Growth defects in metamorphic biotite |
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Authors: | Ian A. Bell C. J. L. Wilson |
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Affiliation: | 1. Department of Physics, Monash University, 3168, Clayton, Victoria, Australia 2. School of Earth Sciences, University of Melbourne, 3052, Parkville, Victoria, Australia
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Abstract: | Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R 1=±1/3 [010], R 2=±1/6 [310] or R 3=±1/6 [3 (bar 1) 0]. Structural considerations indicate that a stacking fault R 1, R 2 or R 3 is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults. |
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