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

Abstract

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