The transition of orthoferrosilite to high-pressure C2/c clinoferrosilite at ambient temperature

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 P2₁lc) with a few small islands of metastable C2lc clinoferrosilite. The presence of the C2/c 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 1.5 GPa) → low-clinoferrosilite (P2₁lc) The transformation of orthoferrosilite to the high-pressure C2lc 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.