Abstract
In “ideal” microcline twinning, the twin axis [010] of the pericline twins is exactly perpendicular to the twin plane (010) of the albite twins (i. e. b is parallel to b *). Small deviations from this law have been found in several microcline perthites; angles of up to 20′ between b and b * have been measured. The causes of this distortion are discussed in connection with the “asymmetric” unmixing of albite in “monoclinic” cryptoperthites.

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