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.

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