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

A detailed investigation of the as-grown layer-stacking microstructures and the attached microcomposition has been carried out by combined KREM-AEM in a Chinese biotite single crystal. 1M, 2M(1), faulted 3T and semi-random 1Mr n(120) coexist as coherent intergrowths. If no strictly periodic complex polytype is found, we evidence oscillatory stacking modes as quasi-periodic 1D-modulated structures at 1M-2M(1) and 1M-3T binary junctions. The transition from order (1M,2M(1)) to disorder is 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 between the layer-stacking microstructure and the composition. Both of them seem to be fingerprints of the environmental conditions of growth of the biotite mica.