Single-crystal metastable high-temperature C2/c clinoenstatite quenched rapidly from high temperature and high pressure

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Vol. 69; no. Pt 6; p. 541
• Main Authors: Yoshiasa, Akira, Nakatsuka, Akihiko, Okube, Maki, Katsura, Tomoo
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.12.2013
• Subjects: Crystal structure; Crystallography; High pressure; High temperature; Single crystals; X-ray diffraction; phase transitions; pressure–temperature quenching; polymorphism; clinoenstatite
• ISSN: 2052-5192; 2052-5206
• DOI: 10.1107/S2052519213028248

The high-temperature clinoenstatite (HT-CEn) is one of the most important MgSiO3 pyroxene polymorphs, but the details of its structure and stability have been uncertain. The single crystal of the C2/c HT-CEn end-member is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 1173-2173 K. The single-crystal X-ray diffraction analysis shows unusual bonding distances and static disorder of the atoms frozen in this metastable structure. The degree of kinking of the silicate tetrahedral chains is 175° for HT-CEn. The chain angle for HP-CEn is substantially smaller (135°), but the angle for L-CEn is in the opposite direction at -160° (= 200°). The degree of kinking increases by being curved by more than 180° for the transition from HT-CEn to L-CEn. As for the reverse change from the expansion to the stretch, a potential barrier exists at the point of the continuity. It is suggested that the reason why a structure can be quenched under ambient conditions is as follows: the present HT-CEn single crystal has been formed by the isosymmetric phase transition from the high-pressure C2/c clinoenstatite (HP-CEn). The presence of HT-CEn from HP-CEn in natural rocks is an indicator of quenching history, which leads to the possibility that it exists in shocked meteorites and impact materials.