The first crystals to form on heating anorthite glass at temperatures of ~ 1100–1400 °C have substantial Al-Si disorder. This metastable disorder is reduced by prolonged annealing, and the ordering process has been followed by transmission electron microscopy. Ordering initially gives rise to an incommensurate superstructure with diffuse type e reflections in electron diffraction patterns. As a function of annealing time the e reflections change orientation and become sharper and closer together. The measured spacings and orientations of the incommensurate repeat are remarkably similar to those shown by crystals of the ordered intermediate plagioclase feldspar solid solution and depend primarily on the degree of order attained rather than on composition. At longer annealing times, the incommensurate structure gives way to the stable l1 ordered state in an apparently continuous manner; the activation energy for this transition was found to be 135 ± 14 kcal/mol. Type b APDs then coarsen according to the expected rate law
where n is constrained to have values between ~2 and ~2.6, k is between 0.44 × 10−4 Å2/h(for n = 2) and 32.6 Å2.6/h (for n = 2.6), and ΔH* is between 121 and 152 kcal/mol. A qualitative picture of the incommensurate structure as arising from the interaction of two component ordering schemes with C1 and I1 symmetry is outlined. The order parameters for these two component structures could have strong interactions via their associated elastic strains.
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