Trigonalrhomboedrisch

TrigonalRhomboedrisch, or trigonal rhombohedral, refers to crystals that combine trigonal symmetry with a rhombohedral lattice. In crystallography, the trigonal crystal system is characterized by a threefold rotation axis along a principal direction. The rhombohedral lattice is a Bravais lattice in which the conventional unit cell can be described as a rhombohedron with equal edges and equal interaxial angles that are not right angles. For practical descriptions, crystallographers often use the hexagonal setting for this family, in which the lattice has a = b ≠ c and the angle gamma = 120°, highlighting the threefold symmetry more clearly.

Many minerals crystallize in the trigonal rhombohedral lattice, including calcite and dolomite, which belong to the rhombohedral lattice type and display rhombohedral crystal forms. Minerals such as quartz are commonly described as trigonal in terms of symmetry, but they are often represented in the hexagonal setting for convenience; in essence, the same symmetry can be described with different cell conventions.

In terminology, trigonal and rhombohedral are sometimes used interchangeably in older literature, but modern crystallography distinguishes between the trigonal crystal system and the rhombohedral lattice within the hexagonal family. The trigonal rhombohedral description is important for understanding the arrangement of atoms, symmetry operations, and phase relationships in crystals with equal-length axes and oblique angles.