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

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 (Delta DA). The calibrations for band A and Delta DA are most Suitable for the determination of composition. They show two linear trends with all intersection at about K/(Na+K) = 0.32, which is probably due to the phase transition between the monoclinic and triclillic 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 call 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 Delta DA, for which the correlations K/(Na+K) = - 8.3608 + 0.0228 - Delta DA and K/(Na+K) = -21.5327 + 0.0573 - Delta 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.