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Index Minerals

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Definition: Just as index fossils are pointers to a specific period of geologic time, index minerals are minerals that point to particular conditions of temperature and pressure. Index minerals are essential to studies of metamorphic rocks.

The concept of index minerals relies on the chemical stability of minerals. Most minerals are actually metastable at the Earth's surface, ready to be degraded by the processes of weathering and turn into a small number of surface minerals. Index minerals mark the reverse process, appearing at a particular minimum of pressure and/or temperature as sediments are turned into rock by lithification and then further transformed by diagenesis and metamorphism to become metamorphic rocks.

The most widely used index minerals are biotite, chlorite, garnet, glaucophane, hornblende, prehnite, sillimanite, staurolite, and the zeolites. In rough ascending order of pressure/temperature, they are zeolites, chlorite, prehnite, biotite, hornblende, garnet, staurolite, sillimanite, glaucophane. But which index mineral appears also depends on the composition of the protolith, or original source rock. For instance, glaucophane is found only in metamorphosed mafic rocks like basalt, and sillimanite appears only in metasedimentary rocks. There are dozens of additional minerals that supplement the major ones, and all of them together produce typical assemblages, or metamorphic facies.

Index minerals are generally used to mark the lower end of a metamorphic facies; that is, we look for rocks that have undergone a simple upward progression in pressure and temperature. Many index minerals are stable well above the conditions where they first form, so their value is limited only to marking the point where they first appeared. Garnet is an example: once it forms, it remains throughout all higher stages of metamorphism. The calcite found in limestone has no value as an index mineral because it never changes. The opposite case—an index mineral that exists exclusively in one metamorphic facies—is called a critical mineral. Retrograde metamorphism—changes in minerals that happen as pressure and temperature decrease—may add confusion by changing some high-grade minerals while leaving others in a metastable state.

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