High pressure transitions in the system MgAl2O4-CaAl2O4:: a new hexagonal aluminous phase with implication for the lower mantle

Phase transitions in MgAl2O4 and CaAl2O4 and in the binary system MgAl2O4-CaAl2O4 were established at high pressures and high temperatures. MgAl2O4 spinel dissociates to periclase and corundum at 15-16 GPa and 1200 degrees C-1600 degrees C with an equilibrium boundary of P (GPa)= 12.3 + 0.0023T (degrees C). The mixture recombines into a phase with calcium ferrite structure at about 26.5 GPa at 1600 degrees C. At 780 degrees C-1400 degrees C, CaAl2O4 crystallizes with the calcium ferrite structure at pressure above 8-9 GPa. At lower pressures, phases LII and IV are stable. On the join MgAl2O4-CaAl2O4, solubility of MgAl2O4 component into CaAl2O4,-rich calcium ferrite is limited to about 10 mol% at 20-21 GPa and 1200 degrees C, and a new phase with composition of (Mg-x,Ca1-x)Al2O4 (x = 0.66-0.8) was found to be stable at pressure above 15 GPa. Powder X-ray diffraction data show that the new phase possesses hexagonal symmetry. Moreover, it is interpreted that an Al-rich phase observed in high pressure assemblage of mid-ocean ridge basalt (MORB) above 25 GPa by Irifune and Ringwood [Irifune, T., Ringwood, A.E,, 1993. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600-800 km in the mantle. Earth Planet. Sci. Lett. 117, 101-110.] has the same structure as the hexagonal phase found in this study, rather than calcium ferrite structure. And another Al-rich phase synthesized at 24-27 GPa in high pressure experiments from natural pyrope-rich garnet starting material is also similar to the hexagonal phase. These results suggest that aluminous phase of the hexagonal structure would be one of major constituent minerals of subducted oceanic crust in the upper part of the lower mantle. (C) 1999 Elsevier Science B.V. AU rights reserved.