SINGLE-CRYSTAL X-RAY DIFFRACTION OF HIGH-PRESSURE PLANETARY MATERAIL

Single-crystal diffraction is central to modern mineralogy. With the high flux of the superconducting wiggler beam line X17C at the National Synchrotron Light Source, a micro single crystal x-ray diffraction technique was developed in the 1990s for studies of earth and planetary materials, particularly high pressure phases. Since then, this technique has been successfully applied to mineral single crystals under pressure, mineral inclusions, and meteorites from the Earth and moon [1-6]. At 18 GPa single crystal wüstite transforms from a cubic structure to a rhombohedral phase containing four twin domains, each associated with one of the four body diagonals<111> of the original cubic crystal [1]. Single crystal stishovite in hydrogen pressure medium undergoes a reversible phase transition from the rutile to the CaCl2 structure at 58 GPa, reverting to the original structure at 40 GPa on decompression [2]. The lattice parameters of coesite inclusions in diamond, obtained from single crystal x-ray diffraction, show that the inclusions are under 3.44 GPa pressure and the c/a ratio reveals that the material is highly anisotropic [3]. A high-pressure polymorph of chromite has been discovered in the shock veins of the Suizhou meteorite, and is 9.4% denser than chromite-spinel [4]. Single crystal data from the lunar mineral hapkeite can give some clues to the mechanisms of space weathering on airless planetary bodies [5]. The technique is also very useful for studies of light elements in condensed phases. A single crystal of methane clathrate was found to adopt two new high-pressure phases: the hexagonal sH structure at 600 MPa and the cubic sII phase at 250 MPa [5]. These results have impacted a broad range of scientific problems associated with the Earth's crust and mantle as well as the interiors of other planetary bodies.

[1] Mao, H. K., et al, Phys. Earth Planet. Inter, 96, 135 -145. 1996 [2] Hemley, R. J., et al, Solid State Comm., 114, 527-532, 2000 [3] Sobolev, N.V., et al, Proc. Nat. Acad. Sci., 97, (22), 11875-11879 2000 [4] Chen, M., et al, PNAS., 100, 14651-14654, 2003. [5] Anand, M. L, et al, PNAS, 101, 6847–6851, 2004 [6] Chou, I. M., et al, PNAS, 97, 13484-13487, 2000