Metamorphic reaction

A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions resulting in the final stable state of the observed metamorphic rock.

[1] Examples include the production of talc under varied metamorphic conditions:

Schematic representation of a metamorphic reaction. Abbreviations of minerals: act = actinolite ; chl = chlorite ; ep = epidote ; gt = garnet ; hbl = hornblende ; plag = plagioclase . Two minerals represented in the figure do not participate in the reaction, they can be quartz and K-feldspar . This reaction takes place in nature when a mafic rock goes from amphibolite facies to greenschist facies .
Epidotisation in Argyll and Bute , U.K