Hexagonal system
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The hexagonal (or trigonal) 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.
Crystals are generally prismatic or columnar, with a rounded triangular or hexagonal cross section; characteristic forms are three- or six-sided prisms, pyramids, and rhombohedrons. The very similar trigonal system is often included within the hexagonal system.
Sources
- Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987), pp. 33-34