Rhombohedral
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In crystallography, the rhombohedral (or trigonal) crystal system is one of the 7 lattice point groups. A crystal system is described by three basis vectors. In the rhombohedral system, the crystal is described by vectors of equal length, all three of which are not mutually orthogonal. The rhombohedral system can be thought of as the cubic system stretched along a body diagonal.
In some classification systems, the rhombohedral system is grouped into a larger hexagonal system.
There exists only one rhombohedral Bravais lattice.
The point groups that fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and example crystals.
| name | international | Schoenflies | example |
| rhombohedral holohedral | <math>\overline{3}\frac2m<math> | D3d | calcite |
| rhombohedral hemimorphic | <math>3m<math> | C3v | tourmaline |
| rhombohedral tetartohedral | <math>\overline{3}<math> | S6 | dolomite |
| trapezohedral | 32 | D3 | quartz |
| rhombohedral tetartohedral | <math>3<math> | C3 |