The equilibrium Chlorite = Cordierite + Forsterite + Spinel + H₂0 has been experimentally determined with a chlorite of composition Mg₄.₇₅Al₂.₅Si₂.₇₅O₁₀(OH)₈. Brackets have been obtained between 605 and 640° C at 0.5 kb, 644 and 670° C at 1.0 kb and 690 and 704° C at 2.0 kb. These data are not notably displaced from Chernosky's (1974) data for the same equilibrium with a chlorite of clinochlore composition (Mg₅Al₂Si₃O₁₀(OH)₈), however they are more constraining. A thermodynamic analysis of the above data and data on related equilibria included ideal solution models describing compositional variability in cordierite, orthopyroxene and chlorite. The H₂0 content of cordierite was described using a model based on that of Newton and Wood (1979). The hydrous end-member has two moles of H₂0 and the volumes of the end-members are different; this allows the full range of data to be described with one function. Al-content of orthopyroxene was calculated with Gasparik and Newton's (1984) model. Solid solution in chlorite was modelled by choosing the end-members, Mg₆Si₄O₁₀(OH)₈ and Mg₄Al₄Si₂O₁₀(OH)₈, and using ideal configurational entropy to describe the free energy of mixing. Disordering phenomena in cordierite and spinel were accounted for by adding small entropies of disorder to the third law entropies. Linear programming was used to calculate consistent thermochemical properties for all phases considered. Experimental results indicate that the upper thermal stability of Mg-chlorite is affected by only a few degrees for the composition used here. The thermochemical properties derived allow more complete modeling of systems that include chlorite.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat
