A Mössbauer study of thermal decomposition of biotites

Summary The effect of heat treatment in air at temperatures from 200°C to 1000°C of two Cornish biotites has been studied using the M6ssbauer effect in 57 Fe. One of the micas was also studied after heating in vacuo . Progressive changes in the Mössbauer 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 Fe 3+ (O s OH); oxidation of Fe 2+ ions in adjacent a-octahedral (cis OH − ) sites to give Fe 3+ (O 6 ); and oxidation of Fe z+ in adjacent a and b (trans OH − ) sites to give either Fe 3+ (O 5 OH) or Fe 3+ (O 6 ) depending on whether a shared hydroxyl group remains intact or otherwise (these processes occur at temperatures below 500°C Also, at higher temperatures, dehydroxylation of Fe 3+ (O 5 OH) to give more Fe 3+ in the Fe 3+ (O 6 ) configuration. Finally, in the temperature range 900 to 1000°C structural breakdown yielding α-Fe 2 O 3 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.