vein, in geology, ore body that is disseminated within definite boundaries in unwanted rock or minerals (gangue). The term, as used by geologists, is nearly synonymous with the term lode, as used by miners. There are two distinct types: fissure veins and ladder veins.

Fissure veins, the earliest described bedrock deposits, occupy one or more fissures; they are tabular, with two dimensions much greater than the third. Fissure veins are formed in two stages, sometimes greatly separated in time; first the fissure is formed, and then it is filled with ore. There are several varieties: simple, with relatively straight, parallel walls; chambered, with irregular, fragmented walls; dilation, or lenticular, with fat lenses in a string or roughly parallel in schists; sheeted, with several distinct, closely spaced, parallel fractures; and composite, with several roughly parallel fissures and connecting diagonals in partially replaced rock.

Ladder veins are short, rather regularly spaced, roughly parallel fractures that traverse dikes (tabular bodies of igneous rocks) from wall to wall. Their width is restricted to the width of the dike, but they may extend great distances along it. Ladder veins are not as numerous or important as fissure veins.

hot springs and epithermal veins
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mineral deposit: Veins
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mineralogy, scientific discipline that is concerned with all aspects of minerals, including their physical properties, chemical composition, internal crystal structure, and occurrence and distribution in nature and their origins in terms of the physicochemical conditions of formation.

A brief treatment of mineralogy follows. For further discussion, see geology: Study of the composition of the Earth.

The goals of mineralogical studies may be quite diverse, ranging from the description and classification of a new or rare mineral, to an analysis of crystal structure involving determination of its internal atomic arrangement, or to the laboratory or industrial synthesis of mineral species at high temperatures and pressures. The methods employed in such studies are equally varied and include simple physical and chemical identification tests, determination of crystal symmetry, optical examination, X-ray diffraction, isotopic analysis, and other sophisticated procedures.

Cross section of Earth showing the core, mantle, and crust
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Although much mineralogical research centres on the chemical and physical properties of minerals, significant work is conducted on their origin as well. Investigators are frequently able to infer the way in which a mineral species forms on the basis of data obtained from laboratory experiments and on theoretical principles drawn from physical chemistry and thermodynamics.

This article was most recently revised and updated by Adam Augustyn.