Al,Si ordering in cordierite using “magic angle spinning” NMR |
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| Authors: | Andrew Putnis Ross J Angel |
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| Institution: | 1. Department of Earth Sciences, University of Cambridge, CB2 3EQ, Cambridge, England
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| Abstract: | A method of determining the number of Al-O-Al bonds per unit cell from 29Si nuclear magnetic resonance (NMR) data of synthetic cordierites with increasing Si, Al order is described. The number of Al-O-Al bonds is found to vary linearly with the logarithm of the annealing time. This may be correlated with previously published heat of solution data on similar samples (Carpenter et al. 1983) to determine the enthalpy change Δh, associated with a single Al?Si interchange in cordierite. Δh is found to be 8.1 kcal/mole. The NMR data show that the short range Al, Si order cannot be described in terms of twin domains of ordered orthorhombic cordierite. An ordering model derived from group theoretical constraints on possible Al, Si distributions within the hexagonal symmetry of the cordierite is found to provide a better fit to the NMR data. |
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