An index mineral is a mineral used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of the protolith and the pressure and temperature it experienced, solid-state reactions between minerals produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have reached for that mineral to form. The higher the pressure and temperature of formation, the higher the metamorphic grade.
The concept traces its roots to 1912, when G. M. Barrow mapped zones of metamorphism in Scotland. Each zone is named for the index mineral that appears in it, for example the chlorite zone is named for chlorite. This work established the Barrovian zones, which became a classic framework for studying regional metamorphism.
Mudrock is a fine-grained sedimentary rock rich in aluminium-bearing minerals. Under increasing metamorphism it develops a succession of mineral assemblages. In the Scottish Barrovian sequence, the zones from low to high grade are:
These assemblages reflect dehydration of hydrous minerals, polymorphic transitions of aluminosilicates and the appearance of garnet, staurolite, kyanite and sillimanite as temperature rises. Each zone is named for the index mineral that appears or becomes abundant in it, so index minerals are both temperature-pressure indicators and the basis for dividing metamorphic zones.
Index minerals are closely linked to the concept of metamorphic facies. A facies is an assemblage formed within a particular pressure-temperature range, and index minerals are its most diagnostic members. Greenschist facies is characterised by chlorite, epidote, actinolite and albite, amphibolite facies by hornblende and plagioclase, and granulite facies by anhydrous assemblages.
Pressure type also affects which index minerals appear. The Barrovian sequence is a medium-pressure sequence with kyanite and staurolite, low-pressure metamorphism may produce andalusite and cordierite, and high-pressure metamorphism may produce glaucophane, lawsonite and jadeite. Index minerals therefore indicate not only grade but also the pressure regime of metamorphism.
Index minerals are basic tools for geological mapping and metamorphic studies. Identifying them allows metamorphic zones to be divided, isograds to be drawn and geothermal gradients and tectonic histories to be inferred. An isograd is the line joining points where a particular index mineral first appears, and it can be used to reconstruct regional metamorphic patterns.
Index minerals are also used in mineral exploration and engineering geology. Some ore deposits are associated with particular metamorphic grades, such as gold mineralisation in greenschist facies or certain base-metal deposits in amphibolite facies. In engineering, metamorphic grade affects rock strength, anisotropy and drillability, so index minerals help assess conditions for tunnels, slopes and foundations.
Interpretation must account for protolith composition. Mudrock, basalt and carbonate rocks form different assemblages at the same pressure and temperature, so the pelitic index-mineral sequence cannot be applied directly to other rock types. Fluid activity, deformation and retrograde metamorphism may also modify or erase index minerals and blur zone boundaries.
Mineral appearance depends not only on temperature and pressure but also on reaction kinetics, fluid composition and inherited minerals. Some index minerals may survive metastably at lower temperatures or be replaced during retrograde metamorphism. Index minerals should therefore be used together with other evidence such as metamorphic facies, isotopic dating and structural analysis rather than as a stand-alone thermometer or barometer.