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Raman spectroscopic characterisation of disordered alkali feldspars along the join KAlSi₃O₈–NaAlSi₃O₈: application to natural sanidine and anorthoclase

Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstraße 1, D-37077, Göttingen, Germany

^* Corresponding author, e-mail: burkhard.schmidt{at}geo.uni-goettingen.de

This study reports a calibration of Raman band positions for the determination of the Na/K ratio in disordered alkali feldspars. Crystals along the join sanidine–analbite were synthesised in steps of 10 mol% by ion exchange from a natural sanidine from Volkesfeld and were analysed by electron microprobe. Calibrations were made for the positions of three Raman bands located at 120–142 cm^–1 (band A), 454–461 cm^–1 (band B) and 510–514 cm^–1 (band D) and for the difference between D and A (DA). The calibrations for band A and DA are most suitable for the determination of composition. They show two linear trends with an intersection at about K/(Na+K) = 0.32, which is probably due to the phase transition between the monoclinic and triclinic crystal structures of sanidine and anorthoclase, respectively. For anorthoclase (K/(Na+K) < 0.32) the Raman shift (RS) of band A is related to composition by K/(Na+K) = 4.5077 – 0.0316·RS. For sanidine (K/(Na+K) > 0.32) the correlation K/(Na+K) = 7.3675 – 0.0550 · RS was obtained. Both correlations allow the determination of K/(Na+K) with an accuracy of better than ± 0.04 for the synthetic anorthoclase and sanidine. The application of this calibration to natural samples was tested by analysing a series of alkali feldspars (K/(Na+K) = 0.28 – 0.86) from various volcanic rocks. These data suggest that the calibration can be used to estimate the K/(Na+K) ratio in natural disordered alkali feldspars with an accuracy of better than 0.1. A further improvement can be reached by using DA, for which the correlations K/(Na+K) = –8.3608 + 0.0228 · DA and K/(Na+K) = –21.5327 + 0.0573 · DA were obtained for anorthoclase and sanidine, respectively. These correlations allow the determination of K/(Na+K) with an accuracy of about ± 0.03 for the synthetic anorthoclase and sanidine and ±0.07 for the natural samples.

Key-words: Raman spectroscopy, alkali feldspar, composition.