ZrO 2 -EMF measurements were carried out on natural biotite samples between about 400 and 1100 degrees C in a closed system in a pure argon atmosphere (99.9999%). The EMF patterns are complex and cannot be compared to classical equilibrium 1/T-log f O2 data. Instead, the electrochemical method, when applied to hydrous phases, is a non-equilibrium, dynamic technique, comparable to differential thermal analyses. The data are interpreted from the perspective of atomic processes that control EMF readings and in light of mica-breakdown reactions known from the literature. These breakdown reactions occur at mainly three temperatures marking the onset of dehydroxylation, of oxidation, and of breakdown to oxides. The new application of ZrO 2 -EMF measurements proposed herein demonstrates the advantage of dynamic tracking of mica breakdown reactions; so that in a single experiment, all breakdown reactions may be studied sequentially for a specific mineral sample.

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