Raman spectroscopy has been employed to investigate possible compositional effects on the high-pressure phase transition of Mg-rich orthoenstatite to a newly discovered P21/c phase. Three natural orthoenstatite (OEN) samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), Al-free, Fe-bearing orthoenstatite (Morogoro, Tanzania) and Al-rich, Fe,Ca-bearing orthoenstatite (Kilbourne Hole, New Mexico). Experiments were carried out at room temperature. For all samples, the high-pressure phase transition is characterized by a splitting of the 660–680 cm−1 doublet in the Raman spectrum into a triplet, with a corresponding change of peak intensities. These spectral changes are caused by the lowered symmetry of the high-pressure phase, as indicated by structural refinement from single-crystal X-ray diffraction results. The high-pressure phase of all samples appears to have space group P21/c. No evidence for a C2/c phase was observed. Our results indicate that upon compression, the presence of 10 mol% Fe decreases the onset pressure of formation of the high-pressure P21/c phase by about 1 GPa. Results for the Kilbourne Hole OEN show that upon compression, a combined enrichment of Al and Ca contents increases the onset pressure of formation of high-pressure clinoesntatite (HPCEN2) by over 3 GPa relative to Tanzania OEN. Upon decompression, all samples revert to single crystals of the orthoenstatite starting phase. Our measurements suggest that orthoenstatite is the prevalent phase of Mg-rich pyroxene throughout the uppermost mantle, whereas the newly discovered P21/c phase might be present near the bottom of uppermost mantle, slightly shallower than the top of the transition zone.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.