Gitterformen

Gitterformen, in crystallography and mineralogy, are the external shapes of crystals that arise from the arrangement of atoms in a Bravais lattice. A lattice form is defined by a family of lattice planes and the symmetry operations of the crystal, such that the faces of the form lie parallel to those planes and are reproduced on all equivalent positions by the crystal’s symmetry. The term is used to describe the specific sets of crystal faces a crystal can display under favorable growth conditions, independent of chemical composition, though growth rates and environmental factors influence their visibility.

Formation and classification of Gitterformen depend on the lattice parameters a, b, c and the interaxial angles, which determine which crystal faces are compatible with the lattice. Forms are linked to the crystal system (such as cubic, tetragonal, hexagonal, etc.) and to the underlying Bravais lattice type. A form may be holohedral, showing all faces of the form, or hemihedral, revealing only a portion of them due to lower symmetry. The observed habit results from which lattice forms are expressed and which are suppressed during growth.

In practice, lattice forms are described by crystallographic indices and are associated with the 14 Bravais lattices. Common lattice forms include cube- and octahedron-like shapes in the cubic system, as well as prismatic and pyramidal forms in other systems. Gitterformen provide a framework for understanding and classifying crystal shapes, while real crystals may deviate from ideal forms because of impurities, solid solution, or environmental conditions.