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 (parent rock), 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 concept traces its roots to 1912, when G. M. Barrow mapped zones of metamorphism in Scotland.Mudrock, a fine-grained sedimentary rock often containing aluminium-rich minerals, produces these minerals after being metamorphosed, from low to high grade:[1]
Map of index minerals of the Barrovian metamorphic zones of Scotland
