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

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 P 6 3 / m ) and the composition [(M + ,Ca) 1 (Mg,Fe) 2 ] ∑3 (Al,Si) 5.5–6.0 O 12 , where M = Na + , K + . The alkali-free phase with the complex solid solution, [Ca 0.79 Mg 0.12 ] ∑ 0.91 [Mg] 2.00 [Al 4.09 Si 1.48 ] ∑5.57 □ 0.43 O 12 , has a unit cell with a = 8.765 (3) A, c = 2.762 (3) A, V = 183.7 (2) A 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.