An index mineral is used in geology to determine the degree of

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the

protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock an ...

(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 higher the pressure and temperature in which the rock formed, the higher the grade of the rock. The concept traces its roots to 1912, when G. M. Barrow mapped zones of metamorphism in southern

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...

. Each zone is named for the index mineral that appears in it. E.g. the chlorite zone is named for

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorou ...

Mineralogic zones

Mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...

, a fine-grained

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

often containing

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

-rich minerals, produces these minerals after being metamorphosed, from low to high grade:Blatt, Harvey and Robert J. Tracy, ''Petrology,'' Freeman, 1996, 2nd ed., p. 375 * Chlorite zone: quartz, chlorite, muscovite, albite * Biotite zone: quartz, muscovite, biotite, chlorite, albite * Garnet zone: quartz, muscovite, biotite, garnet, sodic plagioclase * Staurolite zone: quartz, muscovite, biotite, garnet, staurolite, plagioclase * Kyanite zone: quartz, muscovite, biotite, garnet, kyanite, plagioclase, +/- staurolite * Silimanite zone: quartz, muscovite, biotite, garnet, sillimanite, plagioclase

References

*Marshak, Stephen. ''Earth: Portrait of a Planet,'' Norton, 3rd ed. 2007 Mineralogy Metamorphic petrology {{petrology-stub

Mineralogic zones

See also

References