Phase Equilibria in the System NiO–Al2O3–SiO2

Equilibrium diagrams for the systems NiO-SiO2, NiO-Al2O3, NiAl2O4-SiO2, Ni2SiO4-NiAl2O4, and NiAl2O4-Al6Si2O13 were drawn from data obtained by quenching and direct observational techniques. The only intermediate compound in the binary system NiO-SiO2 is Ni2SiO4, which has the olivine structure. Unlike other olivines which melt congruently, nickel olivine has an upper temperature of stability (1545°C) and at temperatures between 1545° and 1650°C, NiO and SiO2 coexist in equilibrium. The only compound in the binary system NiO-Al2O3 is NiAl2O4, which has a spinel structure. The nickel aluminate spinel varies in composition from 50 to 35 mole % Al2O3 at 1800°C, and the stoichiometric NiAl2O4 composition has a melting point near 2110°C. Of the joins within the ternary system NiO-Al2O3-SiO2 which were studied, only Ni2SiO4-NiAl2O4 is not binary. In this join, crystals of NiO exist in equilibrium with liquid and a ternary assemblage of NiO + NiAl2O4+ liquid is stable to 1775°C. The decomposition temperature of Ni2SiO4 is decreased from 1545°C in the binary system to approximately 1490°C, presumably the result of solubility of NiAl2O4 in Ni2SiO4. The join NiAl2O4-SiO2 is binary in that the compositions of crystalline phases can be expressed in terms of the chosen components. The eutectic temperature in the system is 1495°C. The join NiAl2O4-Al6Si2O13 is binary for the same reasons and has a eutectic temperature at 1720°C. Using the data obtained in this study and those published for the well-known system Al2O3-SiO2, a liquidus surface diagram for the system NiO-Al2O3-SiO2 is proposed. Nickel olivine, even though it has an upper limit of stability in the binary system, has a primary field in the ternary system NiO-Al2O3-SiO2. This is the only refractory oxide system known to illustrate this so-called “typical case,” the governing principles of which have been clearly presented in discussions of phase equilibria.