The system NiAl2O4Ni2SiO4 at high pressures and temperatures: Spinelloids with spinel-related structures

Phase relations in the system NiAl₂O₄Ni₂SiO₄ 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 Ni₂SiO₄-spinel, NiO, and Al₂O₃ 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 NiAl₂O₄·Ni₂SiO₄, whereas phase III spreads from NiAl₂O₄·Ni₂SiO₄ to the NiAl₂O₄-rich side. All the phases I ～ V are structurally considered to be spinel derivatives, “spinelloids,” with three kinds of tetrahedral groups; isolated tetrahedra TO₄, linked ones T₂O₇, and triply linked ones T₃O₁₀. 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 O₃AlOAlO₃.