Investigation of short-range Al,Si order in synthetic anorthite by ²⁹Si MAS NMR spectroscopy

Spectroscopic data from ²⁹Si MAS NMR are presented for a series of synthetic anorthite samples crystallized from a glass of the same composition and annealed at 1400°C for times of 1 min to 179 h. The NMR spectra resemble those of natural Al,Si ordered An-rich feldspars but contain peaks at more negative chemical shifts, indicating the presence of Al-O-Al linkages. The short-range order parameter, σ, varies linearly with the square of the macroscopic order parameter determined by X-ray diffraction measurements on the same samples. The correlation of σ with Q_od implies that An₁₀₀ with no long-range order (C1̄) would still be about 65% short-range ordered. Comparison with calorimetric data for similar synthetic anorthite samples yields an enthalpy of -39⊥12 kJ per mole of Al-Si exchanges in the reaction. -from Authors