PHASE-EQUILIBRIA IN THE SYSTEM NIO-CAO-SIO2 AND GIBBS ENERGY OF FORMATION OF CANISI2O6

Phase relations in the pseudoternary system NiO-CaO-SiO2 at 1373 K are established. The coexisting phases are identified by X-ray diffractio n and energy-dispersive X-ray analysis of equilibrated samples. There is only one quaternary oxide CaNiSi2O6 with clinopyroxene structure. T he Gibbs energy of formation of CaNiSi2O6 is measured using a solid st ate galvanic cell incorporating stabilized zirconia as the solid elect rolyte in the temperature range of 1000 to 1400 K: Pt, Ni + SiO2 + CaS iO3 + CaNiSi2O6 \ (Y2O3)ZrO2 \ Ni + NiO, Pt From the electromotive for ce (emf) of the cell, the Gibbs energy of formation of CaNiSi2O6 from NiO, SiO2, and CaSiO3 is obtained. To derive the Gibbs energy of forma tion of the quaternary oxide from component binary oxides, the free en ergy of formation of CaSiO, is determined separately using a solid sta te cell based on single crystal CaF2 as the electrolyte: Pt, O-2, CaO + CaF2 \ CaF2 \ CaSiO3 + SiO2 + CaF2, O-2, Pt The results can be expre ssed by the following equations: NiO (r.s) + CaO (r.s) + 2SiO(2) (qz) --> CaNiSi2O6 (pyr) Delta G degrees = -115,700 + 10.63T (+/-100) J mol (-1) CaO (r.s) + SiO2 (qz) --> CaSiO3 (wol) Delta G degrees = -90,030 -0.61T (+/-60) J mol(-1).