Abstract

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 P63/m) and the composition [(M+,Ca)1(Mg,Fe)2]∑3 (Al,Si)5.5–6.0O12, where M = Na+, K+. The alkali-free phase with the complex solid solution, [Ca0.79Mg0.12] ∑ 0.91[Mg]2.00 [Al4.09Si1.48] ∑5.570.43O12, has a unit cell with a = 8.765 (3) Å, c = 2.762 (3) Å, V = 183.7 (2) Å3, Z = 1, a formula weight = 429.31, and a calculated density = 3.88 g/cm3 at 0 GPa and 4.16 g/cm3 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.

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