Microstructures

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Twinning in Calcite

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61. Growth and deformation twins in calcite - Like many samples of calcite, both deformed and undeformed, this vein-fill calcite from the Elbrook Formation in Virginia shows e-twins in many of the grains. Where twins are narrow, straight, and extend all the way across a grain, they may be impossible to distinguish from growth twins. However, twins that are lensoid, irregular in width, or that do not extend all the way across the grain (for example, in the grains at lower right and at left of center), are likely to be deformation twins. The only way to be sure that the twins are deformation-induced, is to measure the poles to e-twin lamellae using a Universal Stage, and plot them on a stereonet. If they are deformation-related, they will show a distinct pattern related to the paleo-maximum stress direction.

FOV 1.5 mm, Nicols Crossed.

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62. Deformation twins in calcite - This sample of coarse-grained, green marble from the Banus Quarry, Cordoba Province, central Argentina shows deformation twinning in large calcite grains. Deformation was at cordierite-sillimanite grade. The twins planes are curved, change their width, and often taper to a point within the grain. Twins also appear to have nucleated at high stress sites, such as the boundaries with other mineral phases (see upper right corner of image). Another part of the same sample is shown in image #63.

FOV 3.2 mm, Nicols Crossed.

Click here to view Flash animation in a new window.

63. Deformation twins and undulatory extinction in calcite - Another part of the same sample as image #62. In addition to the deformation twins in calcite, many of the grains show undulatory extinction, indicative that dislocation glide was also operative. Several of the calcite grains are flattened and elongate, aligned horizontal in the image, and their sutured boundaries suggest that grain boundary migration was also operative (see also image #64).

FOV 3.2 mm, Nicols Crossed.

64. Twin boundary migration in calcite - In this part of the same sample as images #62 and #63, deformation twins, undulatory extinction and grain boundary migration features are visible. In addition, the twins in the large calcite grain across the center of the image have highly irregular boundaries. Twin boundary migration, like grain boundary migration, occurs in calcite deformed at high temperatures and assists the grain in recovering from its high internal strain energy to a lower energy, more stable state.

FOV 3.2 mm, Nicols Crossed.

Twinning in Calcite	Click image to enlarge
61. Growth and deformation twins in calcite - Like many samples of calcite, both deformed and undeformed, this vein-fill calcite from the Elbrook Formation in Virginia shows e-twins in many of the grains. Where twins are narrow, straight, and extend all the way across a grain, they may be impossible to distinguish from growth twins. However, twins that are lensoid, irregular in width, or that do not extend all the way across the grain (for example, in the grains at lower right and at left of center), are likely to be deformation twins. The only way to be sure that the twins are deformation-induced, is to measure the poles to e-twin lamellae using a Universal Stage, and plot them on a stereonet. If they are deformation-related, they will show a distinct pattern related to the paleo-maximum stress direction. FOV 1.5 mm, Nicols Crossed. Click here to view Flash animation in a new window.
62. Deformation twins in calcite - This sample of coarse-grained, green marble from the Banus Quarry, Cordoba Province, central Argentina shows deformation twinning in large calcite grains. Deformation was at cordierite-sillimanite grade. The twins planes are curved, change their width, and often taper to a point within the grain. Twins also appear to have nucleated at high stress sites, such as the boundaries with other mineral phases (see upper right corner of image). Another part of the same sample is shown in image #63. FOV 3.2 mm, Nicols Crossed. Click here to view Flash animation in a new window.
63. Deformation twins and undulatory extinction in calcite - Another part of the same sample as image #62. In addition to the deformation twins in calcite, many of the grains show undulatory extinction, indicative that dislocation glide was also operative. Several of the calcite grains are flattened and elongate, aligned horizontal in the image, and their sutured boundaries suggest that grain boundary migration was also operative (see also image #64). FOV 3.2 mm, Nicols Crossed.
64. Twin boundary migration in calcite - In this part of the same sample as images #62 and #63, deformation twins, undulatory extinction and grain boundary migration features are visible. In addition, the twins in the large calcite grain across the center of the image have highly irregular boundaries. Twin boundary migration, like grain boundary migration, occurs in calcite deformed at high temperatures and assists the grain in recovering from its high internal strain energy to a lower energy, more stable state. FOV 3.2 mm, Nicols Crossed.