What is the hexagonal crystal system in mineralogy ?

Hexagonal crystal system : definition

The hexagonal system is a crystalline system whose unit cell is a straight prism with a rhombus base and whose edges make an angle of 120°. The combination of 3 of these meshes forms a hexagonal prism. This system comprises 2 equal crystallographic axes (a = b) forming between them an angle of 120°, the third axis c, is perpendicular to the plane formed by "a" and "b" (a = b = 90°). The characteristic element of the system is an axis of symmetry of order 6. This system comprises 7 crystalline classes. The faces and shape of the hexagonal system are noted with 4 Miller indices and noted (hkil). Some hexagonal meshes are pseudo-orthorhombic because "a" is slightly different from "b".

The hexagonal system represents 10% of mineral species (19% with the rhombohedral system), among which beryl, pyrrhotite, molybdenite, apatite, etc...