THE LOCATION AND CONTENT OF IRON IN NATURAL B IOTITES AND PHLOGOPITES: A COMPARISON OF SEVERAL METHODS

A B S T R A C T: M/Sssbauer spectra have been obtained at room temperature on a series of phlogopites and biotites which have also been characterized bychemical analyses, I.R. and N.M.R. The distribution of the Fe 2+ ions on the two possible octahedral sites is on the average nearly random. A clear example of tetrahedrally coordinated Fe a + ions in a natural phlogopite is given. This phlogopite has a very high vermiculitization ability. The Fe z + content of the samples has been determined by two different chemical methods and it is shown that even when precautions are taken to minimize oxidation, the agreement with M6ssbauer spectra is poor. During the weathering process of micas, iron plays an important role and therefore information concerning the amount, valence and site occupation of this element is essential. M6ssbauer spectroscopy which directly probes the 57Fe nucleus is of course an extremely effective technique, chemical analysis can supply, besides a certain value for the concentration of these ions, some indirect evidence for the occupation by iron of the various coordination sites, finally I.R. spectroscopy can be useful for probing the com-position and structure of the octahedral layer. Some comparisons between these thre