Editing Metamorphism (section)

===Neocrystallization===
Neocrystallization involves the creation of new mineral crystals different from the protolith. [[Chemical reaction]]s digest the minerals of the protolith which yields new minerals. This is a very slow process as it can also involve the diffusion of atoms through solid crystals.{{sfn|Vernon|1976|p=149}}

An example of a neocrystallization reaction is the reaction of [[fayalite]] with [[plagioclase]] at elevated pressure and temperature to form [[garnet]]. The reaction is:{{sfn|Yardley|1989|pp=110, 130–131}}

{{NumBlk|:
|{{overset|fayalite|3 {{chem|Fe|2|SiO|4}}}} + {{overset|plagioclase|{{chem|CaAl|2|Si|2|O|8}}}} → {{overset|garnet|2 {{chem|CaFe|2|Al|2|Si|3|O|12}}}}
|{{EquationRef|Reaction 1}}}}

Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism. These reactions are possible because of rapid diffusion of atoms at elevated temperature. Pore fluid between mineral grains can be an important medium through which atoms are exchanged.{{sfn|Yardley|1989|page=5}}

A particularly important group of neocrystallization reactions are those that release [[Volatile (astrogeology)|volatiles]] such as water and [[carbon dioxide]]. During metamorphism of [[basalt]] to [[eclogite]] in [[subduction zones]], hydrous minerals break down, producing copious quantities of water.{{sfn|Stern|2002|pp=6–10}} The water rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating [[magma]] via [[flux melting]].{{sfn|Schmincke|2003|pp=18, 113–126}} The mantle-derived magmas can ultimately reach the Earth's surface, resulting in volcanic eruptions. The resulting [[Volcanic arc|arc volcanoes]] tend to produce dangerous eruptions, because their high water content makes them extremely explosive.{{sfn|Stern|2002|pp=27–28}}

Examples of ''dehydration reactions'' that release water include:{{sfn|Yardley|1989|pp=75,102}}

{{NumBlk|:
|{{overset|[[hornblende]]|7{{chem2|Ca2Mg3Al4Si6O22(OH)2}}}} + {{overset|quartz|10{{chem2|SiO2}}}} → {{overset|[[cummingtonite]]|3{{chem2|Mg7Si8O22(OH)2}}}} + {{overset|[[anorthite]]|14{{chem2|CaAl2Si2O8}}}} + {{overset|water|4{{chem2|H2O}}}}
|{{EquationRef|Reaction 2}}}}

{{NumBlk|:
|{{overset|[[muscovite]]|2{{chem2|KAl2(AlSi3O10)(OH)2}}}} + {{overset|quartz|2{{chem2|SiO2}}}} → {{overset|sillimanite|2{{chem2|Al2SiO5}}}} + {{overset|[[potassium feldspar]]|2{{chem2|KAlSi3O8}}}} + {{overset|water|2{{chem2|H2O}}}}
|{{EquationRef|Reaction 3}}}}

An example of a decarbonation reaction is:{{sfn|Yardley|1989|p=127}}

{{NumBlk|:
|{{overset|calcite|{{chem2|CaCO3}}}} + {{overset|quartz|{{chem2|SiO2}}}} → {{overset|[[wollastonite]]|{{chem2|CaSiO3}}}} + {{overset|carbon dioxide|{{chem2|CO2}}}}
|{{EquationRef|Reaction 4}}}}