Phase Equilibria in the System NiO–Al ₂ O ₃ –SiO ₂

Equilibrium diagrams for the systems NiO‐SiO 2 , NiO‐Al 2 O 3 , NiAl 2 O 4 ‐SiO 2 , Ni 2 SiO 4 ‐NiAl 2 O 4 , and NiAl 2 O 4 ‐Al 6 Si 2 O 13 were drawn from data obtained by quenching and direct observational techniques. The only intermediate compound in the binary system NiO‐SiO 2 is Ni 2 SiO 4 , 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 SiO 2 coexist in equilibrium. The only compound in the binary system NiO‐Al 2 O 3 is NiAl 2 O 4 , which has a spinel structure. The nickel aluminate spinel varies in composition from 50 to 35 mole % Al 2 O 3 at 1800°C, and the stoichiometric NiAl 2 O 4 composition has a melting point near 2110°C. Of the joins within the ternary system NiO‐Al 2 O 3 ‐SiO 2 which were studied, only Ni 2 SiO 4 ‐NiAl 2 O 4 is not binary. In this join, crystals of NiO exist in equilibrium with liquid and a ternary assemblage of NiO + NiAl 2 O 4 + liquid is stable to 1775°C. The decomposition temperature of Ni 2 SiO 4 is decreased from 1545°C in the binary system to approximately 1490°C, presumably the result of solubility of NiAl 2 O 4 in Ni 2 SiO 4 . The join NiAl 2 O 4 ‐SiO 2 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 NiAl 2 O 4 ‐Al 6 Si 2 O 13 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 Al 2 O 3 ‐SiO 2 , a liquidus surface diagram for the system NiO‐Al 2 O 3 ‐SiO 2 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‐Al 2 O 3 ‐SiO 2 . 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.