Nmr-study of Minerals .3. the Distribution of Mg-2+ and Fe-2+ Around the Oh Groups in Micas

For pt.II see Am. Mineral., vol.64, p.119 (1979). The proton NMR absorption lines of eleven micas have been examined. The crystalline samples (phlogopites and biotites with continuously variable amounts of fluorine and iron) are oriented in the applied magnetic field so that the different existing OH-Mg2+-Fe2+ associations (each giving rise to a shifted line) can easily be differentiated. The results are quantified by taking the integrated value of each line and comparing these with expectation values based on a random distribution of elements. It is found that the Fe2+ ions tend to cluster around the OH groups. A complete picture is obtained by taking into account the distribution in the lattice of the fluorine ions. These are found to be preferentially associated with Mg2+. The fluorine content therefore has a net influence on the OH-Fe2 associations which are detected. Infrared spectra run on the same samples tend to support these local fluctuations which exist in the chemical composition of the octahedral sheet of micas.