| Chlorite group | |
|---|---|
 | |
| General | |
| Category | Phyllosilicates |
| Formula | (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 |
| IMA symbol | Chl[1] |
| Crystal system | Monoclinic 2/m; with sometriclinic polymorphs. |
| Identification | |
| Color | Various shades of green; rarely yellow, red, or white. |
| Crystal habit | Foliated masses, scaley aggregates, disseminated flakes. |
| Cleavage | Perfect 001 |
| Fracture | Lamellar |
| Mohs scale hardness | 2–2.5 |
| Luster | Vitreous, pearly, dull |
| Streak | Pale green to grey |
| Specific gravity | 2.6–3.3 |
| Refractive index | 1.57–1.67 |
| Other characteristics | Folia flexible – not elastic |
| References | [2][3] |
Thechlorites are the group ofphyllosilicateminerals common in low-grademetamorphic rocks and inalteredigneous rocks.Greenschist, formed by metamorphism ofbasalt or other low-silica volcanic rock, typically contains significant amounts of chlorite.
Chlorite minerals show a wide variety of compositions, in which magnesium, iron, aluminium, and silicon substitute for each other in the crystal structure. A completesolid solution series exists between the two most common end members, magnesium-richclinochlore and iron-richchamosite. In addition, manganese, zinc, lithium, and calcium species are known. The great range in composition results in considerable variation in physical, optical, andX-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.
The namechlorite is from theGreekchloros (χλωρός), meaning "green", in reference to its color. Chlorite minerals do not contain the elementchlorine, also named from the same Greek root.
Chlorite forms blue-green crystals resemblingmica. However, while the plates are flexible, they are not elastic like mica, and are less easily pulled apart.Talc is much softer and feels soapy between the fingers.[4][5]
The typical general formula for chlorite is(Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6. This formula emphasizes the structure of the group, which is described asTOT-O and consists of alternatingTOT layers andO layers.[3] TheTOT layer (Tetrahedral-Octahedral-Tetrahedral =T-O-T) is often referred to as a talc layer, since talc is composed entirely of stackedTOT layers. TheTOT layers of talc are electrically neutral and are bound only by relatively weakvan der Waals forces. By contrast, theTOT layers of chlorite contain some aluminium in place of silicon, which gives the layers an overall negative charge. TheseTOT layers are bound together by positively chargedO layers, sometimes calledbrucite layers. Mica is also composed of aluminium-rich, negatively chargedTOT layers, but these are bonded together by individual cations (such as potassium, sodium, or calcium ions) rather than a positively charged brucite layer.[6]
![Crystal structure of chlorite viewed along [100] (looking along the layers)](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aChlorite_100.jpg)
![Chlorite structure viewed along [001] showing pseudohexagonal structure](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aChlorite_001.jpg)
Chlorite is considered aclay mineral. It is a nonswelling clay mineral,[7] since water is not adsorbed in the interlayer spaces, and it has a relatively lowcation exchange capacity.[8]
Chlorite is a common mineral, found in metamorphic, igneous, and sedimentary rocks. It is an important rock-forming mineral in low- to medium-grade metamorphic rock formed by metamorphism ofmafic orpelitic rock.[9] It is also common in igneous rocks, usually as a secondary mineral, formed by alteration of mafic minerals such asbiotite,hornblende,pyroxene, orgarnet.[10] The glassy rims ofpillow basalt on the ocean floor is often altered to pure chlorite, in part by exchange of chemicals with seawater.[11] The green color of many igneous rocks,slates, andschists is due to fine particles of chlorite disseminated throughout the rock.[10] Chlorite is a commonweathering product and is widespread inclay and in sedimentary rock containing clay minerals.[9] Chlorite is found inpelites along withquartz,albite,sericite, andgarnet, and is also found in associate withactinolite andepidote.[10]
In his pioneering work onmetamorphic facies in the Scottish Highlands,G.M. Barrow identified the chlorite zone as the zone of mildest metamorphism.[12] In modern petrology, chlorite is the diagnostic mineral of thegreenschist facies.[10] This facies is characterized by temperatures near 450 °C (840 °F) and pressures near 5 kbar.[13] At higher temperatures, much of the chlorite is destroyed by reactions with eitherpotassium feldspar orphengitemica which producebiotite,muscovite, andquartz. At still higher temperatures, other reactions destroy the remaining chlorite, often with release of water vapor.[14]
Chlorite is one of the most common minerals produced bypropylitic alteration byhydrothermal systems, where it occurs in the "green rock" environment with epidote, actinolite, albite,hematite, andcalcite.[15]
Experiments indicate that chlorite can be stable inperidotite of theEarth's mantle above the oceanlithosphere carried down bysubduction, and chlorite may even be present in the mantle volume from whichisland arcmagmas are generated.[16][17]
| Baileychlore | IMA1986-056 | (Zn,Fe2+,Al,Mg)6(Al,Si)4O10(O,OH)8 |
|---|---|---|
| Borocookeite | IMA2000-013 | LiAl4(Si3B)O10(OH)8 |
| Chamosite | year: 1820 | (Fe,Mg)5Al(Si3Al)O10(OH)8 |
| Clinochlore | year: 1851 | (Mg,Fe2+)5Al(Si3Al)O10(OH)8 |
| Cookeite | year: 1866 | LiAl4(Si3Al)O10(OH)8 |
| Donbassite | year: 1940 | Al2[Al2.33][Si3AlO10](OH)8 |
| Gonyerite | year: 1955 | (Mn,Mg)5(Fe3+)2Si3O10(OH)8 |
| Nimite | year: 1968 | (Ni,Mg,Al)6(Si,Al)4O10(OH)8 |
| Pennantite | year: 1946 | (Mn5Al)(Si3Al)O10(OH)8 |
| Ripidolite | chlinochlore var. | (Mg,Fe,Al)6(Al,Si)4O10(OH)8 |
| Sudoite | IMA1966-027 | Mg2(Al,Fe)3Si3AlO10(OH)8 |
Clinochlore, pennantite, and chamosite are the most common varieties. Several other sub-varieties have been described. A massive compact variety of clinochlore used as a decorative carving stone is referred to by the trade nameseraphinite. It occurs in the Korshunovskoye ironskarn deposit in theIrkutsk Oblast of EasternSiberia.[18]
Chlorite does not have any specific industrial uses of any importance. Some rock types containing chlorite, such as chlorite schist, have minor decorative uses or as construction stone. However, chlorite is a common mineral inclay, which has a vast number of uses.[9]
Chlorite schist has been used as roofing granules, the mineral granules adhered to asphalt composition shingles due to the green color. It was quarried near Ely, Minnesota, US, until superseded by synthetic materials.[citation needed]
