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

Nobuyoshi Miyajima^1,*, Takehiko Yagi¹, Kei Hirose², Tadashi Kondo^1,, Kiyoshi Fujino³ and Hiroyuki Miura³

¹ Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa 277-8581, Japan
² Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo 152-8551, Japan
³ Division of Earth and Planetary Sciences, Graduate school of Science, Hokkaido University, Kita-10 Nishi-8, Sapporo 060-0810, Japan

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, [Ca_0.79Mg_0.12] _0.91[Mg]_2.00 [Al_4.09Si_1.48] _5.570.43O₁₂, has a unit cell with a = 8.765 (3) Å, c = 2.762 (3) Å, V = 183.7 (2) ų, 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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