- journal_title：American Mineralogist
- Contributor：Jin S. Zhang ; Bruno Reynard ; Gilles Montagnac ; Ru Cheng Wang ; Jay D. Bass
- Publisher：Mineralogical Society of America
- Date：2013-05-01
- Format：text/html
- Language：en
- Identifier：10.2138/am.2013.4345
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：98
- issue：5-6
- firstpage：986
- section：Articles
Raman spectroscopy has been employed to investigate possible compositional effects on the high-pressure phase transition of Mg-rich orthoenstatite to a newly discovered P2<sub>1sub>/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<sup>−1sup> 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 P2<sub>1sub>/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 P2<sub>1sub>/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 P2<sub>1sub>/c phase might be present near the bottom of uppermost mantle, slightly shallower than the top of the transition zone.