LAYER STACKING MICROSTRUCTURES IN A BIOTITE SINGLE-CRYSTAL - A COMBINED HREM-AEM STUDY

A detailed investigation of the as-grown layer-stacking microstructure s and the attached microcomposition has been carried out by combined K REM-AEM in a Chinese biotite single crystal. 1M, 2M(1), faulted 3T and semi-random 1Mr n(120) coexist as coherent intergrowths. If no strict ly periodic complex polytype is found, we evidence oscillatory stackin g modes as quasi-periodic 1D-modulated structures at 1M-2M(1) and 1M-3 T binary junctions. The transition from order (1M,2M(1)) to disorder i s progressive. Careful study of isolated stacking faults in 1M, 2M(1) and 3T matrices shows that they do not follow the least stacking fault energy requirements. Rather they are embryos of alternate polytypes. Microcompositional data show differences between 1M, 2M(1) and 1 Mr n( 120) structures. Our investigations suggest a significant coupling bet ween the layer-stacking microstructure and the composition. Both of th em seem to be fingerprints of the environmental conditions of growth o f the biotite mica.