Al-Si ordering in albite : a combined single-crystal X-ray diffraction and Raman spectroscopy study

Raman spectra of a low albite single crystal were collected during annealing at 1076 degrees C for up to 46 days. At seven annealing stages, the single crystal structure was refined by X-ray intensity data to determine the degree of Al-Si order using the tetrahedral bond distances. Single-crystal X-ray diffraction showed that residuals in the difference-Fourier map of the electron density and atomic displacement parameters of Si, O, and, most, Na, increase with Al-Si disorder. The Raman spectra show a significant broadening with disorder, and some slight peak shift. Three strong peaks, at 290 cm(-1) (nu(c)), 478 cm(-1) (nu(b)), and 507 cm(-1) (nu(a)) in ordered albite, were examined in further detail. nu(c) and nu(b) show a red-shift with broadening and Al-Si disorder; nu(a) blue-shifts with disorder and shows only a minor broadening. The broadening and shifts in Raman spectra are caused by structural deformation associated with Al-Si disorder. The nu(a) peak at 507 cm(-1) is the least affected by Al-Si disorder and is suitable to assess compositional changes in plagioclase. The Al-Si order can be determined in albite by the wavenumber difference Delta(ab) between the two main peaks nu(a) and nu(b) as follows: Q(od)(2) = 9.50(75) - 0.307(25)Delta(ab), R-2 = 0.94, where Q(od) is an order parameter derived from average tetrahedral Al-O and Si-O bond distances.