Plagioclase composition by Raman spectroscopy

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 true1¯, I true1¯, and P true1¯ 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.

Al—Si ordering in albite: A combined single‐crystal X‐ray diffraction and Raman spectroscopy study

“…The strongest peaks in plagioclase are found at 478 and 507 cm −1 ( ν b and ν a ). The ν a peak is modified less by changing Al—Si order, but more with composition . It is therefore most suitable to assess the compositional changes in plagioclase.…”

“…The ν a peak is modified less by changing Al-Si order, but more with composition. [6,28] It is therefore most suitable to assess the compositional changes in plagioclase. On the other hand, the peaks ν c and ν b are most suitable to determine the degree of Al-Si order in a given feldspar.…”

“…Ordering and disordering processes have the potential to constrain the thermal history of a feldspar‐bearing rock, but the need of crystallographic characterization of the mineral hindered the application of Al—Si order as a petrologic tool. Recent advances in Raman spectroscopy in the calibration of peak position‐linewidth relations with composition, and in the theoretical description of the vibrational modes, now can provide the basis to determine the degree of order of feldspars by Raman methods . Raman investigation is quick and simple, it can be done on the same thin section used for optical and Scanning Electron Microscopy with Energy Dispersive X‐ray Spectroscopy (SEM‐EDS) investigations, and, if calibrated, could allow rapid determination of the degree of Al—Si order.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…Bersani and co‐workers reported plagioclase composition by Raman spectroscopy. They found that the Raman compositional results for a plagioclase from Marsili submarine volcano agree with composition and zoning found from analysis by laser ablation . Dufresne et al measured the Raman spectra of the eight natural carbonate minerals of calcite structure and analyzed the results using a model based on a graphical approach .…”

Structural insights of hierarchically engineered feldspars by confocal Raman microscopy

“…By means of this approximation, the deviation of the measurements in intermediate plagioclases is estimated to be up to 10%. To determine more accurately the composition of the sintered glass–ceramic, the peaks width and positions of the Raman spectra are compared with some plagioclases with different content of anorthite reported in the literature . Thus, from a mineralogical point of view, the composition of the fast‐sintered glass–ceramic is placed between an oligoclase (~An25) and an andesine (~An45) .…”