Crystal Twinning

Examining The Types of Twin Forms

- BOB JONES

Crystal twinning is a fascinating and common phenomenon in mineral specimens, often enhancing both their visual appeal and scientific interest.

By and large, we collect and display specimens dominated by groups of a particular mineral species, sometimes accompanied by other species. Such clusters of minerals are called crystal aggregates. Under the right circumstances and within those aggregate mineral groups are three general forms of crystals that exhibit twinning.

Crystals can twin in parallel or routine ways involving two crystals, or as in complex twinning, several crystals. To be considered twins, the crystals must be in a fixed identifiable relationship to each other.

Doubly-terminated, twinned, greenish-yellow titanite crystal from the famed locality at Capelinha, Brazil.

HOW TWINS FORM

As crystals grow in solution, individual ions gather to form a crystal. This is an orderly geometric process that produces a geometric pattern that results in an external crystal form that we recognize. Several factors influence how crystals grow, including the direction of the flow of the solution. This brings ions in contact with the growing crystal from a path that can influence growth. It causes ions to accumulate first on the side that receives the flow and can affect the orderly process of crystallization.

CONTACT TWINS

Sometimes, one ion may not quite fit right, appearing slightly offset. This doesn't disturb the lattice structure but can affect later arriving ions. Especially under rapid crystal growth, the next arriving ions attach to the slightly offset ion. This results in a second crystal that grows in a different direction determined by the orderly rules of crystal growth.