Iron (II) -magnesium exchange equilibrium between olivine and calcium˗free pyroxene over a temperature range 800° C to 1 300° C

Experimental data on coexisting olivine, (Mg, Fe)₂SiO₄, and calcium-free pyroxene, (Mg, Fe)₂ Si₂O₆, were critically reviewed on the basis of assumption that both olivine and pyroxene behave as the simple mixtures. New data, obtained by the equilibration at 30 kbar and 1 000° C, were included also. Independent data from several authors gave a consistent picture that : 1) the simple mixture assumption is sufficient to describe the exchange reaction of Fe and Mg between olivine and pyroxene ; 2) excess enthalpy of mixing for olivine is larger than that for pyroxene, the difference between them appearing to be accounted for largely by intracrystalline distribution of Fe and Mg ions in pyroxene ; and : 3) the exchange reaction is singularly insensitive to variations in both temperature and pressure. Thus the Fe-Mg partition between olivine and calcium-free pyroxene is decidedly unsuitable as an indicator of physical condition of chemical equilibrium.