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Line 1: − #REDIRECT [[Hexagonal system]]+ + + + + + + + + +
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The '''trigonal''' (or '''rhombohedral''') [[crystal system]] has four [[Axis|axes]] of [[symmetry]]: three, of equal length, lie in a plane at 120°; the fourth axis is longer or shorter and is at [[right angle]]s 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 [[triangle|triangular]] cross section; characteristic forms are three-sided [[pyramid]]s, and [[rhombohedron]]s. 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''
[[Category:Crystallography]]