Two-phase data for the join monticellite (CaMgSiO₄)-forsterite (Mg₂SiO₄): Experimental results and numerical analysis

Subsolidus equilibria along the CaMgSiO₄-Mg₂SiO₄ olivine join are governed by partial immiscibility between the end-member phases. The solvi bounding the two-phase region are determined over the temperature range 800-1300°C at 2, 5, and 10 kbar. Within the limits of experimental error, the two-phase data are symmetric with respect to composition. Nurnerical analysis of the experimental data in terms of a one-constant Margules equation (Thompson, 1967) yields the following relationship for the mixing parameter of the Gibbs Free Energy Function:

W_G(cal/mole) = 20,338 ± 2355 - 13,560 ± 3412 × T(°K)/1000 + 3627 ± 1228 ×(T/1000)² - 355.4 ± 87.5 P(kbar) + 308.7 ± 65.4×P(T/1000), S.D. = 297.4 Temperature-composition sections of the two-phase region calculated from this equation are in good agreement with the experimental data, The results imply that the maximum amountof CaMgSiO₄ which may be dissolved in forsteritic olivine is less than 5 mole percent under physico-chemical conditions likely to exist in natural systems.