CRYSTAL-CHEMISTRY AND MICROSTRUCTURES OF PLUTONIC BIOTITE

Transmission electron microscope and single-crystal X-ray diffraction studies were carried out on biotite from the Valle del Cervo plutonic body (northwestern Italy) with the aim of characterizing their structures and microstructures and also of establishing possible relationships between polytypism and crystal chemistry. The crystals, from the same lithologic complex, do not present significant chemical variations. On the other hand, they contain many different polytypic sequences, such as 1M and 2M1, which represent the most common polytypes, 3T and disordered and long-period stacking sequences, which often coexist in the same crystal. Four-, five-, six-, seven-, eight-, nine-, 13-, 14-, and 21-layer polytypes, often interrupted by stacking faults, were also observed. No relationship was found between polytypic sequences and chemistry, whereas the observed microstructures seem to be related to the petrological environment. The microstructural study facilitated the interpretation of difference-Fourier maps obtained from structure refinements of biotite from the same rock samples. The crystal-chemical study of two biotite samples, 1M (space group C2/m) and 2M1 (space group C2/c), from the same rock sample and with very similar chemical composition did not reveal any substantial differences in the mean bond lengths, whereas the octahedral and tetrahedral distortion parameters are higher in 2M1 than in 1M. Both polytypes exhibit M1 mean bond lengths greater than M2 mean bond lengths, suggesting partial cation ordering, whereas no tetrahedral cation ordering can be detected in the 2M1 polytype.