Isograd
Encyclopedia
In geology, an isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zone
s of different metamorphic index mineral
s. On geologic map
s focusing on metamorphic terranes (or landscapes underlain by metamorphic rocks), the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd lines. The garnet
isograd, for example, would mark the first occurrence of garnet in the rocks.
The mineral
s present in a metamorphic rock
are important because laboratory experiments at high pressures and temperatures have provided a lot of information on the pressure and temperature conditions under which certain metamorphic minerals form. For example, with increasing temperature and pressure the first minerals to form from a shale
are mica
s, particularly chlorite
and biotite
. With increasing temperature and pressure garnet
appears, and then kyanite
(at relatively high pressure) or sillimanite
(at relatively high temperature). The metamorphic zone with chlorite can be referred to as the chlorite zone, the zone with garnet as garnet zone, and so forth. To communicate this easily, the dominant metamorphic minerals in schist
s are usually included in the name, as, for example, garnet schist, or garnet-staurolite schist, and so forth.
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....
s of different metamorphic index mineral
Index mineral
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...
s. On geologic map
Geologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
s focusing on metamorphic terranes (or landscapes underlain by metamorphic rocks), the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd lines. The garnet
Garnet
The 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...
isograd, for example, would mark the first occurrence of garnet in the rocks.
The mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s present in a metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
are important because laboratory experiments at high pressures and temperatures have provided a lot of information on the pressure and temperature conditions under which certain metamorphic minerals form. For example, with increasing temperature and pressure the first minerals to form from a shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
are mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
s, particularly chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
and biotite
Biotite
Biotite 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...
. With increasing temperature and pressure garnet
Garnet
The 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...
appears, and then kyanite
Kyanite
Kyanite, 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...
(at relatively high pressure) or sillimanite
Sillimanite
Sillimanite 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....
(at relatively high temperature). The metamorphic zone with chlorite can be referred to as the chlorite zone, the zone with garnet as garnet zone, and so forth. To communicate this easily, the dominant metamorphic minerals in schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s are usually included in the name, as, for example, garnet schist, or garnet-staurolite schist, and so forth.