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A number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band. The results show systematic differences some of which are related to the crystallinity of the material, some to the presence of transition metal ions other than Fe3+ and hole-trapping defects.The results provide evidence for at least three sites for substitution of Fe3+ having large crystal fields : (i) a site with near maximum rhombic character having crystal field parameters D > 1·2, λ = ⅓ the occurrence of which is correlated with lack of crystalline perfection due to stacking defects or to the proximity of surfaces, (ii) a site with less rhombic character with λ = 0·234, D = 0·585 as likely parameters, and (iii) a site with parameters near the values λ = 0·207, D = 0·322.Mössbauer and ESR evidence suggests that the principal sites of Fe3+ substitution are octahedral. The presence of Fe3+ in adjacent cation sites leads to a very broad resonance centred near geff = 2. Other resonances in the spectra are attributed to the effect of trapped holes, some at least of which are situated at oxygen sites adjacent to Al3+ ions substituting in sites normally occupied by Si4+.Hyperfine effects due to the presence of the transition ions Mn2+ and (VO)2+ are also observed.

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SUMMARY. The effect of heat treatment in air at temperatures from 200 ~ to iooo ~ of two Cornish biotites has been studied using the M6ssbauer effect in 57Fe. One of the micas was also studied after heating in vacuo.Progressive changes in the M6ssbauer spectra of the materials after heat treatment can be correlated with the following processes: Oxidation of Fe 2+ ions in isolated octahedral sites to give Fe 3+ in the co-ordination Fe3+(OsOH); oxidation of Fe ~+ ions in adjacent a-octahedral (cis OH-) sites to give FeS+(O~); and oxidation of Fe z+ in adjacent a and b (trans OH-) sites to give either Fe3+(O~OH) or Fe3+(O6) depending on whether a shared hydroxyl group remains intact or otherwise (these processes occur at temperatures below 5oo ~ Also, at higher temperatures, dehydroxylation of Fe3+(O~OH) to give more Fe 3+ in the Fez+(Os) configuration. Finally, in the temperature range 9o0 to IOOO ~ structural breakdown yielding a-Fe~Oz as the iron-containing phase.The results and interpretation are in good agreement with thermogravimetric data and with a previous infra-red study. The mechanism of charge diffusion in the lattice during oxidation is discussed in the light of the results.ALTHOUGH much work has beend one on the thermal decomposition of hydrous silicates in general, there has been little published on biotites in particular, most of the mica work having been confined to muscovite. Brindley (1963) has summarized much of the work prior to that date and has emphasized the importance of X-ray techniques in elucidating the broader outlines of the various changes, but the more detailed mechanisms require other techniques for their elucidation. Hodgson, Freeman, and Taylor (I965a) have studied the thermal decomposition of crocidolite and amosite (I965b) by differential thermal analysis and thermogravimetry, and have interpreted their results in terms of the original detailed investigation of amphiboles by Addison et al. (1962). Gibb and Greenwood (I965) have also investigated these two amphiboles using the MSssbauer effect and their results generally support the results of previous workers. Vedder and Wilkins (1969) have studied the thermal decomposition of muscovites and biotites by infra-red spectroscopy, and the interpretation of the results to be presented here has been made along the lines of their arguments.A M~Sssbauer study of high-temperature reactions in kaolinite and halloysite has been undertaken by MacKenzie (1969) and M~Sssbauer work on oxidation of biotite by weathering has been published by Rice and Williams (I969).

Partially oxidized tin(I1) niobate and tantalate ~jyrochlores are homogeneous cubic phases. They have been shown by Mossbauer '19Sn spectroscopy to contain both Sn(I1) and Sn(1V). Each valency state is segregated on its own, crystallographically distinct sublattice as a consequence of the size disparity of Sn2+ and Sn4+ atoms. The strong temperature dependence of the Mossbauer Sn2+:Sn4+ area ratios between room temperature and 4.2"K indicates that the recoil-free fractions of the SnZ+ and Sn4+ atoms in these structures are noticeably different. The values of the recoil-free fractions at 0 OK, which were estimated from simple Einstein and Debye models, were used to obtain Snz+:Sn4+ population ratios from the Mossbauer area ratios extrapolated to 0 OK. The area ratios were shown to differ from the population ratios by 15-20% at 0 OK and by as much as 70% at room temperature. The formulae of the two nonstoichiometric pyrochlores based on the population ratios are Snz-2.Z+(Sn,4+Bz-,5+)- Can. J . Chem.. 51, 1041 (19731 Previously we reported the preparation of cubic pyrochlores of the intended compositions Snz2+-N b z 5 + 0 , and Sn,2+Ta,5t0, (1).