EFFECT OF CA2-PYROXENE( AND FE2+ ON THE EQUATION OF STATE OF MGSIO3 ORTHO)

Room temperature equations of state (EOSs) have been determined for sy nthetic and natural orthopyroxenes on and near the (Mg,Fe)SiO3 join by single-crystal X-ray diffraction to a maximum pressure of 11 GPa. The values of the room pressure bulk moduli, K-0,K-T, and its pressure de rivative, K-0', are shown to be independent of the form of the equatio n of state used to fit the data. Variation in the Fe2+ content of the orthopyroxene has very little effect on either K-0,K-T, or K-0', with the EOS parameters of synthetic FeSiO3 orthoferrosilite being K-0,K-T = 95.1 (3.0) GPa, and K-0' = 10.6 (1.6), both within one combined expe rimental estimated standard deviation of those previously determined f or synthetic MgSiO3 orthoenstatite. However, substitution of even very small amounts of Ca2+ (and Al3+) to the orthopyroxene structure cause s an increase in K-0,K-T by some 14%. The density difference between t he synthetic MgSiO3 and natural orthoenstatites (calculated from the e quations of state at 10 GPa) is of the order of 3.4%, with a correspon ding unit-cell volume difference of the order of similar to 0.3%. It i s concluded that pure MgSiO3 orthoenstatite is not a good representati on of the pyroxene component of the Earth's upper mantle.