A Theoretical Investigation of Deformation Mechanisms in Calcite

A theoretical model for the plastic deformation of calcite, based on symmetry arguments and dislocation theory, is presented. Flow mechanisms suggested by the model conflict with those previously determined by Turner. Predictions from the model include translation gliding on e., ri, f and mI parallel to <110 ( i = 1,2,3; j = 1,2,3,4 ), and twin gliding on r (+) and (-) senses; s = 2.065, 0.630 respectively parallel to <110) , and twin gliding on f. (-) sense parallel to both C110> (s' 2 ) and. No conflict arises for e. twinning (+) sense parallel to 100 ( s = 0.695 ) in either model. Reevaluation of previously deformed calcite crystals ( including those compresses parallel to (11 = (0001) confirms the theoretical prediction of twin gliding on r. ( both senses ) and f. Experimental data interpreted as confirming translation gliding according to the Turner model may be explained by the theoretical model. Examination of the calcite structure permits the prediction that the dissociation of complete dislocations in the (11 = [2110 ( b = 4.99A ) directions into extended dislocations along the [3212] directions for the f. planes and the [121] directions for the r. planes is both structurally and energetically favorable. As dissociation occurs, the resulting particals move apart leaving behind them a sheet of unsymmetrical stacking fault with the resulting abnormalities being confined to next-nearest neighbors only. Use of this dissociation scheme leads to the development of a dislocation theory of mechanical twinning in calcite. The relative ease of e. twinning over the other potential twin systems as well as the polar character of each of the twinning systems can easily be predicted with this theory. Recent etch figure studies of deformed calcite crystals are summarized and evaluated in light of the theoretical model. Such studies tend to confirm the predictions of the theoretical model.