Understanding metamorphism

Understanding metamorphism – chemical constancy:

The topic of metamorphism is one that has the potential to confuse confuse students and teachers alike. Even the apparently simple metamorphic changes – as when clay is fired in a kiln to produce pottery – involve complex chemical interactions.

Whilst the idea of a limestone re-crystallising at high temperature to form a marble (with the same essential mineralogy) is fairly simple, the changes undergone by most rocks containing silicate minerals are complex, leading not only to changes in rock texture, but also the formation of new minerals. These new minerals result from chemical reactions that take place in the solid state (i.e. not involving melting) and are isochemical.

Whilst the main controls are the temperatures & pressures that they are subjected to, reactions between minerals are greatly assisted by the presence of even the tiniest quantities of water - either present in the original rock or released by the breakdown of hydrous minerals at high temperature. The bulk chemistry of metamorphic rocks is thus similar to their igneous or sedimentary parents, though their actual mineralogy may be completely different. Take, for example, the changes that occur when a typical mudstone is metamorphosed.

Clay minerals in the mudstone are hydrous alumino-silicates which, when heated, react to produce micas and quartz. Other alumino-silicates that appear at higher grades include garnet, andalusite, kyanite and feldspar, which form at the expense of some of the mica. As these new minerals appear, the texture of the rock also becomes gradually coarser, with an increasing tendency for minerals to separate into light and dark bands – but still the overall rock composition is chemically the same as the mudstone we started with. This last point is vital to geologists in trying to unravel the past history of metamorphic rocks.