Chlorite group


Chlorite group
Chlorite group
General
Category Mineral
Chemical formula (Mg,Fe)3(Si,Al)4O10
(OH)2·(Mg,Fe)3(OH)6
Identification
Color Various shades of green; rarely yellow, red, or white.
Crystal habit Foliated masses, scaley aggregates, disseminated flakes.
Crystal system Monoclinic 2/m; with some triclinic polymorphs.
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

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.

  • Clinochlore: (Mg5Al)(AlSi3)O10(OH)8
  • Chamosite: (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 rocks and deeply buried sediments.

Contents

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 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:

-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

Quartz crystal with chlorite inclusions from Minas Gerais, Brazil (size: 4.2 x 3.9 x 3.3 cm)

Chlorite is commonly found in igneous rocks 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 minerals. 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.

Chlorite pseudomorph after garnet from Michigan (size: 3.5 x 3.1 x 2.7 cm)

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 magmas 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.[1] Chlorite is found in large boulders scattered on the ground surface on Ring Mountain in Marin County, California.[2]

Members of the chlorite group

Chlorite schist
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.

The name chlorite is from the Greek 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 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.

See also

References

  1. ^ Greenly, E. (1902). "The Origin and Associations of the Jaspers of South-eastern Anglesey". Quarterly Journal of the Geological Society 58: 425–440. doi:10.1144/GSL.JGS.1902.058.01-04.29. 
  2. ^ [1] C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed A. Burnham

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Look at other dictionaries:

  • Chlorite — For the mineral type, see chlorite group. For the neutral chemical compound, see chlorine dioxide. The chlorite ion The chlorite ion is ClO2−. A chlorite (compound) is a compound that contains this group, with chlorine in oxidation state +3.… …   Wikipedia

  • chlorite — chlorite1 chloritic /klaw rit ik, kloh /, adj. /klawr uyt, klohr /, n. a group of minerals, hydrous silicates of aluminum, ferrous iron, and magnesium, occurring in green platelike crystals or scales. [1595 1605; CHLOR 1 + ITE1] chlorite2 /klawr… …   Universalium

  • Chlorite — Chlo rite, n. [Gr. ? (sc. ?), fr. chlwro s light green.] (Min.) The name of a group of minerals, usually of a green color and micaceous to granular in structure. They are hydrous silicates of alumina, iron, and magnesia. [1913 Webster] {Chlorite… …   The Collaborative International Dictionary of English

  • Chlorite slate — Chlorite Chlo rite, n. [Gr. ? (sc. ?), fr. chlwro s light green.] (Min.) The name of a group of minerals, usually of a green color and micaceous to granular in structure. They are hydrous silicates of alumina, iron, and magnesia. [1913 Webster]… …   The Collaborative International Dictionary of English

  • chlorite — chlorite1 [klôr′īt΄] n. a salt of chlorous acid containing the monovalent, negative radical ClO2 chlorite2 [klôr′īt΄] n. [L chloritis < Gr chlōritis < chlōros, pale green] any of a group of usually greenish, soft, monoclinic minerals,… …   English World dictionary

  • chlorite — I. noun Etymology: German Chlorit, from Latin chloritis, a green stone, from Greek chlōritis, from chlōros Date: 1794 any of a group of usually green silicate minerals associated with and resembling the micas • chloritic adjective II. noun… …   New Collegiate Dictionary

  • chlorite — I. /ˈklɔraɪt/ (say klawruyt) noun a group of minerals consisting of hydrous silicates of aluminium, ferrous iron, and magnesium, occurring in green plate like crystals or scales, and common in some metamorphic rocks. {Greek chlōritis kind of… …   Australian English dictionary

  • chlorite — n. mineral group; salt of chlorous acid …   English contemporary dictionary

  • chlorite — I chlo•rite [[t]ˈklɔr aɪt, ˈkloʊr [/t]] n. mir a group of usu. green minerals, hydrous silicates of aluminum, ferrous iron, and magnesium, occurring in platelike crystals or scales • Etymology: 1785–95; prob. < Lchlōrītīsa green precious… …   From formal English to slang

  • Chalk Group — The Needles, part of the extensive Southern England Chalk Formation. The Chalk Group (often just called the Chalk) is a lithostratigraphic unit (a certain number of rock strata) in the northwestern part of Europe. It is characterised by thick… …   Wikipedia


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