Al-Si ordering in albite: a combined single-crystal XRD and Raman spectroscopy study

Raman spectra of a low albite single crystal were collected during annealing at 1076°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 (νc), 478 cm−1 (νb), and 507 cm−1 (νa) in ordered albite, were examined in further detail. νc and νb show a red‐shift with broadening and Al―Si disorder; νa 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 νa 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 Δab between the two main peaks νa and νb as follows: Qod^2= 9.50(75) − 0.307(25)Δab, R^2=0:94; where Qod is an order parameter derived from average tetrahedral Al―O and Si―O bond distances