Chlorite layers interstratified in biotite from altered granite

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Biotite is transformed to chlorite in hydrothermal alteration. Considering the structural difference between the two minerals, one may suspect that chloritization of biotite proceeds by replacing the potassium interlayer in biotite by the hydroxide sheet in chlorite. However, this idea is not realistic because it must accompany considerable volume increase, which seems difficult in a hard rock. Actual chloritization proceeds mainly by removing the tetrahedral sheets along with the potassium interlayer. See Kogure and Bnafield (2000) for the detail.　The image was taken with the incident beam along Xi-axis.
（TEM: JEOL JEM-2010 UHR at 200 kV. Camera: LHESA LH72LL. Sample preparation: ion-milling. ）

Kogure, T. and J. F. Banfield: “New insights into the biotite chloritization mechanism via polytype analysis”, Am. Mineral., 85 (2000) 1202-1208.