Phase Relations at Liquidus Temperatures in the System MgO-NiO-SiO2

The well-known "quenching technique" has been used to determine liquidus and solidus phase relations in the system MgO-NiO-SiO₂. Oxide mixtures were heated at selected temperatures for sufficient lengths of time for attainment of equilibrium, followed by quenching to room temperature, and phase identification by optical microscopy (transmitted-and reflected-light examination) and X-ray diffraction. Two ternary liquidus invariant points are present in the system. One is characterized by the coexistence of pyroxene (Mg-SiO₃-NiSiO₃ solid solution), olivine (Mg₂SiO₄-Ni₂SiO₄ solid solution), silica (cristobalite), and liquid at 1547°C. The other eutectic is at 1633°C and is characterized by the coexistence of olivine, oxide (MgO-NiO solid solution), silica, and liquid. There is a temperature maximum on the olivine-silica liquidus boundary curve at 1639°C. Reversals in the distribution ratios of magnesium and nickel between solid and liquid phases coexisting in equilibrium during crystallization of melts in this system may occur, depending on the compositions of the mixtures.