HR: 1340h
AN: MR23C-0087    [Abstracts]
TI: On the track of 5-fold silicon signature in the high pressure CAS phase CaAl₄Si₂O₁₁
AU: * Gautron, L
EM: gautron@univ-mlv.fr
AF: Laboratoire des Géomatériaux, FRE 2455 CNRS, Université de Marne-la-Vallée, 5 boulevard Descartes, Champs-sur-Marne, Marne-la-Vallée, 77454 France
AU: Daniel, I
EM: Isabelle.Daniel@univ-lyon1.fr
AF: Laboratoire des Sciences de la Terre, UMR 5570 CNRS, ENS Lyon - UCB Lyon1, 46 allée d'Italie, Lyon, 69364 France
AU: Beck, P
EM: pierre.beck@ens-lyon.fr
AF: Laboratoire des Sciences de la Terre, UMR 5570 CNRS, ENS Lyon - UCB Lyon1, 46 allée d'Italie, Lyon, 69364 France
AU: Guignot, N
EM: guignot@esrf.fr
AF: European Synchrotron Radiation Facility, Grenoble, 6 rue Jules Horowitz, Grenoble, 38043 France
AU: Andrault, D
EM: andrault@impmc.jussieu.fr
AF: Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590 CNRS, Campus Boucicaut Bat7, 140 rue de Lourmel, Paris, 75015 France
AU: Greaux, S
EM: greaux@univ-mlv.fr
AF: Laboratoire des Géomatériaux, FRE 2455 CNRS, Université de Marne-la-Vallée, 5 boulevard Descartes, Champs-sur-Marne, Marne-la-Vallée, 77454 France
AB: High pressure experiments proposed that the CAS phase of composition CaAl₄Si₂O₁₁ is one of the phases candidate for hosting aluminium in subducting slabs, which present Al content up to ten times that of the surrounding mantle. First recognized by Irifune et al. (1994), the CAS phase was further synthesized as a single phase, and fully characterized by Gautron et al. (1996, 1997, 1999) at ambient conditions. The CAS phase is hexagonal with a = 5.4223(4) Å and c = 12.7041(6) Å. The space group is P6₃/mmc, with Z = 2; its room-pressure density is 3.905 g.cm^-3. The CAS phase recently became a mineral (although not labeled yet), since Beck et al. (2004) first reported the occurrence of CAS in a Shergottite shocked Martian meteorite. With a structure related to that of hexagonal barium ferrites, CAS is expected to display silicon in 5-fold coordination in a trigonal bipyramidal site. But X-ray diffraction on quenched CAS single crystals revealed that at room pressure Si is split in 4-fold and 6-fold coordination. Nonetheless, it is likely that a transition from 5 to 4-fold coordination occurred during quenching from run conditions. The exceptional 5-fold coordination of Si in minerals could be intermediate stage between the 4-fold and 6-fold coordinations observed in minerals in the upper and lower mantle, respectively. Hence, a transition toward a phase with silicon only six-fold coordinated, might also occur at higher pressure. SiO₅ polyhedra are believed to play an important role in the transport properties of minerals because of their relation to oxygen vacancies, which are essential for diffusion phenomena. We performed X-Ray Diffraction (XRD) and Raman spectroscopy measurements on the CAS phase at high pressure in situ in a diamond anvil cell, in order to determine possible signatures of 5-fold coordinated silicon in this phase. XRD experiments allowed us to characterize the CAS phase upon compression and decompression, up to 50 GPa. Raman spectra at ambient pressure were fully interpreted. The possible transition for silicon from 4 to 5-fold coordination was investigated through the evolution of Raman spectra of the CAS phase upon compression.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005