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# Rhombohedral crystal system

In crystallography, the rhombohedral (or trigonal) crystal system is one of the seven lattice point groups, named after the two-dimensional rhombus. A crystal system is described by three basis vectors. In the rhombohedral system, the crystal is described by vectors of equal length, of which all three are not mutually orthogonal. The rhombohedral system can be thought of as the cubic system stretched diagonal along a body. a = b = c; $\alpha = \beta = \gamma \neq 90^\circ$. In some classification schemes, the rhombohedral system is grouped into a larger hexagonal system.

There exists only one rhombohedral Bravais lattice.

## List of particulars

The point groups which fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and example crystals.

 name international Schoenflies examples rhombohedral holohedral $\overline{3}m$ D3d calcite, corundum, hematite rhombohedral hemimorphic 3m C3v tourmaline, alunite rhombohedral tetartohedral $\overline{3}$ S6 dolomite, ilmenite trapezohedral 32 D3 quartz, cinnabar rhombohedral tetartohedral 3 C3 none verified