Abstract
The partitioning of chromium and aluminium between coexisting orthopyroxene and spinel in equilibrium with forsterite in the system MgO–Al2O3–SiO2–Cr2O3 (MAS–Cr) has been experimentally determined as a function of temperature, pressure and Cr/(Cr + Al) ratio. Experiments were conducted at temperatures between 1300 and 1500 °C and at pressures from 5 to 54 kbar. Previous experimental results on the (Al, Cr)2O3 and Mg(Al, Cr)2O4 solid solutions have been combined with the present results plus relevant data from the CMAS system to derive a thermodynamic model for Al–Cr-bearing orthopyroxenes, spinels and corundum–eskolaite solid solutions. The orthopyroxene solid solution can be modelled within the accuracy of all experimental constraints as a ternary solid solution involving the components Mg2Si2O6 ( E), MgAl2SiO6 (M) and MgCr2SiO6 (C), in which the activities are related to composition through the equations:
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