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 Anrich feldspars but contain peaks at more negative chemical shifts, indicating the presence of Al-O-Al linkages. We describe a simple method to determine the average number of Al-O-Al linkages per formula unit (NAl-Al) based on the first moment of the 29Si NMR spectrum. For the synthetic An100 samples, NAl-Al ranges from 0.40 after 1 min at 1400°C to 0.19 after 179 h, corresponding to large degrees of short-range order, σ = 0.80–0.91. 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 Qod implies that An100 with no long-range order (C1) would still be about 65% short-range ordered. The values of NAl-Al also correlate well with the inverse of the spacing of type b and type e antiphase boundaries, indicating that much of the change in the number of Al-O-Al linkages with short annealing time is related to the reduction in total surface area of the antiphase domains.

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
Si-O-Si + Al-O-Al 2(Si-O-Al).
This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.