Lattice Parameter and Redox Equilibria in CaO-containing Wustite.

Equilibrium relation among oxygen potential in atmosphere, composition and lattice parameter in CaO-containing wustite have been studied at 1 273 K. Wustite plates with the molar cation fraction MCa/(MFe + MCa) (= MCa–Fe) ranging from 0.0058 to 0.079 were equilibrated with CO–CO2 gas mixtures. Composition of the wustite specimens was determined by chemical analysis and thermo-gravimetric method. Lattice parameter of the CaO-containing wustite was measured by X-ray diffraction method. The relation between the lattice parameter a and the gas ratio Ks(=PCO2/PCO) in the gas mixture was expressed bya = (4.297 + 0.560·MCa–Fe) – 1.470×1022·ln(Ks) (Å)The variation of a with MCa–Fe and molar ratio MFe/MO in the CaO-containing wustite was given bya = 1.916 + 0.565·(MCa–Fe) + 4.719·(MFe/MO) – 2.316·(MFe/MO)2 (Å)Then the lattice parameter may be used as the index of the composition in CaO-containing wustite. The 'iso-lattice-parameter' line was drawn within the wustite field in the isothermal section of the system FeO–Fe2O3–CaO. The molar ratio MFe2O3/MFeO of the CaO-containing wustite on the Fe2O3-rich side slightly increases with an increase in CaO content but the ratio on the FeO-rich side does not change. The solubility of CaO in the wustite equilibrated with both metallic iron and 2CaO·Fe2O3 was estimated to be 9 mol%CaO.