Twinned Minerals

Twinned minerals can add a fascinating side to ordinary minerals or can add another dimension to already complex minerals. There are several minerals that form classic twins, such as chalcocite, fluorite, sanidine, microcline, staurolite, gypsum, cinnabar, spinel and rutile to name a few. Some twins have colloquial names such as “fairy cross”, “iron cross” and “cogwheel” twins. Twins form as a result of an error during crystallization. Instead of a normal single crystal, twins grow out of or into each other.

Accidental relationships are not considered twins, that is, where two distinct crystals grow more or less randomly side-by-side or toward each other. Twin formation is never random and follows certain defined rules called twin laws, usually named for well-known twins. Spinel law, Albite law, etc.

The twin laws are crystallographic in nature and are caused by flaws in the crystal structure occurring during growth or change in phase. Many minerals form with a stacking sequence. If an error occurs during growth, the twin forms as a mis-positioned sequence, which is repeated as if nothing happened. The crystals grow outward in both directions. Twinning has a dramatic effect on the outward symmetry of the mineral. There are two general types of twin styles, contact and penetration. Contact twins have a composition plane that forms the boundary between them, a mirror plane where the twins look like reflected images or an angled plane resulting in a “bend” to the twin forming dove-tails, fishtails and chevrons. Penetration twins look like whoever made the crystal didn’t know how it was supposed to fit and ended up twin crosses, 3-D stars, and complex structures. Twinning is actually rather common in the mineral kingdom, but perfectly formed twins are not.
(From several bulletins via Washington Mineral Council 9/07)