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.
Original language | English |
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Pages (from-to) | 1739-1742 |
Number of pages | 4 |
Journal | American Mineralogist |
Volume | 94 |
Issue number | 11-12 |
DOIs | |
Publication status | Published - Nov 1 2009 |
Keywords
- Al and si coordination
- CAS phase
- High pressure
- NMR spectroscopy
- Si-Al disorder
ASJC Scopus subject areas
- Geophysics
- Geochemistry and Petrology