Structure of a new Al-rich phase, [K, Na]_0.9[Mg, Fe]₂[Mg, Fe, Al, Si]₆O₁₂, synthesized at 24 GPa

A new Al-rich phase, [K_0.56Na_0.32][Ca_0.04Mg_1.66Fe_0.3²⁺][Mg_0.98Fe_0.3³⁺Al _2.62Si_2.1]O₁₂, was synthesized at 24 GPa, 1700 °C, and its structure was determined by single-crystal X-ray diffraction. The unit cell is hexagonal, space group P6₃/m, a = 8.830(1), c = 2.779(1) Å, V = 187.65(5) ų, Z = 1, formula weight = 448.62, calculated density = 3.970 g/cm³. The previously unknown structure consists of alkali- and vacancy-bearing M3 sites in tunnels along c, mostly Mg-bearing linear arrays of trigonal prismatic M2 sites, very unusual for mantle phases, and a framework of edge-linked distorted octahedral M1 sites filled mostly with Al and Si. The observed range of compositional variations and high density suggest complex solid solution behavior at the lower-mantle pressures similar to pyroxenes at the upper-mantle and majorite garnet at the transition-zone pressures. The stability of the Al-rich phase appears to expand to lower Al contents with increasing pressure, suggesting that a chondritic lower mantle could contain up to 24% of the new phase on the molecular basis.