CRYSTAL TWINNING IN CRYSTALLOGRAPHY: FLUORITE

A BLOG BY PUFFINS & PIES

Twinning in crystallography does not simply mean two crystals growing together. Rather, two (or sometimes three or even more) crystals grow in accordance with fixed ‘laws’. They are joined along one of the imaginary axes that run through a crystal and, as a result, share some lattice points at the molecular level, determined by the crystal system.

In cubic minerals there are three principal twinning laws. The most common in fluorite is the penetration twin, though fluorite also displays the ‘spinel law twin’. Another well-known penetration twin is the ‘iron cross law twin’, often found in pyrite. This type of twinning is referred to as growth twinning, as it occurs during the primary growth of the crystal.

The precise circumstances that give rise to twin growth are varied, and the true reasons remain something of an enigma. It may occur by chance, or because twin crystals require less energy to form. Factors such as sudden changes in pressure or temperature, the amount of available space, the saturation of element-rich fluids, the growth rate, and the presence of trace foreign elements can all influence the development of twin crystals.