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 1). 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 as 20Aa in largest dimension, deform the layers of the sheet silicates and apparently dissolve after complete conversion to chlorite has occurred.--Modified journal abstract.