text: Simple twinned crystals may be contact twins or penetration twins. Contact twins meet on a single composition plane, often appearing as mirror images across the boundary. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. An example is metazeunerite. Contact twinning characteristically creates reentrant faces where faces of the crystal segments meet on the contact plane at an angle greater than 180°. 
  A type of twinning involving 180° relationships is called hemitropism or hemitropy.
  In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite, pyrite, and fluorite often show penetration twinning. The composition surface in penetration twins is usually irregular and extends to the center of the crystal.
  Contact twinning can arise from either reflection or rotation, whereas penetration twinning is usually produced by rotation.
  If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called polysynthetic twins. When the multiple twins are not parallel they are cyclic twins. Albite, calcite, and pyrite often show polysynthetic twinning. Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern.
  For rotational twinning the relationship between the twin axis and twin plane falls into one of three types:
  :#parallel twinning, when the twin axis and compositional plane lie parallel to each other, 
  :#normal twinning, when the twin plane and compositional plane lie normally, and 
  :#complex twinning, a combination of parallel twinning and normal twinning on one compositional plane.