Trigonal system

The trigonal (or rhombohedral) crystal system has four axes of symmetry: three, of equal length, lie in a plane at 120°; the fourth axis is longer or shorter and is at right angles to the others. Some authorities class the trigonal system as a sub-set of the hexagonal system.

Crystals are generally prismatic or columnar, with a triangular cross section; characteristic forms are three-sided pyramids, and rhombohedrons. Trigonal crystals maybe described by a hexagonal unit cell, but they lack hexad rotation. They are sometimes called rhombohedral.

Sources

• Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987), pp. 33-34