# Arithmetic crystal classes

### From Online Dictionary of Crystallography

Classes cristallines arithmétiques (*Fr.*). Arithmetische Kristallklasse (*Ge*). Clases cristallinas aritméticas(*Sp*). Classi cristalline aritmetiche (*It*)

## Definition

The arithmetic crystal classes are obtained in an elementary fashion by combining the geometric crystal classes and the corrresponding types of Bravais lattices. For instance, in the monoclinic system, there are three geometric crystal classes, 2, *m* and 2/*m*, and two types of Bravais lattices, *P* and *C*. There are therefore six monoclinic arithmetic crystal classes. Their symbols are obtained by juxtaposing the symbol of the geometric class and that of the Bravais lattice, in that order: 2*P*, 2*C*, *mP*, *mC*, 2/*mP*, 2/*mC* (note that in the space group symbol the order is inversed: *P*2, *C*2, etc...). In some cases, the centring vectors of the Bravais lattice and some symmetry elements of the crystal class may or may not be parallel; for instance, in the geometric crystal class *mm* with the Bravais lattice *C*, the centring vector and the two-fold axis may be perpendicular or coplanar, giving rise to two different arithmetic crystal classes, *mm*2*C* and 2*mmC* (or *mm*2*A*, since it is usual to orient the two-fold axis parallel to *c*), respectively. There are 13 two-dimensional arithmetic crystal classes and 73 three-dimensional arithmetic crystal classes that are listed in the attached table. They do not contain glide or screw elements and are therefore in one to one correspondence with the symmorphic groups.

The group-theoretical definition of the arithmetic crystal classes is given in Section 8.2.3 of *International Tables of Crystallography, Volume A*.

## See also

Section 8.2.3 of *International Tables of Crystallography, Volume A*

Sections 1.3.4 and 1.5.3 of *International Tables of Crystallography, Volume B*

Section 1.4 of *International Tables of Crystallography, Volume C*