The system NiAl 2O 4Ni 2SiO 4 at high pressures and temperatures: Spinelloids with spinel-related structures

Abstract

Phase relations in the system NiAl 2O 4Ni 2SiO 4 were studied in the pressure range 1.5 ∼ 13.0 GPa and in the temperature range 800 ∼ 1450°C. Two new phases, IV and V, were found in regions of pressure higher than 4 GPa. Phase V disproportionates into a mixture of Ni 2SiO 4-spinel, NiO, and Al 2O 3 at approximately 9.5 GPa and 1100°C. Phases III, IV, and V form a solid solution in some compositional range: phases IV and V have a composition around NiAl 2O 4·Ni 2SiO 4, whereas phase III spreads from NiAl 2O 4·Ni 2SiO 4 to the NiAl 2O 4-rich side. All the phases I ∼ V are structurally considered to be spinel derivatives, “spinelloids,” with three kinds of tetrahedral groups; isolated tetrahedra TO 4, linked ones T 2O 7, and triply linked ones T 3O 10. The ratios of isolated tetrahedra to linked ones are large in the higher-pressure phases and small in the lower-pressure phases. The difference of compositional range of phase III from that of phases IV and V is possibly explained by the avoidance of linked tetrahedra such as O 3AlOAlO 3.