The original polarizing prisms of petrographic microscopes were the early 19^th century design of William Nicol, and each was referred to as a Nicol prism (Bloss, 1961). When both Nicol prisms were introduced into the optic path in mutually perpendicular orientations, the examination was said to be using “crossed Nicols”. The phrase “crossed nicols” is still used for this geometry of polarized light, regardless of whether the polarizing optics use Nicol prisms or another polarization device.
See petrographic microscope, crossed polarizers

“Polars” or “polarizers” are devices that permit passage of light with vibrations in only one direction. In an optical “polarizing microscope”, two polarizers (the lower device is often referred to as the “polarizer” and the upper device is the “analyzer”) are oriented such that light vibrations are at 90 degrees from each other (and the two polarizers are said to be “crossed”). If nothing is present in the optical path between the two devices to change the light vibrations, all light is prevented from passing and the image is black. Material placed in the optical path changes the behavior of the light rays, with the behavior dependent on the crystallography of the material (e.g., crystal structure, crystallinity), the chemical composition, the thickness of the material, and refractive indices. If the analyzer is removed from the optical path, then the devices are not considered “crossed”.
Syn., crossed nicols, crossed polars

An obsolete varietal term for zinnwaldite, ferroan trilithionite, and ferroan polylithionite

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