Index mineral
Encyclopedia
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 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
. Each zone is named for the index mineral that appears in it, e.g. the chlorite zone is named for chlorite
.
, a fine-grained sedimentary rock
often containing aluminium
-rich minerals, produces these minerals after being metamorphosed, from low to high grade:
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
(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
George Barrow
George Barrow was a British geologist.Barrow matriculated at London University in 1871, holding a Turner scholarship. Admitted to King's College London, he studied science, winning prizes in mathematics and geology. He was the first to map a metamorphic gradient by determining a sequence of...
mapped zones of metamorphism
Metamorphic zone
In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occurs in the bed rocks. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure....
in southern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. Each zone is named for the index mineral that appears in it, e.g. the chlorite zone is named for chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
.
Mineralogic zones
MudrockMudrock
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 are less than 0.0625 mm and are too small to study readily in the field...
, a fine-grained sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
often containing aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
-rich minerals, produces these minerals after being metamorphosed, from low to high grade:
- ChloriteChlorite groupThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
- BiotiteBiotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
- GarnetGarnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
- StauroliteStauroliteStaurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak.-Properties:It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: 2Al94O204...
- KyaniteKyaniteKyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
- SillimaniteSillimaniteSillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....