Mineral alteration is the set of natural processes that change a mineral's chemical composition or crystal structure. Alteration can occur at any stage after mineral formation, in near-surface weathering environments as well as in deep hydrothermal, metamorphic or late magmatic settings.
Alteration is essentially governed by thermodynamic laws related to energy conservation and by environmental conditions, often in the presence of catalysts. The most common and influential catalyst is water. Water acts both as a reactant and as a medium for ion transport, promoting hydrolysis, hydration, dissolution and reprecipitation, so that the original mineral is replaced by a more stable assemblage.
The degree and timescale of alteration depend on the starting mineral, its physical properties and its susceptibility. Some minerals, such as quartz and zircon, are highly resistant under normal weathering conditions. Yet quartz may alter to stishovite under intense pressure, and zircon may alter to cyrtolite, a metamict zircon, with radioactive components and time. Resistance is therefore relative to environmental conditions rather than absolute.
In some circumstances a mineral alters while maintaining its outer form, producing a pseudomorph. A pseudomorph shows that the original mineral has been replaced while the new mineral inherited the original crystal shape, cleavage or surface texture, and it is important evidence of alteration history and fluid activity.
Mineral alteration is distinctly different from the rock alteration process of metamorphism and from weathering. Metamorphism emphasises solid-state reassembly of mineral assemblages driven by temperature and pressure, weathering emphasises low-temperature breakdown and transport near the surface in contact with water, oxygen and organisms, and mineral alteration is the broader term for the replacement or modification of individual minerals. These processes can overlap and reinforce one another. Some minerals are members of a solid-solution series and represent a continuum of compositional change, so they are not products of mineral alteration.
A common oxidation example is the alteration of a ferrous iron mineral such as pyrite to goethite or other ferric hydroxides or sulfates. Oxidation is often accompanied by acid generation, producing acid mine drainage, and pyrite oxidation is one of the most closely monitored reactions in mine-environment management.
Under prolonged desiccation, the hydrous sulfate mineral gypsum readily alters to the anhydrous sulfate anhydrite: CaSO₄·2H₂O → CaSO₄. The reaction is reversible, and anhydrite rehydrates to gypsum on contact with water. Hydration and dehydration are common in evaporites, soils and building stone.
Kaolinization is the alteration of alkali feldspar to the clay mineral kaolinite in the presence of slightly acidic solutions. Rain dissolves atmospheric carbon dioxide, promoting the weathering of granitic rocks. In the presence of carbonic acid and water, potassium feldspar alters to kaolinite, with potassium ion, bicarbonate and silica released in solution:
2 KAlSi₃O₈ + 2 H₂CO₃ + 9 H₂O → Al₂Si₂O₅(OH)₄ + 4 H₄SiO₄ + 2 K⁺ + 2 HCO₃⁻
Kaolinization is a key process in the formation of granite weathering profiles and ceramic raw materials, and it is a major source of clay minerals in soils.
Epidotization is the alteration of plagioclase feldspar to epidote-group minerals, common in low-grade metamorphism, hydrothermal alteration and burial metamorphism. Calcium and aluminium from plagioclase are recombined into epidote and clinozoisite, with sodium and silica released.
Chloritization is the alteration of pyroxene or amphibole to chlorite-group minerals. It is common during the transition to greenschist facies and during retrograde metamorphism of amphibolite facies, and it is a hallmark of hydrothermal alteration in mafic and ultramafic rocks.
Around impact craters and other astroblemes, ordinary silica or quartz crystals may alter to stishovite and coesite under the extreme pressure and high temperature produced by meteorite impact. These high-pressure polymorphs are important evidence for identifying impact structures.
A common example of radioactive decay alteration is the conversion of radioactive-element-bearing zircon or allanite to a metamict or amorphous state through structural damage. Radioactive particles disrupt the crystal lattice, destroying long-range order and producing a metamict mineral, and heating can partly restore crystallinity.
Serpentinization is the alteration process that forms serpentine-group minerals mainly from olivine-group minerals, with hydration and pressure changes as major factors. Serpentinization involves volume expansion and changes in rock mechanical properties, and it is a key process in the study of ophiolites and ultramafic rocks.
Dolomitization refers to the various ways in which a predominantly calcite-rich sedimentary rock such as limestone may alter to dolomite-rich rock. Diagenesis is a likely mechanism, involving large volumes of water and relatively low heat, with water acting as an ion-exchange catalyst:
2 CaCO₃ (limestone) + Mg²⁺ → CaMg(CO₃)₂ (dolomite) + Ca²⁺
Dolomitization changes porosity and reservoir properties and is important in carbonate hydrocarbon exploration and groundwater studies.
Pyritization involves ionic replacement by iron and sulfur atoms that combine to form pyrite. It is often associated with reducing conditions, organic-matter decomposition and sulfate-reducing bacteria, and it can also form where hydrothermal fluids supply iron and sulfur.
Opalization is the alteration of amorphous silica, often organic remains of siliceous microfossils in lithified sedimentary rocks, to the mineraloid opal. Opal lacks long-range order and is therefore classified as a mineraloid rather than a mineral in the strict sense.
Uralitization is the deuteric alteration of pyroxene, most commonly augite, to amphibole such as actinolite-tremolite. It occurs during late-stage magmatic crystallisation at low temperature, below 500 °C, or during low-grade metamorphic events in sub-greenschist facies. The reaction is paramorphic, meaning that the mineral structure is modified while the original chemistry is retained.
Mineral alteration records the interaction of minerals with fluids, the atmosphere and the biosphere. Altered assemblages, reaction rims, pseudomorphs and fluid inclusions allow past fluid composition, temperature, pH and redox conditions to be reconstructed. Alteration also affects porosity, permeability, strength and chemical stability, and is therefore important in hydrocarbon reservoirs, groundwater, mine environments and engineering geology.