Investigation of Al, Si order in K-feldspars using ²⁷Al and ²⁹Si MAS NMR

This paper presents a ²⁷Al and ²⁹Si MAS NMR study of K-feldspars and demonstrates that the spectra are sensitive to variations in the state of Al,Si order. For synthetically annealed samples, the results are in agreement with previous IR spectroscopy and demonstrate that Al,Si rearrangement continues after the samples have become monoclinic as determined by powder XRD. NMR methods provide a significantly improved picture of the state of local Al,Si order in such samples. For triclinic samples, measures of the state of Al,Si order correlate well with site occupancies determined by powder XRD, but for the monoclinic samples the NMR parameters continue to change whereas the XRD parameters do not. Interpretations based on the NMR results for the synthetically disordered samples are consistent with 1-step disordering, as observed by XRD. ²⁷Al and ²⁹Si MAS NMR is likely to be a useful tool for probing the state of local Al,Si order in a wide variety of natural samples. -from Authors