Abstract--An electron-microscopic and MiSssbauer spectroscopic study of a range of kaolinites has revealed three distinct types of iron contamination within these minerals: a) Ferric ion may substitute for aluminium and be evenly distributed throughout the lattice, b) Ferric ion may be present as a crystalline coating of goethite, as indicated by lattice-imaging studies, or c) as an amorphous coating. The distribution of the iron in the groups b and c is non-uniform and is highest at the flake surfaces. Ferrous ion, when detected, is thought to be evenly distributed throughout the lattice. The size of the contaminating goethite crystaUites and the observed M6ssbauer spectra of these samples suggest that such particles are super-paramagnetic. All kaolinites can be cleaned by acid treatment except those having iron substituting for AI 3+.

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The technique of 67Fe conversion-electron Mossbauer spectroscopy is shown to be a viable method for the study of the early stages of solid-state reactions which start in regions close to solid surfaces. In particular, increases in the Fe3+ : Fe2+ ratio can be detected in a region confined t o within 300 nm of the outer surfaces of a biotite flake which has been subjected to mild oxidation in air. These changes would have gone undetected if conventional transmission Mossbauer methods had been used alone. The reactionswhich occur in the surface regions of biotites during heat treatment in air are discussed. THE use of 57FeMossbauer spectroscopy in the study of minerals and solid-state reactions is now well estab-1ished.l-3 Most Mossbauer spectra are accumulated by monitoring the y-radiation transmitted through a ' thin ' specimen and in this way information relating to bulk properties of the material under examination is obtained. In situations where the process of sample preparation and consequent alteration or destruction of the specimen is impracticable, ' thick ' iron-containing specimens can be studied in a reflection geometry by monitoring the back-scattered Xor y-ray photons which result from the decay of the I = $ nuclear spin state of 57Fe in the sample. This technique has been applied to mineral specimens including lunar samples 4-6 and in the study of corrosion products in sita.7 More attractive to the surface chemist is the possibility of increasing the surface sensitivity of 57Fe Mossbauer spectroscopy by counting the back-scattered conversion and Auger electrons.8-10 In this way the depth probed is liiiiited to ca. 300 nm.9911 The sensitivity of the method is such that (using existing equipment) ca. 80 A of natural iron (ca. 2% 57Fe) can be detected and, with

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