A petrogenetic grid is a

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...

phase diagram A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseous ...

that connects the stability ranges or

metastability In chemistry and physics, metastability denotes an intermediate Energy level, energetic state within a dynamical system other than the system's ground state, state of least energy. A ball resting in a hollow on a slope is a simple example of me ...

ranges of

metamorphic minerals Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

assemblages to the conditions 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 chem ...

. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as

metamorphic reaction A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions r ...

boundaries in a pressure–temperature

cartesian coordinate system A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in t ...

to produce a petrogenetic grid for a particular

rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...

composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in

geothermobarometry Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobarometry is a combination of ''geobarometry'', where a pressure of mineral formation is resolved, an ...

. Figure 1 is an example of a complex petrogenetic grid for metamorphosed pelitic rocks. It shows most of the important reactions that govern the development of aluminous mineral assemblages from the

prehnite-pumpellyite facies The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres. It is a metamorphic grade transitional between zeolite facies and greenschist facies repr ...

to the

granulite facies Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated ...

, as well as the

blueschist facies Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue co ...

and eclogite facies at higher pressures and the contact hornfels facies at lower pressures. As the rock undergoes higher temperatures and pressures, it follows the classic Barrovian sequence from the

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 chlorous ac ...

zone to the

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

zone to the

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

zone to the

staurolite Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc ...

zone. For a metapelitic rock containing

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

, and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

the petrogenetic grid for metapelites (Figure 1) shows that such a rock can only form at relatively low pressures and temperatures. However, if it had carpholite instead of chlorite, then it would have formed at higher pressures, and if it had

pyrophyllite Pyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al2Si4O10(OH)2. It occurs in two forms (habits): crystalline folia and compact masses; distinct crystals are not known. The folia have a pronounced pearly luster ...

instead of kaolinite, it would have formed at higher temperatures. This assumes the rock has a KFMASH (K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) composition because that is what the experimental data was created with. If the composition of the rock differs from this, then the figure is less accurate. Norman L. Bowen proposed the concept of petrogenetic grids in 1940. At the time, he envisioned geologists eventually determining every possible metamorphic reaction and assemblage in nature, but realized that the magnitude of undertaking the necessary experiments was a huge task that would not be finished for a very long time. As such, modern petrogenetic grids are only partially complete. Depending on the level of precision and characterization needed, a petrogenetic grid may be simple, or it may be an extremely large system consisting of a hundred or more reactions.

See also

References

Further reading