Original paper
Raman spectroscopic characterisation of disordered alkali feldspars along the join KAlSi3O8NaAlSi3O8: application to natural sanidine and anorthoclase
Bendel, Verena; Schmidt, Burkhard C.
European Journal of Mineralogy Volume 20 Number 6 (2008), p. 1055 - 1065
published: Dec 15, 2008
Abstract
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 (ΔDA). The calibrations for band A and ΔDA 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 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 ΔDA, for which the correlations K/(Na+K) = -8.3608 + 0.0228 ˙ ΔDA and K/(Na+K) = -21.5327 + 0.0573 ˙ ΔDA 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.