Calcite twinning: the four classes

Calcite (Calcium carbonate) is a very common mineral, but fascinating in the vast variety of crystal forms it assumes, including some beautiful symmetries produced by the phenomenon of crystal twinning.

The crystals are trigonal, meaning that there is a single axis of three-fold rotational symmetry (the c-axis) as well as three equal axes perpendicular to the c-axis. They occur in a myriad of forms, from squat rhombohedra to flat-ended prisms to elongate pointy scalenohedra. Some crystals display very large numbers of faces.

Here is an example of an untwinned calcite crystal, 67 mm tall from the Rotherhope Fell Mine, Alston Moor, Cumbria, England (collected in 1922).

The complex shapes produced by twinning have long been admired and here is a page from a famous mineralogy textbook by Edward Dana (6th Edition of "The System of Mineralogy" 1904). The first edition of this work was produced by James Dwight Dana in 1837.

Then, as now, four distinct classes of calcite twin were recognized, depending on the orientation of the c-axis of each member of the crystal twin relative to the other and the orientation of the twin contact plane.

Here is a sample of four calcite crystals to illustrate the four classes of calcite twinning.

The top crystal is 3 cm tall, comes from the Meikle Mine, Nevada, and has a twin plane across which the crystal form is reflected. The c-axes of each member of the twin are aligned and the formal notation of the twin plane is {00.1}, meaning that the plane is perpendicular to the c-axis.

The gem transparent crystal shown to the right comes from the Whuzhou Mine of Guangxi, China. The c-axes of each member of the twin are inclined at 127 degrees 30 minutes and the formal notation of the twin plane is {01.8}.

The crystal shown to the left comes from the Leiping Mine of Hunan, China. The c-axes of each member of the twin are inclined 90 degrees 46 minutes and the formal notation of the twin plane is {10.4}.

The crystal shown at the bottom comes from the Torr Works Quarry, Somerset, England. The c-axes of each member of the twin are inclined at 53 degrees 46 minutes and the formal notation of the twin plane is {01.2}. This is the rarest of the four classes of twinning in calcite.

Expression of other crystal faces than seen in these four examples (prisms, rhombohedra, scalenohedra) can produce strikingly different looking crystal twins whilst sharing the exact same symmetry relations.

Here, for example is another example of the {01.8} twin form. It is 3 cm tall and comes from Dalnegorsk, Russia. The resemblance to the Whuzhou specimen shown above is quite difficult to detect!

These twins, from Elmwood, Tennessee, USA represent the first class, with twin plane perpendicular to c-axis. The shapes differ due to different faces developing at different rates during crystal growth.

Many other examples of calcite twin forms can be found here.