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. Mudrock, a fine-grained sedimentary rock often containing aluminum-rich minerals, produces these minerals after being metamorphosed: Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i. ... Quartzite, a form of metamorphic rock, from the Museum of Geology at University of Tartu collection. ... Two types of sedimentary rock: limey shale overlaid by limestone. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ...

Chlorite (low grade)
Biotite
Garnet
Staurolite
Kyanite
Sillimanite (high grade) This discusses some chlorine compounds. ... A Biotite slice Biotite is a common phyllosilicate mineral that contains potassium, magnesium, iron and aluminium. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... Staurolite Staurolite is a red brown to black mostly opaque nesosilicate mineral with a white streak. ... Kyanite, whose name derives from the Greek, kyanos, meaning blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. ... Sillimanite: Biotite gneiss (Mesozoic and Paleozoic) Sillimanite is an alumino-sillicate mineral with the chemical formula Al2SiO5. ...

References

Marshak, Stephen. Earth: Portrait of a Planet