A TEM study of the biotite-chlorite reaction and comparison with petrologic observations

A biotite that has substantially altered to chlorite in a granitic rock from south-central Maine has been investigated with high-resolution and analytical transmission electron microscopy. The reaction takes place primarily by the replacement of TOT mica layers in the biotite by brucite-like layers (mechanism 2). This mechanism is different from that observed in other studies, in which the brucite-like layers grow into the interlayer planes of mica (mechanism l). The reaction in the present case is accompanied by the formation of micro-precipitates, possibly of quartz or amorphous silica, on the planes of alteration. These precipitates, which can be as small at 20A in hrgest dimension, deform the layers of the sheet silicates and apparently dissolve after complete conversion to chlorite has occurred. The chemical and physical consequences of the two reaction mechanisms noted above can be predicted and are quite different. The changes in chemistry and sheet silicate volume expected from the observed mechanism (2) match very closely the changes inferred by Ferry (1979) on the basis of textural and chemical arguments. The agreement between chemical and volume changes predicted by mechanism I and Ferry's data are poor. The microscopic crystallographic and larger-scale petrologic observations are thus consistent with each other. A