Spectroscopy of Micas

This chapter is concerned with spectroscopic studies of micas which address the origin of their color and pleochroism, the oxidation states and concentrations of cations, site occupancies and other structural details, thermodynamic properties, and broader aspects of the chemical-physics of spectroscopic interactions in general. The field of mica spectroscopy has not previously been reviewed as a whole, although chapters in Lazarev (1972) and Farmer (1974) discuss the infrared spectroscopy of layer silicates as a group. The three primary spectroscopic methods used in this study are Mössbauer absorption, optical absorption, and infrared absorption. Mössbauer spectra measure the absorption of gamma rays by ^(57)Fe nuclei. The details of a spectrum are determined by the crystallographic site and oxidation state of the iron atoms (see review by Bancroft, 1973). Optical spectra in the ultraviolet, visible, and near-infrared spectral regions respond to oxidation state and coordination environment of a variety of metal ions, and can be strongly influenced by interactions with next-nearest neighbors, especially when adjacent ions are in different oxidation states (see review by Burns, 1970). Absorption in the infrared spectral region occurs when local units of a crystal are set in motion by the energy of incident radiation. Electron spin resonance (ESR) spectra and especially nuclear magnetic resonance (NMR) spectra are finding greater use in the study of micas. ESR spectra respond to metal ions which have unpaired electrons and are especially applicable when they are at low concentrations. NMR spectra can monitor structural details of Al, Si, Na, H, and F.