Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography. Mineral alteration is essentially governed by the laws of thermodynamics related to energy conservation, relevant to environmental conditions, often in presence of catalysts, the most common and influential being water (H2O). The degree and scales of time in which different minerals alter vary depending on the initial product and its physical properties and susceptibility to alteration. Some minerals such as quartz and zircon are highly resistant to alteration under normal weathering conditions. Yet quartz may alter to stishovite with intense pressure, and zircon to crytolite (a metamict zircon) with amount of radioactive components and time. In some circumstances, a mineral alters while maintaining its outer form known as a pseudomorph. Mineral alteration is distinctly different than the rock alteration process metamorphism. It also differs from weathering. However, these processes assist in mineral alteration. Some minerals are members of a solid solution series and are samples of a range of compositional changes in a continuum, and thus are not 'mineral alteration' products. Examples of mineral alterations Oxidation A common oxidation example is when a natural ferrous iron mineral such as pyrite is oxidized to form goethite or other ferric iron hydroxides or sulfates. 5(4x + 3)/2 = y Hydration and dehydration With prolonged desiccation, a common hydrous sulfate mineral called gypsum readily alters to an anhydrous sulfate called anhydrite. CaSO4·2H2O <=> CaSO4 This is a reversible reaction. Kaolinization Kaolinization refers to the alteration of alkali feldspar into the clay mineral kaolinite in the presence of slightly acidic solutions. Rain readily dissolves carbon dioxide (CO2) from the atmosphere, promoting weathering of granitic rocks. As demonstrated in the following reaction, in the presence of carbonic acid and water, potassium feldspar is altered to kaolinite, with potassium ion, bicarbonate, and silica in solution as byproducts. 2 KAlSi3O8 + 2 H2CO3 + 9 H2O => Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3− Epidotization Epidotization is the alteration process in which plagioclase feldspars convert into the epidote group minerals. Chloritization Chloritization is the alteration of pyroxene or amphibole minerals into the chlorite group minerals. Chloritization is a common process in metamorphic transitions to the greenschist facies, and amphibolite facies retrograde metamorphism. Shock induced alteration As observed in and around astroblemes such as impact craters, ordinary silica or quartz crystals may alter to the minerals stishovite and coesite