rhombohedral

Rhombohedral describes shapes or crystal structures in which the basic unit is a rhombohedron: a parallelepiped with all faces as rhombi and all edges congruent. In geometry, a rhombohedron is obtained by skewing a cube; it remains a polyhedron with opposite faces parallel, and its faces are equal rhombuses. If the interaxial angles are 90 degrees, the rhombohedron reduces to a cube; otherwise it is a skewed cube.

In crystallography, rhombohedral refers to a Bravais lattice type and to symmetry that can be described within the trigonal crystal system. The rhombohedral lattice is primitive, with three equal lattice vectors and equal oblique angles between them. In practice, crystals of rhombohedral symmetry can also be described in a hexagonal setting, reflecting a dual description used in mineralogy. The system is characterized by a threefold rotation axis.

Minerals commonly described as rhombohedral include calcite, which forms well known rhombohedral crystals; other minerals in the trigonal system may display rhombohedral habit or symmetry.

The term also appears in materials science to describe phases where a high-symmetry lattice distorts into a rhombohedral configuration, altering lattice parameters and physical properties.

Etymology: from Greek rhombos meaning rhombus and hedra meaning face.