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Earth and Planetary Science Letters 290 (2010) 150-154
Carbon polymorphism in shocked meteorites: Evidence for new natural ultrahard phases
Tristan Ferroir 1, Leonid Dubrovinsky 2, Ahmed El Goresy 2, Alexandre Simionovici 3, Tomoki Nakamura, Philippe Gillet 1
(02/2010)

A slice of the Haverö meteorite which belongs to the ureilite class known to contain graphite and diamond was cut and then polished as a thin section using a diamond paste. We identified two carbonaceous areas which were standing out by more than 10 µm in relief over the surface of the silicate matrix suggesting that the carbonaceous phases were not easily polishable by a diamond paste and would therefore imply larger polishing hardness. These areas were investigated by reflected light microscopy, high-resolution Field Emission SEM (FESEM), energy-dispersive X-ray (EDX) analysis, Raman spectroscopy, and were subsequently extracted for in situ synchrotron microbeam X-ray fluorescence (XRF), imaging and X-ray diffraction (XRD). We report here the natural occurrences of one new ultrahard rhombohedral carbon polymorph of the R3m space group which structure is very close to diamond but with a partial occupancy of some of the carbon sites. We also report the natural occurrence of the theoretically predicted 21R diamond polytype with lattice parameters very close to what has been modelized. These findings are of great interests for better understanding the world of carbon polymorphs and diamond polytypes giving new natural materials to investigate. These natural samples demonstrate that the carbon system is even more complex than what is currently thought based on ab initio static lattice calculations and high-pressure experiments since this new ultrahard polymorph has never been predicted nor synthesized.
1 :  Laboratoire de Sciences de la Terre (LST)
CNRS : UMR5570 – INSU – Université Claude Bernard - Lyon I (UCBL) – École Normale Supérieure (ENS) - Lyon
2 :  Bavarian Research Institute of Experimental Geochemistry and Geophysics (Bayerisches Geoinstitut)
Universitat Bayreuth – University of Bayreuth
3 :  Laboratoire de géodynamique des chaines alpines (LGCA)
CNRS : UMR5025 – OSUG – INSU – Université de Savoie – Université Joseph Fourier - Grenoble I
Planète et Univers/Sciences de la Terre/Planétologie