In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form. Minerals are the basic building blocks of rocks and the solid Earth, and they are central to geochemistry, petrogenesis and resource studies.
The geological definition usually excludes compounds that occur only within living organisms. However, some minerals are commonly biogenic, such as calcite, or organic, such as mellite. Organisms also synthesise inorganic minerals such as hydroxyapatite, which also occurs in rocks. Biomineralisation and geological mineralisation are therefore not sharply separated but form a continuum.
Minerals differ from rocks. A rock is a relatively homogeneous mass of geological material at macroscopic scale and may consist of one mineral or of two or more minerals separated into distinct phases. A granite may contain quartz, feldspar, mica and hornblende, each with its own composition and structure.
Natural solid substances without a definite crystal structure, such as opal or obsidian, are more accurately called mineraloids. If a compound occurs naturally with different crystal structures, each structure is treated as a separate mineral species. Quartz and stishovite, for example, are both silica minerals but represent the low-pressure and high-pressure phases and are therefore separate species.
The International Mineralogical Association is the internationally recognised authority for defining and naming mineral species. As of May 2025 the IMA recognised 6,145 official mineral species. New minerals require naming, description and voting procedures, and type specimens and type localities must be preserved.
The IMA sets the following requirements for a substance to be regarded as a distinct mineral species:
Details of these rules are debated. Several proposals to classify amorphous substances as minerals have not been accepted by the IMA. The IMA is also reluctant to accept minerals that occur naturally only as nanoparticles a few hundred atoms across, but it has not defined a minimum crystal size. Some authors require a substance to be stable or metastable at room temperature, whereas the IMA requires only that it be stable enough for its structure and composition to be well determined. For example, epsomite, a natural magnesium sulfate hydrate, is accepted even though it forms and persists only below 2 °C.
Minerals differ from mineraloids in the degree of atomic order. Opal, obsidian, limonite and mineral wax lack long-range order and are classified as mineraloids. Mineraloids are important in industry, gems and geological research, but they do not occupy distinct mineral-species positions in strict classification.
Minerals differ from rocks in scale and association. A rock is an aggregate of minerals and may contain one mineral, as marble is dominated by calcite, or several, as granite contains quartz, feldspar, mica and amphibole. Rocks may also contain amorphous material, organic matter and pore fluids, so they cannot be described simply by a mineral definition.
The chemical composition of a named mineral may vary slightly because of trace impurities. Particular varieties of a species sometimes have their own traditional or official names. Amethyst is a purple variety of quartz, smoky quartz is a brown variety, and precious opal is a variety of opal.
Some mineral species may contain two or more chemical elements that occupy equivalent positions in the structure. Maucherite, for example, can be written (Fe, Ni)₉S₈, with variable iron and nickel proportions. The olivine group forms a continuous solid solution from magnesium-rich forsterite to iron-rich fayalite, and the end members are named as separate species.
The division of solid solutions is partly arbitrary. Where composition varies continuously, mineralogists draw species boundaries using structure, compositional range and nomenclature rules. Such divisions aid communication but mean that group boundaries may differ between classification schemes.
Minerals are classified mainly by chemical composition. The two principal systems are the Dana classification, based on chemical composition and structural relationships with detailed numbering, and the Strunz classification, organised around chemical formulae and structures into native elements, sulfides, oxides, halides, carbonates, sulfates, phosphates and silicates.
Silicate minerals make up about 90% of the Earth's crust and are the dominant mineral group in the crust and mantle. Other important groups include native elements, sulfides such as galena (PbS), oxides such as quartz (SiO₂), halides such as halite (NaCl), carbonates such as calcite (CaCO₃), sulfates such as gypsum (CaSO₄·2H₂O), molybdates such as wulfenite (PbMoO₄) and phosphates such as pyromorphite (Pb₅(PO₄)₃Cl).
Beyond chemical composition and crystal structure, the description of a mineral species usually includes its common physical properties: habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, crystal system, zoning, parting, specific gravity, magnetism, fluorescence, radioactivity, taste, odour and reaction to acid.
These properties are the basis of mineral identification. Hardness is measured with the Mohs scale or a Vickers indenter, cleavage reflects weak planes in the crystal structure, streak is the colour of the powdered mineral and is often more reliable than bulk colour, specific gravity and density reflect composition and packing efficiency, and fluorescence and radioactivity relate to impurities, defects or radioactive elements.
Minerals form by magmatic crystallisation, hydrothermal precipitation, metamorphic reaction, weathering and alteration, biomineralisation and sedimentary processes. Magmatic minerals crystallise from melt and are common in igneous rocks, hydrothermal minerals precipitate from hot water in veins and ore deposits, metamorphic minerals form in the solid state as temperature and pressure change, weathering minerals form near the surface through water, oxygen and organisms, and biominerals are precipitated under biological control, as in shells, bones and corals.
Mineral stability depends on environmental conditions. The same mineral may be replaced by others at different temperature, pressure, pH and oxygen fugacity, producing alteration, pseudomorphs or secondary assemblages. Minerals are therefore not static objects but dynamic records of geological processes.
Minerals are among the most important natural resources of human society. Metallic minerals are ores for metal production, industrial minerals are used in construction, ceramics, glass, fertilizers and chemicals, gem minerals are used in jewellery and collecting, and mineral fuels and radioactive minerals are energy resources. Minerals are also used in environmental remediation, soil amendment, water treatment and carbon storage.
Scientifically, minerals record the evolution of Earth and other planets. Mineral compositions, isotopes, inclusions and dating can be used to study crustal formation, plate tectonics, palaeoclimate, biological evolution and meteorite impacts. Mineralogy is therefore closely linked to petrology, geochemistry, planetary science and materials science.