Rhombohedral crystal system

A crystal system with three equal-length axes meeting at equal angles other than 90°. Its conventional unit cell is a rhombohedron.

Crystal system: A classification of crystals based on the symmetry and geometry of their unit cells. This is the broader classification defined by the rhombohedral system’s equal axes and non-right angles.
Trigonal crystal system: A crystal system characterized by a principal threefold rotational symmetry axis. Rhombohedral lattices belong to the trigonal crystal system, despite the different naming scheme.
Calcite: A calcium carbonate mineral that commonly crystallizes in the trigonal system. Its crystal forms and structure provide a familiar rhombohedral example.
Hexagonal crystal system: A crystal system with two equal horizontal axes at 120 degrees and a distinct perpendicular axis. Its conventional cell looks different, though hexagonal settings also describe some trigonal structures.
Lattice system: A classification of three-dimensional lattices based on their metric constraints and symmetry. It groups the rhombohedral lattice with lattice types that share compatible cell geometry.
Rhombohedral lattice: A three-dimensional Bravais lattice with equal primitive vectors and equal non-right interaxial angles. It is the lattice type whose metric matches this system’s defining geometry.
Quartz: A crystalline form of silicon dioxide with several trigonal structural varieties. Its trigonal symmetry links a familiar mineral to this crystal classification.
Monoclinic crystal system: A crystal system with three unequal axes and one non-right interaxial angle. It contrasts with the rhombohedral system’s three equal axes and three equal angles.
Rhombohedron: A parallelepiped whose six faces are congruent rhombi. This is the characteristic shape of the system’s conventional primitive cell.
Hexagonal axes: A four-axis coordinate convention used to describe hexagonal and trigonal crystal structures. This alternative setting often describes rhombohedral crystals with a hexagonal-shaped cell.

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