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.
Examples include the production of talc under varied metamorphic conditions:
:serpentine + carbon dioxide → talc + magnesite + water
:chlorite + quartz → kyanite + talc + water