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-oceani c ridge basalt under lower mantle conditions was studied by X-ray diffracti on and analytical transmission electron microscopy. The phase, coexisting w ith silicate perovskites, the Ca-ferrite-structure phase, and stishovite. h as 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-f ree 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 f ormula weight = 429.31, and a calculated density = 3.88 g/cm(3) at 0 GPa an d 4.16 g/cm(3) at 23 GPa. This Al-rich phase is considered to be same as th e hexagonal phases recently reported, and thus the hexagonal phases can pot entially host alkali and alkali-earth elements In the lower mantle.