What is the rhombohedral crystal system ?
Rhombohedral crystal system : definition
The rhombohedral system (or trigonal) is a crystal system which can be described in two ways. Either we describe its unit cell as a rhombohedron whose 6 faces are equal rhombuses, with therefore a = b = c and alpha = beta = gamma, all different from 90°, or we describe it as the hexagonal system (a = b ≠ c) and alpha = beta = 90°, with gamma = 120°). This second way is now preferred for the presentation of crystallographic parameters of minerals. The characteristic element of the system is an axis of symmetry of order 3. This system comprises 5 crystal classes.
The rhombohedral system is that of 10% of mineral species (19% with the hexagonal system), including quartz, calcite and dolomite, tourmalines, hematite, corundum...