The effect of heat treatment in air at temperatures from 200 °C to 1000 °C of two Cornish biotites has been studied using the Mössbauer effect in 57Fe. 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 Fe²⁺ ions in isolated octahedral sites to give Fe³⁺ in the co-ordination Fe³⁺(O₅OH); oxidation of Fe²⁺ ions in adjacent α-octahedral (cis OH⁻) sites to give Fe³⁺(O₆); and oxidation of Fe²⁺ in adjacent a and b (trans OH^-) sites to give either Fe³⁺(O₅OH) or Fe³⁺(O₆) 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 Fe³⁺(O₅OH) to give more Fe³⁺ in the Fe³⁺(O₆) configuration. Finally, in the temperature range 900 to 1000 °C structural breakdown yielding α-Fe₂O₂ 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.