This study presents evidence that for a solid solution of Mg x Fe (sub x-1) SiO 3 perovskite, the shift in volume thermal expansivity, alpha , is small as the index chi is changed. According to data obtained theoretically by Hama and Suito (1998), alpha decreases by 0.3-0.4% as chi changes from 1 to 0.9 in a temperature range of 1900 K. Furthermore, the relative shift in alpha for Fe substitution is -0.4% under lower mantle conditions. Hama and Suito used the Vinet equation of state to calculate thermoelastic properties. Using a thermodynamic approach applied to the properties of a Debye solid, the data show that the relative shift in alpha is less than the relative change in specific volume, and, as chi changes from 1 to 0.9, V increases by 0.6% and alpha decreases by 0.4%.

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