Raman Spectroscopy and Structure of MgSiO3 High Temperature C2/c Clinoenstatite

Abstract:

The high-temperature clinoenstatite (HT-CEn) is one of the important MgSiO3 pyroxene polymorph. The single-crystal of C2/c HT-CEn endmember is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 900-1900 °C [1]. No report that it is produced as single crystal or large domain had been made on the MgSiO3 endmember. The HT-CEn-type modifications were observed in Ca-poor Mg-Fe clinoenstatite and pigeonite and are always found to be unquenchable in rapid cooling.

The high pressure and high temperature experiments of MgSiO3 composition were carried out with a Kawai-type multi-anvil apparatus. The samples were quenched by rapidly releasing the oil pressure load and/or by blow out of anvil cell gasket. The space group of C2/c is strictly determined by Rigaku RAPID Weissenberg photographs and synchrotron radiation. HT-CEn and HP-CEn have the greatly different beta angles of 109° and 101°, respectively. Raman spectra of HT-CEn and OEn single crystals were collected at ambient conditions.

The unusual bonding distances frozen in the metastable structure. The observed average Mg1-O and Si-O distances in HT-CEn [1.997 and 1.620 Å, respectively] are shorter than those in HP-CEn at 7.9GPa. The average Mg2-O distance in HT-CEn [2.311 Å] is significantly longer than that in L-CEn, providing an abnormal larger distance for the Mg2 atom. The Mg2 polyhedron in HT-CEn is more irregular than that in HP-CEn. The Debye-Waller factor of atoms in HT-CEn have abnormally larger amplitude. The static irregularity of the atomic displacement caused by the transition is frozen in the metastable state.

Almost all Raman peaks are broad owing to the large statistical positional arrangement of atoms in HT-CEn. The braod patterns have the common feature which were obserbed by the high temperature Raman spectroscopy for pyroxene. The peaks have been confirmed at 108, 259, 684, and 1097 cm^-1. Peak positions for HT-CEn are different from those for protoenstatite under high temperature. HT-CEn may be found in natural rocks that had rapid quenching history such as a shock-metamorphosed meteorite. Especially the peaks of 108 and 684 cm-1 are clear and Raman spectrra can use for an identification.

[1] A. Yoshiasa, A. Nakatsuka, M. Okube and T. Katsura, Acta Crystallographica Section B, 2013, 69, 541-546