krystallform

Krystallform, or crystal form, is a term used in crystallography and mineralogy to describe the external shape a crystal tends to assume as it grows. This shape arises from the arrangement of atoms in the crystal’s internal lattice and from conditions during growth, such as availability of material, space, and temperature. A crystal form represents a geometric set of faces that share a common crystallographic orientation.

In a crystal, faces that belong to the same orientation form a single crystal form. A crystal can display several different forms simultaneously, and some faces may be absent or underdeveloped because growth was inhibited or environmental conditions changed. The concept of crystal form is distinct from crystal habit, which refers to the overall, visible appearance of a natural crystal and can be influenced by external factors beyond the ideal lattice geometry.

Crystal forms are described in relation to the crystal system and symmetry. In practice, forms are associated with specific groups of faces, which in turn relate to Miller indices used to denote crystal faces. Common forms include cube-like faces in cubic systems, hexagonal prisms and pyramids in hexagonal systems, and other polyhedral shapes such as rhombohedra and octahedra in various systems. Euhedral crystals display well-formed forms, while subhedral or anhedral crystals show less developed faces.

Crystal form is a key feature in mineral identification and in studies of growth mechanisms and material properties. Examples include pyrite, which often forms cubes; quartz, which typically shows hexagonal prisms and pyramids; and calcite, known for rhombohedral forms.