A Raman microprobe study of natural micas

Abstract

AbstractA wide range of natural K-, Na-, Ca- or (K + Li)-micas have been systematically examined by Raman spectrometry. The spectra are interpretable in terms of regular variations in peak positions and chemical parameters. Several vibrations give higher wavenumbers for Na-micas compared to K-micas, in accord with the smaller ionic size of Na+ than K+. The ≈195 cm-1 and ≈270 cm-1 peak wavenumbers and intensities vary as functions of the chemistry of the octahedral sites, i.e. the replacement of Mg2+ by Mn2+, Zn2+, Cr3+, Fe3+, Ti4+, and especially by Al3+, or by a vacancy, and the replacement of (OH)- by F-. The group of ≈700 cm-1 peaks vary in wavenumber and intensity with the replacement of Si by Al in the tetrahedra; distinct Si-O-Si and Si-O-Al vibrations can be recognized. Di- and tri-octahedral micas are distinguished on the basis of certain relative peak intensities which vary considerably with polarization direction, and of trends with increasing Al(iv), Al(vi) or Al(tot.). Calibration of these trends for the chemical analysis of mica microinclusions seems feasible once the uncertainties in the data set are resolved by the determination of further samples selected to highlight the effect of specific elements.