Types of Twinning in Minerals

Types of Twinning in Minerals. A rutile aggregate with a lot of twins from Simpelveld The Netherlands. Photo: © Fred Kruijen

Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations.

Twinned minerals have their own following in the mineral collecting hobby. There are collectors that are only interested in twinned minerals. Twins can add a fascinating side to otherwise boring minerals or can add yet another dimension to an already complex mineral such as calcite. There are several minerals that form classic twins such as chalcocite, fluorite, sanidine, microcline, harmotome, staurolite, gypsum, cinnabar, spinel and rutile to name a few. Some twins have been given colloquial names such as the "fairy cross", "iron cross" and "cog wheel" twins.

Twins form as a result of an error during crystallization. Instead of a "normal" single crystal, twins will often appear doubled where two crystals appear to be growing out of or into each other, like Siamese twins. Some twins however are not even identifiable outwardly and some minerals in fact have been found to be just a twinned variety of another mineral. Accidental relationships are not considered twins, such as when two distinct crystals grow more or less randomly side-by-side or toward each other, etc. Also epitaxial overgrowths are also not twins. These occur when one mineral of similar structure, but different chemistry, grows onto and "continues" the earlier mineral's crystal. A twin's formation is never random and follows certain defined rules called twin laws. Many types of twin laws are given their own unique names and some are well known, such as the Spinel Law or the Albite Law.

A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.

Twinning can often be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern.

Pyrite Twin

Types of twinning

Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. An example is metazeunerite. In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite, pyrite, and fluorite often show penetration twinning.

If these multiple twins are aligned in parallel they are called polysynthetic twins. When the multiple twins are not parallel they are cyclic twins. Albite, calcite, and pyrite often show polysynthetic twinning. Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern.

Twinned crystals (e.g., see Figure ) may be described as follows:

• Simple twins – composed of only two parts.
• Multiple twins – composed of more than two orientations.
• Contact twins – this occur if a definite composition plane is present.
• Penetration twins – occur if two or more parts of a crystal appear to interpenetrate each other with the surface between the parts being indefinable and irregular..
• Polysynthetic twinning – occurs when three or more individuals are repeated alternately on the same twinned plane. If the individuals of polysynthetic twins are thin plates, the twinning is called lamellar e.g. plagioclase feldspars.

Rare Spinel cyclic twin from Pyin-Oo-Lwin District, Mandalay Division, Myanmar. Photo:: by a great mineral photographer.

Modes of formation