Metamorphic reactions

Reactions in a kaolinite-quartz system

A very simple mineralogical system and its response to changing pressure and temperature provide a good illustration of what occurs in metamorphism. An uncomplicated sediment at Earth’s surface, a mixture of the clay mineral kaolinite [Al₄Si₄O₁₀(OH)₈] and the mineral quartz (SiO₂), provides a good example. Most sediments have small crystals or grain sizes but great porosity and permeability, and the pores are filled with water. As time passes, more sediments are piled on top of the surface layer, and it becomes slowly buried. Accordingly, the pressure to which the layer is subjected increases because of the load on top, known as overburden. At the same time, the temperature will increase because of radioactive heating within the sediment and heat flow from deeper levels within Earth.

In the first stages of incremental burial and heating, few chemical reactions will occur in the sediment layer, but the porosity decreases, and the low-density pore water is squeezed out. This process will be nearly complete by the time the layer is buried by 5 km (3 miles) of overburden. There will be some increase in the size of crystals; small crystals with a large surface area are more soluble and less stable than large crystals, and throughout metamorphic processes there is a tendency for crystals to grow in size with time, particularly if the temperature is rising.

Eventually, when the rock is buried to a depth at which temperatures of about 300 °C (572 °F) obtain, a chemical reaction sets in, and the kaolinite and quartz are transformed to pyrophyllite and water: Al₄Si₄O₁₀(OH)₈ + 4SiO₂ → 2Al₂Si₄O₁₀(OH)₂ + 2H₂O.

The exact temperature at which this occurs depends on the fluid pressure in the system, but in general the fluid and rock-load pressures tend to be rather similar during such reactions. The water virtually fights its way out by lifting the rocks. Thus, the first chemical reaction is a dehydration reaction leading to the formation of a new hydrate. The water released is itself a solvent for silicates and promotes the crystallization of the product phases.

If heating and burial continue, another dehydration sets in at about 400 °C (752 °F), in which the pyrophyllite is transformed to andalusite, quartz, and water: Al₂Si₄O₁₀(OH)₂ → Al₂SiO₅ + 3SiO₅ + H₂O.

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After the water has escaped, the rock becomes virtually anhydrous, containing only traces of fluid in small inclusions in the product crystals and along grain boundaries. Both of these dehydration reactions tend to be fast, because water, a good silicate solvent, is present.

Although the mineral andalusite is indicated as the first product of dehydration of pyrophyllite, there are three minerals with the chemical composition Al₂SiO₅. Each has unique crystal structures, and each is stable under definite pressure-temperature conditions. Such differing forms with identical composition are called polymorphs. If pyrophyllite is dehydrated under high-pressure conditions, the polymorph of Al₂SiO₅ formed would be the mineral kyanite (the most dense polymorph). With continued heating, the original andalusite or kyanite will invert to sillimanite, the highest-temperature Al₂SiO₅ polymorph:

The kinetics of these polymorphic transformations are sufficiently sluggish, however, that kyanite or andalusite may persist well into the stability field of sillimanite.

Reactions of other mineral systems

Because of the very simple bulk composition of the protolith in this example (a subset of the pelitic system containing only SiO₂-Al₂O₃-H₂O), no other mineralogical changes will occur with continued heating or burial. The original sediment composed of kaolinite, quartz, and water will thus have been metamorphosed into a rock composed of sillimanite and quartz and perhaps some metastable andalusite or kyanite, depending on the details of the burial and heating history. In the case of a more typical pelite containing the additional chemical components potash, ferrous oxide, and magnesium oxide, the reaction history would be correspondingly more complex. A typical shale that undergoes burial and heating in response to continent-continent collision would develop the minerals muscovite, chlorite, biotite, garnet, staurolite, kyanite, sillimanite, and alkali feldspar, in approximately that order, before beginning to melt at about 700 °C (1,292 °F). Each of these minerals appears in response to a chemical reaction similar to those presented above. Most of these reactions are dehydration reactions, and the shale thus loses water progressively throughout the entire metamorphic event. As discussed above, the total number of minerals present in the rock is controlled by the Gibbs phase rule, and the addition of new minerals generally results from the breakdown of old minerals. For example, the reaction involving garnet, muscovite, and chlorite that yields staurolite, biotite, quartz, and water occurs at temperatures of about 500–550 °C (932–1,022 °F) and typically consumes all the preexisting chlorite in the rock, introduces the new mineral staurolite, and adds more biotite and quartz to the biotite and quartz generated by earlier reactions. Some garnet and muscovite usually remain after the reaction, although examination of the sample under the microscope would probably reveal partial corrosion (wearing away due to chemical reactions) of the garnets resulting from their consumption.