Hexagonal crystal system

An example of the hexagonal crystals, beryl
Hexagonal Hanksite crystal	Hexagonal lattice cell (P)

In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families. They are closely related and often confused with each other, but they are not the same. The hexagonal lattice system consists of just one Bravais lattice type: the hexagonal one. The hexagonal crystal system consists of the 7 point groups such that all their space groups have the hexagonal lattice as underlying lattice. The hexagonal crystal family consists of the 12 point groups such that at least one of their space groups has the hexagonal lattice as underlying lattice, and is the union of the hexagonal crystal system and the trigonal crystal system.

In some cases, it is useful or instructive to redraw a hexagonal structure with orthohexagonal axes, wherein the b axis is redrawn at 90° to the a and c axes.

Graphite is an example of a crystal that crystallizes in the hexagonal crystal system.

Hexagonal lattice system

The hexagonal lattice system is one of the seven lattice systems, consisting of the hexagonal Bravais lattice. It is associated with 45 space groups whose underlying lattice has point group of order 24. It is often confused with the smaller hexagonal crystal system, which consists of the 27 space groups such that all space groups with the same point group are in the hexagonal lattice system, or with the larger hexagonal crystal family, consisting of the 52 space groups in either the hexagonal or rhombohedral lattice systems.

Crystal classes

The point groups (crystal classes) in this crystal system are listed below, followed by their representations in Hermann-Mauguin or international notation and Schoenflies notation, and mineral examples, if they exist.

#	Point group					Example	Space groups
	Class	Intl	Schoen.	Orb.	Cox.
168-173	Hexagonal pyramidal	6	C6	66	[6]+	nepheline, cancrinite	P6, P61, P65, P62, P64, P63
174	Trigonal dipyramidal	6	C3h	3*	[2,3+]	Laurelite and Boric acid	P6
175-176	Hexagonal dipyramidal	6/m	C6h	6*	[2,6+]	apatite, vanadinite	P6/m, P63/m
177-182	Hexagonal trapezohedral	622	D6	226	[2,6]+	kalsilite and high quartz	P622, P6122, P6522, P6222, P6422, P6322
183-186	Dihexagonal pyramidal	6mm	C6v	*66	[6]	greenockite, wurtzite	P6mm, P6cc, P63cm, P63mc
187-190	Ditrigonal dipyramidal	6m2	D3h	*223	[2,3]	benitoite	P6m2, P6c2, P62m, P62c
191-194	Dihexagonal dipyramidal	6/mmm	D6h	*226	[2,6]	beryl	P6/mmm, P6/mcc, P63/mcm, P63/mmc

Hexagonal crystal family

The hexagonal crystal family consists of the 12 point groups such that at least one of their space groups has the hexagonal lattice as underlying lattice, and is the union of the hexagonal crystal system and the trigonal crystal system. There are 50 space groups associated with it, which are exactly those whose Bravais lattice is either hexagonal or rhombohedral.

See also

• Crystal structure
• Close-packing (hcp)
• Wurtzite (crystal structure)