HR: 0830h
AN: V31D-0957    [PDF]
TI: High-Temperature Phase Transition in Enstatite : Raman Spectroscopic Results
AU: * Reynard, B
EM: breynard@ens-lyon.fr
AF: Laboratoire de Sciences de la Terre CNRS UMR 5570 Ecole Normale Superieure, 46 Allee d'Italie, Lyon, 69007 France
AU: Bass, J
EM: jaybass@uiuc.edu
AF: Dept of Geology, Univ. of Illinois, USA, 1301 W Green St., Urbana, IL 61801 United States
AB: (Mg,Fe)SiO$_3$ enstatite has various polymorphs of which orthoenstatite with space group Pbca is the most common in natural rocks. The existence of a high temperature form has been suggested from various experiments but its symmetry remains unknown. Recent high-temperature Brillouin measurements on nearly pure MgSiO$_3$ show that this transition is first order with a strong hysteresis (Tc at about 1200-1250$\deg$C with increasing temperature, Tc around 1000$\deg$C with decreasing temperature; Jackson et al, 2003). It is accompanied by strong pretransitional softening of some elastic constants and has some important consequences in the understanding of upper mantle seismic properties especially in hot regions. In order to more fully understand the nature of this transition and possibly the structural changes associated with it, we have performed in situ Raman spectroscopy on pure enstatite up to the transition temperature. The transition is observed in the same temperature range with increasing temperature, and is characterized by a decrease of the number of Raman modes, which can be interpreted as the transition to a space group with reduced Wigner-Seitz cell. Pretransitional effects are observed especially on a low frequency mode at 80 cm$^{-1}$, which displays pronounced anharmonic behaviour. Possible space groups are Pbcn (protoenstatite), C2/c (high-clinoenstatite) or a previously unreported Cmca structure. The latter is a supergroup of Pbca and could account for the pretransitional softening. On decreasing temperature, backtransformation to orthoenstatite is marked by the appearance of cracks along simple crystallographic directions, which eventually leads to the breaking of the submillimeter-sized single crystals used as starting materials. Areas of untransformed high-temperature phase can be preserved down to about 750$\deg$C. This large hysteresis is strongly controlled by crystal shape and size as well as thermal history. In a parallel experiments, needle shaped thin (5x50 micrometers) crystals with the high-temperature structure have been quenched from a melt down to room temperature. Finally, preliminary experiments on a natural Fe bearing orthoenstatite from San Carlos show that iron increases the transition temperature that could not be observed up to 1260$\deg$C for this composition.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting