A rock-texture term for which the individual mineral grains in an aggregate are too small to be distinguished in an ordinary light microscope. If grains can be distinguished in a light microscope, the texture is “microcrystalline”. The rock is said to have a “compact” texture if grains cannot be distinguished by the naked eye. Carbonate petrographers use a discrete crystal dimension for individual carbonate grains for a rock to be classified as having “cryptocrystalline” texture, although the dimension does not appear to be in universal agreement.

See hollandite.

An element or chemical compound that is crystalline and shows planar faces that express this crystallinity. If the solid lacks faces, it is referred to as “anhedral” and if it is completely bounded by faces, then it is referred to as “euhedral”, and it is “subhedral” if bounded by poorly defined faces. The term “single crystal” is often used in a colloquial sense for crystalline grains which are anhedral.

Crystal chemistry is the study of the solid state with the associated principles and interpretation of atomic structure and related chemical and physical properties (or any structure property relation). Systematic crystal chemistry is the study of how the atomic structure relates to changes in the composition, environmental conditions, and physical properties of a solid. Originally, crystal chemistry related to crystalline solids only, but amorphous or near amorphous states may be described using similar principles.

In thermodynamics, the driving force (F_d) associated with crystal growth is: F_d = Δμ/k_BT = σ, where Δμ is the change in chemical potential, σ is the supersaturation state, k_B is the Boltzmann constant, and T is absolute temperature.

The atomic arrangement for a crystalline material.

Crystal systems are defined based on the symmetry of a crystal. There are six crystal systems, given in decreasing symmetry: cubic (or isometric), hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. Minimum symmetry requirements are: four 3-fold or -3 axes (cubic), one 3- or 6-fold axes (hexagonal), one 4-fold axis (tetragonal), three mutually perpendicular directions with 2-fold and/or mirror plane symmetries (orthorhombic), one 2-fold axis and/or mirror plane (monoclinic), and center of symmetry or identity operation only (triclinic). Consequently, because of the symmetry present, the relative lengths of the crystallographic axes and the values of interaxial angles may be constrained: cubic, a₁ = a₂ = a₃, α = β = γ = 90^o; hexagonal, a₁ = a₂ = a₃ not equal c, α, β = 90^o, γ = 120^o, β = 90^o; tetragonal, a₁ = a₂ not equal c, α = β = γ = 90^o; orthorhombic, a not equal b not equal c, α = β = γ = 90^o; monoclinic, a not equal b not equal c, α = γ = 90^o, β > 90^o; triclinic, a not equal b not equal c, α not equal β not equal γ. The term “isometric” is sometimes used as a morphological term where the measured crystal is equant.
Cf., crystallographic axes

A solid consisting of atoms, ions, or molecules packed together in a periodic arrangement. The material must have sufficient atomic ordering such that a (X-ray, electron, neutron, etc.) diffraction pattern containing well-defined maxima can be indexed using Miller indices (Nickel, 1995).
Cf., non-crystalline

An attempt to describe the state of crystallinity of a solid as a value of some characteristic, usually relating to diffraction. The term is a misnomer because it suggests that the complex idea of crystallinity may be represented by a single value. The use of the term “crystallinity index” should be avoided, although it may be placed within quotation marks when referring in a limited way to previously referenced work (Guggenheim et al., 2002). Some indices are useful to describe e.g., crystallite size or grade of diagenesis. Indices were derived by Hinckley (1963) to distinguish between different samples of kaolinite, by Kübler (1964) to describe certain origins of samples of illite, and by Árkai (1991) to describe different origins of chlorite. It is recommended to refer to the author describing the procedures necessary to define the value, regardless of what the index may actually be describing, such as the Hinckley index.
See Guggenheim et al. (2002) and references therein.
Cf., Árkai index, Kübler index, Hinckley index

A set of reference axes used in crystallography. These axes are usually three in number, although in some cases, they may be four. The axes are generally mutually perpendicular, coincide with symmetry axes or the normals to symmetry planes, and in cases where the crystal lacks symmetry, parallel to lines of intersection of two faces with greatest areas. These axes are designated as a, b, c, and angles between axes are designated α, β, and γ where α is located between axes b and c, β is located between axes a and c, etc. according to the right- hand rule.
See also: crystal system.