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

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 $\mathrm{H}–\mathrm{H}$ 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: $$\begin{array}{c}{\mathrm{Al}}_{\mathrm{I}}{\mathrm{:\; |\; e}}^2\text{qQ/h |=10.6\hspace{0.17em}}\mathrm{MHz}\text{; η=0.38; X ‖ c; Z\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em}Y, 30°\hspace{0.17em}}\mathrm{away\; from}\mathrm{\hspace{0.17em}a\hspace{0.17em}}\mathrm{and}\mathrm{\hspace{0.17em}b.}\\ {\mathrm{Al}}_{\mathrm{II}}{\mathrm{:\; |\; e}}^2\text{qQ/h |=5.6\hspace{0.17em}}\mathrm{MHz}\text{; η=0.34; Y ‖ c; Z\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em}X, 20°\hspace{0.17em}}\mathrm{away\; from}\mathrm{\hspace{0.17em}a\hspace{0.17em}}\mathrm{and}\mathrm{\hspace{0.17em}b.}\end{array}$$ The Al_I and Al_II sites correspond to the distorted tetrahedral T₁ site outside the six‐membered ring and T₅ site within the ring in the cordierite structure. The Al_I 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 $\mathrm{Al}–\mathrm{Si}$ order in low cordierite.