PHASE-EQUILIBRIA IN THE SYSTEM CAO-COO-SIO2 AND GIBBS ENERGIES OF FORMATION OF THE QUATERNARY OXIDES CACOSI2O6, CA2COSI2O7, AND CACOSIO4

Phase relations in the pseudoternary system CaO-CoO-SiO2 have been est ablished at 1323 K. Three quaternary oxides were found to be stable: C aCoSi2O6 with clinopyroxene (Cpx), Ca2CoSi2O7 with melilite (Mel), and CaCoSiO4 with olivine (Ol) structures. The Gibbs energies of formatio n of the quaternary oxides from their component binary oxides were mea sured using solid-state galvanic cells incorporating yttria-stabilized zirconia as the solid electrolyte in the temperature range of 1000-13 24 K. The results can be summarized as follows: CoO (rs) + CaO (rs) 2SiO(2) (Qtz) --> CaCoSi2O6 (Cpx), Delta G(f)(0) = -117920 + 11.26T (/-150) J/mol CoO (rs) + 2CaO (rs) + 2SiO(2) (Qtz) --> Ca2CoSi2O7 (Mel) , Delta G(f)(0) = -192690 + 2.38T (+/-130) J/mol CoO (rs) + CaO (rs) SiO2 (Qtz) --> CaCoSiO2 (Ol), Delta G(f)(0) = -100325 + 2.55T (+/-100 ) J/mol where rs = rock salt (NaCl) structure and Qtz = quartz. The un certainty limits correspond to twice the standard error estimate. The experimentally observed miscibility gaps along the joins CaO-CoO and C aCoSiO4-Co2SiO4 were used to calculate the excess free energies of mix ing for the solid solutions CaxCo1-xO and (CayCo1-y)CoSiO4:Delta G(E) = X(1 - X)[31975X + 26736 (1 - X)] J/mol and Delta G(E) = 23100 (+/-25 0) Y(1 - Y) J/mol. A T-X phase diagram for the binary CaO-CoO was comp uted from the thermodynamic information; the diagram agrees with infor mation available in the literature. The computed miscibility gap along the CaCoSiO4-Co2SiO4 join is associated with a critical temperature o f 1389 (+/-15) K. Stability fields for the various solid solutions and the quaternary compounds are depicted on chemical-potential diagrams for SiO2, CaO, and CoO at 1323 K.