Potential host phase of aluminum and potassium in the Earth's lower mantle

An Al-rich phase produced by phase transformation from a natural mid-oceanic ridge basalt under lower mantle conditions was studied by X-ray diffraction and analytical transmission electron microscopy. The phase, coexisting with silicate perovskites, the Ca-ferrite-structure phase, and stishovite. has hexagonal symmetry (space group P6(3)/m) and the composition [(M+,Ca)(1)(Mg,Fe)(2)](Sigma) (3)(Al,Si)(5.5-6.0)O-12, where M = Na+, K+. The alkali-free phase with the complex solid solution, [Ca0.79Mg0.12] Sigma (0.91)[Mg](2.00) [Al4.09Si1.48](Sigma5.57) O-0.43(12), has a unit cell with a = 8.765 (3) Angstrom, c = 2.762 (3) Angstrom, V= 183.7 (2) Angstrom (3), Z = 1, a formula weight = 429.31, and a calculated density = 3.88 g/cm(3) at 0 GPa and 4.16 g/cm(3) at 23 GPa. This Al-rich phase is considered to be same as the hexagonal phases recently reported, and thus the hexagonal phases can potentially host alkali and alkali-earth elements In the lower mantle.