New insights into the mechanism for chloritization of biotite using polytype analysis

Near-atomic-resolution transmission electron microscopy was used to investi gate the chloritization mechanisms of biotite in a granitic rock and to rev eal polytypic details of the resulting chlorite. Comparison of stacking seq uences in 2M(1) and longer period biotite polytypes with sequences in areas containing chlorite layers revealed that typically, two biotite layers tra nsform to one chlorite layer, losing two potassium interlayer sheets and tw o tetrahedral sheets. In some cases, more than two biotite layers are repla ced by one chlorite layer. Less commonly, a potassium interlayer sheet is r eplaced by a brucite-like sheet. One biotite layer is transformed to one se rpentine layer via loss of a potassium interlayer and a tetrahedral sheet i n places. Based on the relative frequency of the two chlorite layers to one biotite layer vs, one biotite layer to one chlorite layer mechanisms, the net result of chloritization is a considerable volume decrease along c*:. N ear-atomic-resolution images recorded down [010], [310], or [3 (1) over bar 0] revealed that the chlorite polytype in the biotite-chlorite interstrati fications is predominantly IIbb. However mixtures of Ibb, Iab, IIab, and II bb also occur. The "aa" stacking sequences were never found. Chlorite polyt ypes may be determined in part by the chloritization mechanisms and in part by relief of local shear by a/3 displacements.