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Si-Al distribution in high-pressure CaAl₄Si₂O₁₁ phase: A ²⁹Si and ²⁷Al NMR study

¹ Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193 Japan
² School of Earth and Space Sciences, Peking University, Beijing, 100871, P.R. China

Correspondence: ^* E-mail: xianyu{at}misasa.okayama-u.ac.jp

High-resolution ²⁹Si and ²⁷Al NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl₄Si₂O₁₁ (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The ²⁹Si MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400~1600 °C show two broad, asymmetric peaks near –92.7 and –182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The ²⁷Al MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants.

Key Words: NMR spectroscopy • Al and Si coordination • CAS phase • Si-Al disorder • high pressure