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A metastable mineral is a mineral found in a set of conditions (e.g., temperature, pressure, redox environment) where that mineral is not kinetically stable. The presence of such minerals is often used to infer geologic changes and tectonic motions, such as uplift or eduction, which have moved rocks from a situation (e.g., depth) where the minerals were formed to one in which the minerals are no longer stable. Over time, metastable minerals alter to more stable minerals.

Origins of metastable minerals
A number of processes may lead to the formation and/or emplacement of metastable minerals at the Earth's surface.

Biologic Processes
While purely biogenic substances are specifically excluded from being classified as minerals, products of combined biologic and geologic processes may be considered minerals. Distinction is often difficult between purely biogenic products and those whose formation included significant geologic processes.

Some biomineralization products are thermodynamically unstable at standard temperature and pressure. For example, many invertebrates and vertebrates produce aragonite, which alters to calcite on scales of 10^7 to 10^8 years.

Geologic Processes
Tectonic processes such as orogeny and eduction raise rocks from depth. Coupled uplift and erosion expose rocks with minerals that formed at depth at the surface in areas such as the Himalayas.

Extensional faulting can also expose rocks from the middle and lower crust in metamorphic core complexes.

Rocks may also be brought to the surface by volcanic processes. Xenoliths and xenocrysts within igneous rocks are often composed of metastable minerals. Such fragments within surface basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, provide information about the otherwise inaccessible mantle.

Impact events and explosions
The high pressures created by high energy impacts produce minerals that are not stable in conditions commonly found at the earth's surface. Such minerals are often found in meteorites and craters.

Alteration processes
Most alteration processes in metastable minerals at the earth's surface are a form of retrograde metamorphism. Some metastable minerals alter to more stable polymorphs over time, while others decompose or react with surrounding compounds to form chemically distinct species.

Water plays a significant role in alteration of metastable minerals. Some minerals, particularly evaporites and their polymorphs, dissolve easily in water. Other minerals are the result of chemical devolatilization reactions at depth, in which water, carbon dioxide and other volatile compounds are expelled. The reversal of these reactions, and subsequent degradation of the metastable minerals, requires the addition of these compounds, mainly water, back into the system. When water is present in or forced through a rock, reaction rims may form around grains composed of metastable minerals. For example, a rim of chlorite and quartz may be found around many grains of garnet. The presence of reaction rims is sometimes mapped to yield information about fluid pathways in the past.

Examples of metastable minerals
Minerals commonly identified at the earth's surface, where they are metastable:
 * Diamond
 * Aragonite: CaCO3 formed from precipitation and biologic processes. Alters to calcite.
 * Vaterite: μ-CaCO3, less stable than aragonite
 * Ferrihydrite: fine grained and highly defective nanomaterial, alters to hematite and goethite
 * Pyroxferroite
 * Mackinawite: precipitate mineral. Alteration pathway depends on redox state, but usually ends with pyrite.
 * Stishovite
 * Coesite
 * Cristobalite
 * Lonsdaleite
 * Ringwoodite
 * Tridymite: high-temperature polymorph of quartz
 * Garnet
 * Sanidine: high-temperature polymorph of Orthoclase
 * Olivine: weathers to iddingsite (a combination of clay minerals, iron oxides and ferrihydrites) in the presence of water