Phase relations of the Al₂O₃–SiO₂ system at 13–21 GPa and 2300–2800 K and a new high-pressure Al₂Si₂O₇ phase

Phase relations of the Al₂O₃–SiO₂ system were determined by multi-anvil experiments at pressures of 13–21 GPa and temperatures of 2300–2800 K. We obtained a new high-pressure Al₂Si₂O₇ phase (227-phase), which possesses a triclinic unit cell (Z = 3) with a = 7.0237(12) Å, b = 7.1002(14) Å, c = 6.6729(10) Å, α = 103.466(15)°, β = 99.107(15)°, γ = 60.542(12)°, and V = 281.41(24) ų, in addition to the two new high-pressure Al₂SiO₅ phases reported recently, kyanite II and kyanite III. Because of the formation of the three new phases at temperatures above 2400 K, the phase relations are complicated, in contrast to the previously-known phase assemblage of corundum + stishovite stable at the post-kyanite pressures and temperatures below 2400 K. The three new phases and also kyanite are chemically invariant intermediate phases in the binary system and can be formed by the isothermal reactions, Al₂O₃ (corundum) + SiO₂ (stishovite) = Al₂SiO₅ (kyanite II or III) and Al₂SiO₅ (kyanite, II, or III) + SiO₂ (stishovite) = Al₂Si₂O₇ (227-phase). In comparison to kyanite II and kyanite III, 227-phase is formed at higher temperatures. Crystal structure of 227-phase is based on distorted closest packing of O²⁻ and tetrahedrally- and octahedrally-coordinated cations, similar to those of kyanite and kyanite II. The appearance of 227-phase once again suggests the key effect of temperature for stabilizing dense crystal structures fully made of Al–O and Si–O polyhedra at high pressures.