#crystallography

• Acanthite — A mineral belonging to the sulfide class, representing the low-temperature monoclinic crystalline form (stable below 173°C) of silver sulfide (Ag2S). This precious and heavy mineral is frequently found in hydrothermal veins or secondary enrichment zones of silver ore deposits, showcasing a lead-gray to black color with a dull metallic luster. It is dimorphous with argentite, the cubic mineral that pseudomorphically converts into acanthite upon natural cooling.
• Alochromatic — Being colorless in its pure state, but acquiring various colors due to the presence of fine chemical impurities, foreign inclusions, or structural defects in the crystal lattice; it applies to minerals like quartz, fluorite, or corundum, whose color is not an intrinsic property.
• Allocure — A structural phenomenon or a process of thermal or chemical treatment through which the crystalline phase or internal structure of a material is modified in a way that differs from its standard thermodynamic evolution. In advanced metallurgy or solid-state physics, the term indicates a controlled deviation in the solidification or crystallization process, generating alternative mechanical properties.
• Alvanite — A rare natural mineral from the hydrated vanadate class, structurally composed of a hydrated aluminum and zinc vanadate, having the chemical formula Al3(VO4)(OH)6·2H2O or related. It occurs as microcrystalline aggregates, crusts, or rosettes of a pinkish-yellow, white, or grayish color, found in the oxidation zones of uranium and vanadium deposits.
• Alvanite — A formal variant or structural feminine adaptation of the term 'alvanit', sometimes used in older technical literature or mineralogical translations; a rock or mineral aggregate composed predominantly of this hydrated vanadate. It represents a compound of purely scientific and collecting interest due to its extreme rarity in the Earth's crust.
• Amorphism — The state, property, or character of being amorphous; the total absence of an ordered crystalline structure or a regular geometric shape within a solid body. In mineralogy and crystallography, this phenomenon reflects long-range structural molecular disorder, causing isotropic properties that remain constant in all spatial directions.
• Amorphism — A structural state or property of certain solid bodies of lacking an internal crystalline organization and a regular external shape, their constituent atoms or molecules being randomly arranged. In a biological sense, the term indicates the absence of a clearly defined anatomical conformation or structure in certain lower organisms or parasitic cells.
• Anatase — A natural, polymorphic mineral form of titanium dioxide (TiO2), crystallizing in the tetragonal system. Also known historically as octahedrite, anatase is typically found as isolated, sharp crystals ranging in color from dark indigo, brown, yellow, to black, possessing a pronounced adamantine luster. It irreversibly transforms into rutile at high temperatures and is heavily studied in materials science for its photocatalytic and semiconducting properties.
• Anatase — A natural mineral form of titanium dioxide (TiO2), crystallizing in the tetragonal system, acting as a low-temperature stable polymorph alongside rutile and brookite. It typically occurs as isolated crystals, often octahedral or bipyramidal, with color variations ranging from yellowish-brown to dark indigo and black, possessing a pronounced adamantine luster. This mineral is extensively studied in materials engineering for its advanced semiconducting properties, utilized in photovoltaic cells and photocatalytic applications.
• Anisogonic — Having unequal angles; a polygon or geometric figure whose internal angles are not identical in measure, although the sides may sometimes retain symmetrical proportions. The term is used rarely in higher geometry or crystallography to describe asymmetric polygonal structures or crystal lattices that deviate from isogonic or regular shapes.
• Anisometry — A lack of metric symmetry; structural inequality found when measuring paired dimensions, axes, or segments. In crystallography, it denotes the characteristic of a crystal lattice of having axes of different lengths in space. In medicine, the term can be used as a rare synonym or correlate for volume or length discrepancies of symmetrical anatomical segments.
• Anisotropic — Exhibiting physical or mechanical properties (such as thermal conductivity, elasticity, expansion, or light refraction) that vary depending on the direction of measurement or propagation through that substance. This structural characteristic is specific to most crystals and composite materials, where the asymmetric arrangement of atoms determines a physical behavior dependent on vector orientation.
• Anisotropy — The property of certain substances (especially crystals) or physical media to present different mechanical, optical, thermal, or electrical characteristics in different directions. This structural phenomenon results from the non-uniform arrangement of atoms or molecules in space, representing the opposite of isotropy and forming the basis for the operation of many advanced optical and electronic devices.
• Antitropic — Exhibiting inverse symmetry, situated in direct structural opposition, or having physical and geometric properties oriented in the opposite direction relative to a reference element. In anatomy, it describes paired organs or structures facing each other in opposite planes; in crystallography, it refers to symmetrically inverted crystalline axes or faces.
• Aragonite — A mineral from the natural carbonate class, representing an unstable crystalline form of calcium carbonate and a high-pressure polymorph of calcite. It crystallizes in the orthorhombic system, frequently forming long prismatic crystals, acicular aggregates, or specific encrustations such as flos ferri, and constitutes the primary mineral component making up the nacre of mollusk shells and fine pearls.
• Argentite — A mineral from the sulfide class, composed of silver sulfide with the chemical formula Ag2S, structurally stable only at temperatures above 173°C. At ambient temperatures, it transforms through pseudomorphosis into acanthite (its stable monoclinic modification), while retaining its initial outer cubic form. Industrially, it is an extremely important silver ore, featuring a dark grey to black color, a bright metallic luster, and high density.
• Automorphic — Regarding minerals or crystals, having their own regular, well-developed geometric shapes, bounded by plane crystalline faces reflecting the internal symmetrical structure of the atomic lattice, without being modified or restricted by space constraints from neighboring minerals. In petrography, the term describes the first minerals that separate and solidify from an igneous melt, having full freedom of structural growth within the fluid residual liquid.