A mineral is a naturally occurring, inorganic solid substance with an ordered internal atomic structure and a characteristic chemical composition that falls within a defined range. Minerals are fundamental components of rocks, soils, sediments, planetary crusts, and many geological deposits. The term naturally occurring distinguishes minerals from substances manufactured artificially, although laboratory-grown crystals can have essentially the same chemical and structural properties as natural minerals.
The inorganic requirement traditionally excludes substances produced directly by living organisms, although the boundary can become complicated when biological processes contribute to mineral formation. The ordered atomic structure means that atoms, ions, or molecules occupy a repeating arrangement at the microscopic scale. This crystalline structure gives minerals characteristic physical properties such as hardness, cleavage, density, lustre, optical behaviour, and crystal form. Chemical composition and atomic arrangement together determine mineral identity, so two substances with similar compositions can still be different minerals if their structures differ.
More than five thousand mineral species have been formally recognised by mineralogical organisations, although a much smaller number make up most common rocks. Silicate minerals are the dominant mineral group in Earth's crust and include feldspars, quartz, micas, pyroxenes, amphiboles, and olivine. Silicates are built primarily from silicon and oxygen combined with other elements.
Other major mineral groups include carbonates such as calcite and dolomite, oxides such as hematite and magnetite, sulfides such as pyrite, sulfates such as gypsum, halides such as halite, phosphates such as apatite, and native elements such as gold, copper, sulfur, and carbon in forms including diamond and graphite. Mineral classification is largely based on chemical composition and structural relationships. These groups are important because mineral chemistry provides information about the conditions under which rocks formed and subsequently changed.
A mineral's chemical composition is usually expressed by a chemical formula, although many minerals permit limited substitution of one element for another. For example, olivine occurs as a compositional series between magnesium-rich and iron-rich end members, while feldspars form several extensive solid-solution systems. Such substitution can occur because elements with similar ionic sizes and charges can occupy equivalent positions in a crystal lattice. Temperature, pressure, fluid composition, and geological environment influence which mineral compositions are stable.
Mineral stability is therefore closely connected with thermodynamics and phase equilibria. A mineral assemblage can record the physical and chemical conditions that existed when a rock formed. Geologists use these assemblages to infer temperature, pressure, fluid activity, oxidation conditions, and geological history. Minerals can subsequently become unstable when conditions change and may react to form new minerals.
Physical properties are central to mineral identification. Hardness describes resistance to scratching and is commonly compared using the Mohs hardness scale. Cleavage describes the tendency of a mineral to break along particular crystallographic planes where bonding is relatively weak, while fracture describes irregular or non-cleavage breakage. Lustre describes how a mineral surface reflects light and can be metallic, glassy, pearly, silky, earthy, or described by other terms.
Streak is the colour of the mineral's powdered form and can be more diagnostically useful than surface colour. Specific gravity measures density relative to water. Transparency, magnetism, reaction with dilute acid, fluorescence, crystal habit, and optical properties can provide additional identification evidence. No single physical property is always sufficient, so mineral identification normally combines several observations with chemical or analytical tests.
Minerals form through numerous geological processes. They can crystallise from cooling magma, precipitate from water-rich fluids, form through evaporation, develop during metamorphism, or be produced through chemical reactions during weathering and alteration. Igneous minerals crystallise as molten material cools, with different minerals becoming stable at different temperatures and compositions. Sedimentary minerals may form through direct precipitation from water or through weathering and deposition of pre-existing mineral grains.
Metamorphic minerals form when existing minerals become unstable under new temperature, pressure, or chemical conditions and react to create new assemblages. Hydrothermal fluids can transport dissolved elements through fractures and pores and deposit minerals when temperature, pressure, acidity, or chemical composition changes. These processes produce economically important concentrations of metals, salts, gemstones, and industrial minerals.
Minerals have major practical importance because they provide many of the raw materials used by modern societies. Iron-bearing minerals supply iron for steel, copper minerals supply copper for electrical systems, and aluminium is extracted from aluminium-rich ores such as bauxite. Quartz is used in glass manufacture, electronics, optical technologies, and engineered materials.
Gypsum is used in plaster and plasterboard, halite provides common salt, calcite is important in cement and lime production, and numerous clay minerals are essential to ceramics and construction. Rare earth element minerals are important sources of elements used in electronics, magnets, batteries, and other technologies. A mineral deposit becomes an ore when valuable material can be extracted economically under particular technological and market conditions. Thus, not every occurrence of a useful mineral constitutes an ore deposit.
Minerals are closely related to rocks but are not synonymous with rocks. A rock is a naturally occurring aggregate of one or more minerals or mineraloid materials, whereas a mineral is an individual substance with a characteristic composition and crystalline structure. Granite, for example, commonly contains quartz, feldspars, and micas. Limestone can be dominated by calcite, although it may also contain other minerals and biological material.
Some naturally occurring substances do not satisfy all traditional mineral criteria and are classified as mineraloids. Glass is an important example because it lacks the long-range ordered crystalline structure characteristic of minerals. Organic materials such as coal also have complex geological origins but are not normally classified as minerals. This distinction is important in geology, because mineral composition controls many of the physical, chemical, and mechanical properties of rocks.
Mineral science, or mineralogy, examines the composition, structure, properties, formation, classification, and occurrence of minerals. Modern mineral identification can involve X-ray diffraction, electron microscopy, spectroscopy, chemical analysis, and crystallographic measurements in addition to traditional field observations. X-ray diffraction is particularly important because the diffraction pattern reflects the arrangement of atoms within a crystalline structure. Mineralogy connects geology with chemistry, physics, materials science, environmental science, planetary science, and economic geology. Minerals also provide records of geological change because particular mineral assemblages form under particular conditions. By studying minerals in rocks, scientists can reconstruct processes such as volcanic crystallisation, mountain building, metamorphism, weathering, hydrothermal alteration, and planetary differentiation.
