Difference between revisions of "Trigonal 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'''. | 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 [[ | + | [[Crystals]] are generally [[prism]]atic 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. |
Latest revision as of 17:28, July 15, 2007
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