Exploring Rocks: Twinning

Crystal twins cause distinctive striped and crisscrossed patterns found in some minerals. Twinning occurs when two or more parts of a crystal grow together in a specific geometric relationship around either a shared axis or a shared plane. A twin axis is a common line of rotation around which one part of a crystal can be brought into the orientation of its twin. A twin plane is a plane of reflection across which twinned crystals appear as mirror images.


The striped twinning, characteristic of plagioclase, helps distinguish it from other members of the feldspar group. The thin lines separating the blue and yellow parts of this crystal mark planes of mirror symmetry. To better understand this type of twinning, put your palms together as if to pray. You create a plane of symmetry where your hands touch; one hand is the mirror image of the other. Picture the symmetry you see in plagioclase as many praying hands, side by side, with all the left hands painted blue and all the right hands painted yellow.


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Anorthosite, Piscataquis County, Maine
Crossed polarizers with gypsum plate

At lower magnification, many twinned plagioclase grains are visible. Each color reflects a different orientation of the plagioclase atomic structure. Multiple twins repeat the colors, because they repeat the structure.


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Anorthosite, Piscataquis County, Maine
Crossed polarizers with gypsum plate

For a more complex type of twinning see Scapolite in Microcline.

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