Structure of a new Al-rich phase, [K, Na](0.9)[Mg, Fe](2)[Mg, Fe, Al, Si](6)O-12, synthesized at 24 GPa

A new Al-rich phase, [K0.56Na0.32][Ca0.04Mg1.66Fe0.32+][Mg0.98Fe0.33+Al2.62Si2.1]O-12, was synthesized at 24 GPa, 1700 degrees C, and its structure was determined by single-crystal X-ray diffraction. The unit cell is hexagonal, space group P6(3)/m, a = 8.830(1), c = 2.779(1) Angstrom, V = 187.65(5) Angstrom(3), Z = 1, formula weight = 448.62, calculated density = 3.970 g/cm(3). 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.