The transition of orthoferrosilite to high-pressure C2/c clinoferrosilite at ambient temperature
Hugh-Jones, Demelza; Sharp, Thomas; Angel, Ross; Woodland, Alan
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1337 - 1346
published: Jan 8, 1997
manuscript accepted: Jul 15, 1996
manuscript received: Jun 10, 1996
ArtNo. ESP147050806012, Price: 29.00 €
Abstract High-pressure single-crystal X-ray diffraction experiments on FeSiO3 orthoferrosilite have revealed that at pressures above ~4.2GPa, the orthopyroxene transforms to a new phase. Examination by transmission electron microscopy of transformed material recovered from the diamond-anvil cell shows that the final product is predominantly low-clinoferrosilite (space group P2\lc) with a few small islands of metastable C2/c clinoferrosilite. The presence of the Cllc clinoferrosilite, and the fact that this phase transforms readily to low-clinoferrosilite upon decompression below - 1.5 GPa in hydrostatic conditions, confirms that the high-pressure clinoferrosilite was formed by the breakdown of orthoferrosilite at ~ 4.2 GPa. The sequence of transformations is therefore: orthoferrosilite (Pbca) → (P increase above 4.2 GPa) → high-P clinoferrosilite (C2/c) → (P release below 1.5GPa) → low-clinoferrosilite (P21/c) The transformation of orthoferrosilite to the high-pressure C2/c clinoferrosilite is highly time-dependent and first-order in character, accompanied by a -2.6% volume discontinuity. The morphology and microstructure of the recovered transformation products is indicative of a reconstructive transformation mechanism similar to that proposed previously for other orthopyroxene to clinopyroxene transformations.