary, and materials sciences, including the fundamental changes of silicates, oxides, ices, and condensed gases that are the main components of deep planetary interiors.