Plagioclase composition by Raman spectroscopy. (8th February 2018)

• Record Type: Journal Article
• Authors: Bersani, Danilo; Aliatis, Irene; Tribaudino, Mario; Mantovani, Luciana; Benisek, Artur; Carpenter, Michael A.; Gatta, Giacomo Diego; Lottici, Pier Paolo
• Is Part Of: Journal of Raman spectroscopy. Volume 49:Number 4(2018)
• Page Start: 684
• Page End: 698
• Publication Date: 2018-02-08
• Subjects: composition -- feldspars -- plagioclase -- Raman spectroscopy; Raman spectroscopy -- Periodicals; 535.846
• DOI: 10.1002/jrs.5340
• Languages: English
• ISSNs: 0377-0486

Abstract

Abstract: Plagioclase undergoes complex exsolution and ordering and phase transition processes during their evolution in nature, and this has hindered attempts to define simple trends relating the major peaks of their Raman spectra with composition. Here, the peak position and linewidth of major Raman features have been calibrated for a set of 20 plagioclases, spanning from albite to anorthite in composition, with symmetry and ordering states that were already well characterized. Point group symmetry is the most important factor determining the Raman peak behaviour with composition, though C 1 ¯, I 1 ¯, and P 1 ¯ plagioclases show different trends for the position of the main peak νa at ~500 cm −1 . Using a simplifying approach, which merges the effect of Al–Si ordering and incommensurate modulations, a method has been developed to estimate the plagioclase composition from calibration of a few determinative Raman peaks. This makes use of the wavenumber difference Δab between the most intense peaks νa and νb around 500 cm −1, the linewidth Гa of the strongest νa peak, and the wavenumber difference Δcb between νc and νb peaks, where νc is a Raman feature at ~560–580 cm −1 . The calibration was completed from data sets composed of spectra from metamorphic to pegmatitic plagioclase. The results were then tested against a further data set, mostly made by volcanic plagioclase. In most samples, the difference between electron micro probe analysis (EMPA) and Raman compositions is less than 5%. Higher residuals (beyond 10%) are observed for intermediate plagioclase, suggesting that some differences in Δab exist between volcanic and metamorphic plagioclase of intermediate compositions. The Raman compositional results for a plagioclase from Marsili submarine volcano agree with composition and zoning found from the analysis by laser ablation. Abstract : Plagioclase is a key mineral in the crust of the Earth and other rocky planets. The peak position and linewidth of major Raman features have been calibrated for a set of 20 plagioclases, spanning from albite to anorthite in composition, with well characterized symmetry and ordering states. A method has been developed to estimate the plagioclase composition from few determinative Raman peaks. The method has been successfully tested on a second set of plagioclases coming from volcanoes, plutonic rocks, and meteorites.