Nuclear Magnetic Resonance of 1H and 27Al and Al–Si Order in Low Cordierite, Mg2Al4Si5O18 · nH2O

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Abstract
Proton resonance data in the low-temperature form of cordierite indicate the presence of isolated water molecules in the channel formed of the AlSi5O18 rings, with H–H directions along the crystallographic c axis and with large vibration amplitudes. Two crystallographically nonequivalent aluminum sites have been observed. The quadrupole coupling constant e2qQ/h, the asymmetry parameter η, and the principal axes directions are: AlI: | e2qQ/h |=10.6 MHz; η=0.38; X ‖ c; Z and Y, 30° away from a and b.AlII: | e2qQ/h |=5.6 MHz; η=0.34; Y ‖ c; Z and X, 20° away from a and b.The AlI and AlII sites correspond to the distorted tetrahedral T1 site outside the six-membered ring and T5 site within the ring in the cordierite structure. The AlI NMR data also agree with Fe3+ paramagnetic resonance data, indicating that the Fe3+ ions preferentially occupy the larger T1 sites. The 27Al NMR results indicate a high degree of Al–Si order in low cordierite.

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