What Are Index Minerals?

As rocks are subjected to heat and pressure, they change or metamorphose. Different minerals appear in any given rock depending upon the type of rock and the amount of heat and pressure the rock undergoes.

Geologists look at the minerals in rocks to determine how much heat and pressure — and thus how much metamorphosis — the rock has undergone. Certain minerals, called "index minerals," only appear in certain rocks at certain pressures, Thus, index minerals can tell geologists how much the rock has metamorphosed.

Examples of Index Minerals

The most widely used index minerals are, in ascending order of pressure/temperature, are biotite, the zeolites, chlorite, prehnite, biotite, hornblende, garnet, glaucophane, staurolite, sillimanite, and glaucophane. 

When these minerals are found in particular types of rocks, they can indicate the minimum amount of pressure and/or temperature the rock has experienced.

For example, slate, when it undergoes metamorphosis, changes first to phyllite, then to schist, and finally to gneiss. When slate is seen to contain chlorite, it is understood to have undergone a low grade of metamorphosis.

Mudrock, a sedimentary rock, contains quarts at all grades of metamorphosis. Other minerals, however, are added as the rock undergoes different "zones" of metamorphosis. The minerals are added in the following order: biotite, garnet, staurolite, kyanite, sillimanite. If a piece of mudrock contains garnet but no kyanite, it has probably undergone only a low grade of metamorphosis. If, however, it contains sillimanite, it has undergone extreme metamorphosis.