Rhomboedriske

Rhomboedriske, or rhombohedral in English, describes both a geometric form and a crystallographic category associated with rhombohedra. Geometrically, a rhombohedron is a six-faced polyhedron with congruent rhombic faces. Opposite faces are parallel, and all edges have the same length. The dihedral angles are generally not 90 degrees; a cube is a special case where the rhombohedron’s faces are squares and the angles are right angles.

In crystallography, rhombohedral refers to crystals that can be described by a rhombohedral lattice or, more broadly, by trigonal symmetry. The term is used in two senses: as a lattice type with equal edges and non-right interaxial angles, and as a description of crystals that display threefold rotational symmetry at a principal axis. In practice, some minerals that appear rhombohedral belong to the trigonal crystal system and are commonly described using rhombohedral forms such as rhombohedra.

Calcite is the most familiar mineral with a rhombohedral form; its crystals commonly grow as rhombohedra with rhombic faces. Other minerals in the trigonal system may exhibit rhombohedral habits or rhombohedral cleavage. The term also appears in broader crystallographic discussions to distinguish a lattice cell or symmetry that favors a rhombohedral geometry from purely cubic or orthogonal systems.

A rhombohedral geometry provides a straightforward extension of the cube, allowing deformations that preserve edge length while changing angles away from 90 degrees. In crystallography, the term captures both lattice description and the associated symmetry.