Cation distribution in some amphiboles by spectroscopic methods

A new spectroscopic study of the orthorhombic amphiboles has been undertaken, using Mandouml;ssbauer and infrared spectroscopic techniques to determine site occupancies. The crystal structure of amphiboles is reviewed, and a new approach to the structures of chain silicates is outlined. Hydroxyl spectra of the specimens are of better quality than in the previous study of the orthoamphiboles (Law, 1969/70) and this allows a hyperfine structure to be seen in spectra of anthophyllites. Spectra are generally more difficult to interpret qualitatively than are spectra of related minerals, because of their complexity or because of low absorption intensities which characterise gedrite spectra. The development of a computer model for investigating the relationship between site occupancies and peak intensities allows, in principle, separate site populations for M1 and M3 sites to be determined from hydroxyl spectra. The Mandouml;ssbauer spectra, particularly of aluminium-rich gedrites, are too complex to be resolved completely, and this limits the accuracy of the site population information which can be obtained. Site occupancies are presented for the orthoamphiboles, however. A distinction is observed between high- and low-aluminium gedrites, and this empirical distinction is useful in discussing the properties of these amphiboles.