Description: Pericline law is defined by a composition plane that contains the b axis, with a variable angle with c axis. Sometimes, when the composition plane is parallel to the basal pinacoid (001), the twin show an excess of vertex (acline, figure at left) and other times the twin plane is an irrational plane -the so called "rhombic section"- that is oblique to the pinacoid in a variable angle. Image © Klockmann-Ramdohr

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