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Crystals can be classified in three ways: lattice systems, crystal systems and crystal families. The various classifications are often confused: in particular the trigonal crystal system is often confused with the rhombohedral lattice system, and the term "crystal system" is sometimes used to mean "lattice system" or "crystal family".
The trigonal crystal system consists of the 5 point groups that have a single three-fold rotation axis, which includes space groups 143 to 167. These 5 point groups have 7 corresponding space groups (denoted by R) assigned to the rhombohedral lattice system and 18 corresponding space groups (denoted by P) assigned to the hexagonal lattice system.
Rhombohedral: R 3 m (No. 166) 105 (rh.) 315 (hex.) Partly due to its complexity, whether this structure is the ground state of Boron has not been fully settled. α-As: A7: Rhombohedral: R 3 m (No. 166) 2 (rh.) 6 (hex.) in grey metallic form, each As atom has 3 neighbours in the same sheet at 251.7pm; 3 in adjacent sheet at 312.0 pm. [18]
Liquid crystal – a state of matter with properties of both conventional liquids and crystals Patterson function – a function used to solve the phase problem in X-ray crystallography Periodic table (crystal structure) – (for elements that are solid at standard temperature and pressure) gives the crystalline structure of the most ...
It can be used to define the rhombohedral lattice system, a honeycomb with rhombohedral cells. A rhombohedron has two opposite apices at which all face angles are equal; a prolate rhombohedron has this common angle acute, and an oblate rhombohedron has an obtuse angle at these vertices.
R rhombohedral; A reflection plane m within the point groups can be replaced by a glide plane, labeled as a, b, or c depending on which axis the glide is along. There is also the n glide, which is a glide along the half of a diagonal of a face, and the d glide, which is along a quarter of either a face or space diagonal of the unit cell.
Rhodochrosite forms a complete solid solution series with iron carbonate . Calcium (as well as magnesium and zinc, to a limited extent) frequently substitutes for manganese in the structure, leading to lighter shades of red and pink, depending on the degree of substitution. This is the reason for the rose color of rhodochrosite.
In geometry, a trigonal trapezohedron is a polyhedron with six congruent quadrilateral faces, which may be scalene or rhomboid. [1] [2] The variety with rhombus-shaped faces faces is a rhombohedron. [3] [4] An alternative name for the same shape is the trigonal deltohedron. [5]