• journal_title：American Mineralogist
• Contributor：Hiroshi Kojitani ; Takemi Iwabuchi ; Makoto Kobayashi ; Hiroyuki Miura ; Masaki Akaogi
• Publisher：Mineralogical Society of America
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2011.3638
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：96
• issue：8-9
• firstpage：1248
• section：Articles

Two hexagonal aluminous phases, which could serve as potential Na- and K-host minerals in the lower mantle, with compositions K_1.00Mg_2.00Al_4.80Si_1.15O₁₂ and Na_1.04Mg_1.88Al_4.64Si_1.32O₁₂ were synthesized at 22–25 GPa and 1500 °C. The K-rich hexagonal aluminous phase was synthesized for the first time. Crystal structures of both hexagonal aluminous phases were refined using the Rietveld method. Obtained interatomic distances and bond angles were compared to published data on the hexagonal aluminous phase CaMg₂Al₆O₁₂. The general chemical formula of the hexagonal aluminous phase is represented as [M3][M2]₂[M1]₆O₁₂, where the small-, middle-, and large-sized cations occupy the M1, M2, and M3 sites, respectively. Changes of size and shape of M1O₆ octahedra by the substitution of Si⁴⁺ for Al³⁺ in the M1 site make it possible to adjust the size of the M2 and the M3 sites to accommodate Na⁺ and Mg²⁺ in the M2 sites and Na⁺ and K⁺ in the M3 sites, respectively. The stability of hexagonal aluminous phases in a relatively wide compositional range of 30–50 mol% in NaAlSiO₄ component along the NaAlSiO₄-MgAl₂O₄ join can be explained by possible replacement of Mg²⁺ by Na⁺ in the M2 site and by shrinkage and deformation of M1O₆ octahedra with the coupled substitution: ^M2Mg²⁺ + ^M1Al³⁺ → ^M2Na⁺ + ^M1Si⁴⁺.