EFFECTS OF PRESSURE ON MG-FE ORDERING IN ORTHO-PYROXENE SYNTHESIZED AT 11.3 GPA AND 1600-DEGREES-C

A single crystal of intermediate orthopyroxene, (Mg0.56Fe0.44)2Si2O6, has been recovered from a synthesis experiment at approximately 1.1 GPa and 1600-degrees-C. This rapidly quenched crystal displays a high degree of disorder for orthopyroxene M1 and M2 octahedral sites (K(d) = 3.9). Comparison with low-pressure orthopyroxene quenched from similar temperatures indicates that pressures corresponding to the Earth's mantle transition zone have little effect on Mg-Fe ordering between M1 and M2, unlike intracrystalline ordering in several other dense magnesium iron silicates.