Abstract

Studies of dislocation microstructures are being carried out on (I) experimentally deformed calcite crystals, (2) experimentally deformed limestones and (3) carbonates from the Central Alps deformed at different metamorphic grades. The aims of the work are to study deformation mechanisms in carbonates, the calcite/aragonite transformation, and to investigate sub-microscopic features in relationship to metamorphic grade.

Normal twinning (e) generally produces numerous dislocations and twin intersection generates very high local dislocation densities. Twinning in the cleavage plane(r) is also found and gliding on this plane is very disruptive at low temperatures producing small mosaic blocks by microcracking. The results of a limited study of limestones from the Bergell Alps show a good correlation between dislocation structures and supposed metamorphic grade and can be compared with a study of quartzites from the same region (Liddell et al., 1974).

Differences in the behaviour of dislocations in calcite and dolomite are apparent. Dislocations in recovered dolomite tend to be oriented in crystallographic directions, indicating directions of minimum line energy in the ordered structure. Effects of disorder and non-stoichiometry are seen in some grains, which contain lamellar features and branching arrays of faults, as yet unidentified.

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