Chlorite group
Encyclopedia
The chlorites are a group of phyllosilicate
mineral
s. 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.
In addition, zinc, lithium, and calcium species are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal
rocks and deeply buried sediments.
Chlorites have a 2:1 sandwich structure (2:1 sandwich layer = tetrahedral-octahedral-tetrahedral = t-o-t...), this is often referred to as a talc
layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space (the space between each 2:1 sandwich filled by a cation) is composed of (Mg2+, Fe3+)(OH)6. This (Mg2+, Fe3+)(OH)6 unit is more commonly referred to as the brucite
-like layer, due to its closer resemblance to the mineral brucite (Mg(OH)2). Therefore, chlorite's structure appears as follows:
An older classification divided the chlorites into two subgroups: the orthochlorites and leptochlorites. The terms are seldom used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic.
s as an alteration product of mafic
minerals such as pyroxene
, amphibole
, and biotite
. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a metasomatism
product via addition of Fe, Mg, or other compounds into the rock mass. Chlorite is a common mineral associated with hydrothermal ore
deposits and commonly occurs with epidote
, sericite
, adularia and sulfide mineral
s. Chlorite is also a common metamorphic
mineral, usually indicative of low-grade metamorphism. It is the diagnostic species of the zeolite
facies and of lower greenschist
facies. It occurs in the quartz
, albite
, sericite
, chlorite, garnet
assemblage of pelitic schist
.
Within ultramafic rocks, metamorphism can also produce predominantly clinochlore chlorite in association with talc
.
Experiments indicate that chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere
carried down by subduction
, and chlorite may even be present in the mantle volume from which island arc
magma
s are generated.
Chlorite occurs naturally in a variety of locations and forms. For example, chlorite is found naturally in certain parts of Wales
in mineral schists. Chlorite is found in large boulders scattered on the ground surface on Ring Mountain
in Marin County, California
.
Clinoclore, pennantite, and chamosite are the most common varieties. Several other sub-varieties have been described. A gem-grade form of clinochlore is popularly referred to by the trade name Seraphinite
.
The name chlorite is from the Greek
chloros (χλωρός), meaning "green", in reference to its color.
Talc
is much softer and feels soapy between fingers. The powder generated by scratching is white.
Mica
plates are elastic whereas chlorite plates are flexible without bending back.
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
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. Chlorites can be described by the following four endmembers
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.
- Clinochlore: (Mg5Al)(AlSi3)O10(OH)8
- ChamositeChamositeChamosite is the Fe2+end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore. Like other chlorites, it is a product of the hydrothermal...
: (Fe5Al)(AlSi3)O10(OH)8 - Nimite: (Ni5Al)(AlSi3)O10(OH)8
- Pennantite: (Mn,Al)6(Si,Al)4O10(OH)8
In addition, zinc, lithium, and calcium species are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal
Hydrothermal circulation
Hydrothermal circulation in its most general sense is the circulation of hot water; 'hydros' in the Greek meaning water and 'thermos' meaning heat. Hydrothermal circulation occurs most often in the vicinity of sources of heat within the Earth's crust...
rocks and deeply buried sediments.
Chlorite structure
The typical general formula is: (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6. This formula emphasises the structure of the group.Chlorites have a 2:1 sandwich structure (2:1 sandwich layer = tetrahedral-octahedral-tetrahedral = t-o-t...), this is often referred to as a talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space (the space between each 2:1 sandwich filled by a cation) is composed of (Mg2+, Fe3+)(OH)6. This (Mg2+, Fe3+)(OH)6 unit is more commonly referred to as the brucite
Brucite
Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...
-like layer, due to its closer resemblance to the mineral brucite (Mg(OH)2). Therefore, chlorite's structure appears as follows:
- -t-o-t-brucite-t-o-t-brucite ...
An older classification divided the chlorites into two subgroups: the orthochlorites and leptochlorites. The terms are seldom used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic.
Occurrence
Chlorite is commonly found in igneous rockIgneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s as an alteration product of mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
minerals such as pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, 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...
. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
product via addition of Fe, Mg, or other compounds into the rock mass. Chlorite is a common mineral associated with hydrothermal ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
deposits and commonly occurs with epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
, sericite
Sericite
Sericite is a fine grained mica, similar to muscovite, illite, or paragonite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars in areas that have been subjected to hydrothermal alteration typically associated with copper, tin, or other hydrothermal ore deposits...
, adularia and sulfide mineral
Sulfide mineral
The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...
s. Chlorite is also a common metamorphic
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...
mineral, usually indicative of low-grade metamorphism. It is the diagnostic species of the zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
facies and of lower greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies. It occurs in the quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
, sericite
Sericite
Sericite is a fine grained mica, similar to muscovite, illite, or paragonite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars in areas that have been subjected to hydrothermal alteration typically associated with copper, tin, or other hydrothermal ore deposits...
, chlorite, 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...
assemblage of pelitic 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...
.
Within ultramafic rocks, metamorphism can also produce predominantly clinochlore chlorite in association with talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
.
Experiments indicate that chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
carried down by subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
, and chlorite may even be present in the mantle volume from which island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
s are generated.
Chlorite occurs naturally in a variety of locations and forms. For example, chlorite is found naturally in certain parts of Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
in mineral schists. Chlorite is found in large boulders scattered on the ground surface on Ring Mountain
Ring Mountain (California)
Ring Mountain is an elevated landform on the Tiburon Peninsula in Marin County, California. This mountain was named for George E. Ring, who served as a Marin County Supervisor from 1895 to 1903.A number of rare and endangered flora inhabit Ring Mountain...
in Marin County, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
.
Members of the chlorite group
| Baileychlore | (Zn,Fe+2,Al,Mg)6(Al,Si)4O10(O,OH)8 |
|---|---|
| Chamosite | (Fe,Mg)5Al(Si3Al)O10(OH)8 |
| Clinochlore | (Mg,Fe2+)5Al(Si3Al)O10(OH)8 |
| Cookeite | LiAl4(Si3Al)O10(OH)8 |
| Donbassite | Al2[Al2.33][Si3AlO10](OH)8 |
| Gonyerite | (Mn,Mg)5(Fe+3)2Si3O10(OH)8 |
| Nimite | (Ni,Mg,Al)6(Si,Al)4O10(OH)8 |
| Odinite | (Fe,Mg,Al,Fe,Ti,Mn)2.4(Al,Si)2O5OH4 |
| Orthochamosite | (Fe+2,Mg,Fe+3)5Al(Si3Al)O10(O,OH)8 |
| Pennantite | (Mn5Al)(Si3Al)O10(OH)8 |
| Ripidolite | (Mg,Fe,Al)6(Al,Si)4O10(OH)8 |
| Sudoite | Mg2(Al,Fe)3Si3AlO10(OH)8 |
Clinoclore, pennantite, and chamosite are the most common varieties. Several other sub-varieties have been described. A gem-grade form of clinochlore is popularly referred to by the trade name Seraphinite
Seraphinite
Seraphinite is a trade name for a particular form of clinochlore, a member of the Chlorite group.Seraphinite apparently acquired its name due to its resemblance to feathers, such as one might find on a bird's wing...
.
The name chlorite is from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
chloros (χλωρός), meaning "green", in reference to its color.
Distinguishing from other minerals
Chlorite is so soft that it can be scratched by a finger nail. The powder generated by scratching is green. It feels oily when rubbed between the fingers. The plates are flexible, but not elastic like mica.Talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
is much softer and feels soapy between fingers. The powder generated by scratching is white.
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...
plates are elastic whereas chlorite plates are flexible without bending back.