Minerals disappear and appear Minerals are known to disappear and new minerals appear as metamorphism progresses. This in essence is how the piece of rock in my hand evolved. The original basalt did not need to contain garnets in order to be transformed into garnet gneiss. The almandite crystals were formed by chemical action during metamorphism as hot rich fluids migrated through the rock. Such transportation indicates the use of volatiles such as water and carbon dioxide.
Volatiles are an essential part of the process of metamorphism.
During metamorphisim, the original basalt is helped by volatiles to form a new mineral, which is chlorite. It is known that both chlorite and garnet can exist together in thermodynamic equilibrium at a particular temperature. But this reaction cannot be that simple. With increasing metamorphic grade, there can be a change in the ratio of iron and magnesium in the chemical environment of the surrounding rock. As temperature rises the chlorite is encouraged to shed some of its iron and in so doing becomes more magnesium-rich.