The compressibility of hexagonal Al-rich NAL phase: similarities and differences with calcium ferrite-type (CF) phase with implications for the lower mantle

We have determined the equation of state of a hexagonal Al-rich type NAL phase [K0.07Na0.81Ca0.12]Σ1.01[Mg1.62Fe0.38]Σ2.00[Al4.98Fe0.10Ti0.05Si0.88]Σ6.01O12, an aluminum host mineral that is likely to occur in subducted oceanic crusts (mid-oceanic ridge basaltic (MORB)) in the Earth’s lower mantle. Angle-dispersive X-ray powder diffraction experiments were conducted up to 36 GPa at room temperature at the BL04-B2 at Spring-8. Fitting a third-order Birch–Murnaghan equation of state to the data gives a bulk modulus of K0=214 (±2) GPa with K0′=3 (±0.1), a volume V0=184.55(6) Å3 and a zero-pressure density of ρ0=4 g/cm3. When K0′ is fixed at 4, the value of K0=202.3 (±0.9) GPa. These values are comparable to those for similar anhydrous hexagonal aluminous phases reported previously by Ono et al. [Phys. Chem. Minerals 29 (2002a) 527]. The density of NAL phase is lower than that of other minerals (excluding the CF phase) co-existing in the subducted oceanic crust, such as Mg-perovskite, Ca-perovskite and stishovite. We find that the elastic properties of the hexagonal Al-rich phase do not change despite substantial variations in Si, Al, Fe, Mg, Na and K content in the NAL chemical composition.