An index mineral
is used in geology to determine the degree of metamorphism
a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith
, chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.
In 1912 G.M. Barrow mapped zones of metamorphism in S. Scotland
. Each zone is named on the first appearance of the mineral. The zone is then named on the basis of this index mineral, E.g index mineral for the chlorite zone is chlorite.
Examples of Index Minerals (low grade to high grade)
Mudrock, a fine-grained sedimentary rock
often containing aluminum
-rich minerals, produces these minerals after being metamorphosed:
Marshak, Stephen. Earth: Portrait of a Planet