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
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)