Microstructures

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Twinning in Feldspar

Click image to enlarge

59. Undeformed twins in plagioclase - Although this plagioclase grain from the Borrego Springs tonalite in southern California is essentially undeformed, there are indications that the twin boundaries have migrated, probably in response to deformation in the immediate vicinity of this sample. Vertical, narrow, albite twin lamellae in the upper half of the image are irregular and appear to have 'propagated' into the broader, subhorizontal twin lamella.

FOV 1.5 mm, Nicols Crossed.

60. Deformation twins in feldspar - Irregular and lensoid twin lamellae are evident along the margins of polygonal feldspar grains from the Ponaganset augen gneiss in the Hope Valley Shear Zone of eastern Connecticut. These deformation twins propagate from high stress sites such as cracks or grain boundaries, into the grain. They do not extend all the way across the grain and often terminate in a zone of undulatory extinction.

FOV 0.8 mm, Nicols Crossed.

Twinning in Feldspar	Click image to enlarge
59. Undeformed twins in plagioclase - Although this plagioclase grain from the Borrego Springs tonalite in southern California is essentially undeformed, there are indications that the twin boundaries have migrated, probably in response to deformation in the immediate vicinity of this sample. Vertical, narrow, albite twin lamellae in the upper half of the image are irregular and appear to have 'propagated' into the broader, subhorizontal twin lamella. FOV 1.5 mm, Nicols Crossed.
60. Deformation twins in feldspar - Irregular and lensoid twin lamellae are evident along the margins of polygonal feldspar grains from the Ponaganset augen gneiss in the Hope Valley Shear Zone of eastern Connecticut. These deformation twins propagate from high stress sites such as cracks or grain boundaries, into the grain. They do not extend all the way across the grain and often terminate in a zone of undulatory extinction. FOV 0.8 mm, Nicols Crossed.