THERMODYNAMICS OF MULTICOMPONENT PYROXENES .2. PHASE-RELATIONS IN THE QUADRILATERAL

The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg2Si2O6, CaMgSi2O6, CaFeSi2O6, and Fe2Si2O6. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi2O6-Mg2Si2O6 and CaFeSi2O6-Fe2Si2O6, (3) calorimetric data for CaMgSi2O6-Mg2Si2O6 Pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene + augite, and miscibility gap features, over the temperature and pressure ranges 800-1500-degrees-C and 0-30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M 2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe2+ -Mg2+, M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe2+ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe + Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca + Fe + Mg) ratios. The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthopyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constituent subsystems are calculated for temperatures and pressures over the range 800-1700-degrees-C and 0-50 kbar and compare favorably with experimental constraints.