| Several projects have been completed related to the deformation of biotite and phlogopite crystals and the crystal structure of the mineral ganophyllite. Crystals of mylonitic biotite were examined by TEM to determine directly the region in which dislocations are present in the basal planes. Experimental TEM images of dislocations were compared to calculated dislocation simulations, and it was found that dislocations occur in either the octahedral or tetrahedral sheet, but not in the interlayer as has been previously assumed.; Crystals of biotite and phlogopite were experimentally deformed to determine their mechanical properties under conditions of pure shear. Modification of an interferometric strain/displacement gauge allowed for the testing of crystals with very low compressive stresses between the basal planes. No obvious correlation could be found between strength and either composition or growth spiral densities, but variations in strength may be related to the presence of small, gas- or fluid-filled inclusions that are ubiquitous in micas. Previous experiments suggested that there may be a relationship between overall strength and F content; although F could not be directly correlated with overall strength in these tests, it did correlate with orientational differences in strength.; Incommensurate modulation was found in selected-area electron diffraction patterns (SAED) of ganophyllite crystals. Although modulation in ganophyllite was previously described, the incommensurate nature was not previously noted. To determine the nature of the incommensurate modulation, an X-ray refinement of the ganophyllite subcell was performed with a substantial increase in precision over previous refinements. Models of incommensurate supercells were created and simulated SAED patterns were calculated. Comparison of experimental and simulated SAED patterns indicates that modulation occurs due to periodic offsets of adjacent, three-fold tetrahedral strips. |
