What is a unit cell in mineralogy ?

Unit cell : definition

The unit cell is the smallest unit of elementary volume of the crystal lattice retaining all the geometric, physical and chemical properties of the crystal ; it contains enough atoms to match the chemical formula of the mineral.

It is generally represented by a parallelogram, the apices of which are formed by nodes. Its translation to infinity in the 3 directions of space gives the crystal lattice. The primitive mesh can be single or multiple. There are 14 possible types of mesh, the 14 Bravais lattices, grouped into 7 crystal systems : cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic.