trigonalrhombic

Trigonalrhombic is a historical or informal descriptor used in crystallography and mineralogy for crystals that are associated with the trigonal (rhombohedral) system and display a rhombohedral external form. In practice, it refers to crystals whose common habit shows rhombohedra, a parallelepiped with equal edge lengths and oblique angles.

In modern terminology, the relationship between trigonal and rhombohedral terminology is clarified: the rhombohedral lattice is the characteristic lattice of the trigonal system, and many crystals with rhombohedral faces are described as belonging to this system. The term trigonal rhombic is largely obsolete in current crystallography, where crystals are typically described as trigonal or rhombohedral depending on the context, and the lattice symmetry is distinguished from the external rhombohedral habit.

Morphologically, a rhombohedron is a three-dimensional shape with four pairs of parallel faces that are rhombuses, each face identical, and with edge lengths equal but interior angles that are not 90 degrees. Crystals of the trigonal system often exhibit rhombohedral faces or habits, making the rhombohedron a common morphological form for these minerals. External shape can vary with growth conditions and impurities, but the inherent symmetry relates to the threefold axis characteristic of the trigonal system.

Notable examples commonly cited in older literature include minerals like calcite, which frequently forms rhombohedral crystals. Modern mineral descriptions typically favor the terms trigonal or rhombohedral to reflect the underlying symmetry and lattice, rather than the combined descriptor trigonalrhombic.

See also: rhombohedron, rhombohedral lattice, trigonal crystal system, mineral morphology.