Chlorite group

For the chemistry term, see chlorite.

The chlorites are a group of phyllosilicate minerals.Chlorites can be described by the following fourendmembers based on their chemistry via substitution ofthe 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 considerablevariation in physical, optical, and X-ray properties. Sim-ilarly, the range of chemical composition allows chloritegroup minerals to exist over a wide range of tempera-ture and pressure conditions. For this reason chloriteminerals are ubiquitous minerals within low and mediumtemperature metamorphic rocks, some igneous rocks,hydrothermal rocks and deeply buried sediments.The name chlorite is from the Greek chloros (),meaning green, in reference to its color.

1 Chlorite structureThe typical general formula is:(Mg,Fe)3(Si,Al)4O10(OH)2(Mg,Fe)3(OH)6. Thisformula emphasizes the structure of the group.Chlorites have a 2:1 sandwich structure (2:1 sandwichlayer = tetrahedral-octahedral-tetrahedral = t-o-t...), thisis often referred to as a talc layer. Unlike other 2:1clay minerals, a chlorites interlayer space (the spacebetween each 2:1 sandwich lled by a cation) is com-posed of (Mg2+, Fe3+)(OH)6. This (Mg2+, Fe3+)(OH)6unit is more commonly referred to as the brucite-likelayer, due to its closer resemblance to the mineral brucite(Mg(OH)2). Therefore, chlorites structure appears asfollows:

-t-o-t-brucite-t-o-t-brucite ...

An older classication divided the chlorites into two sub-groups: the orthochlorites and leptochlorites. The terms

are seldom used and the ortho prex is somewhat mis-leading as the chlorite crystal system is monoclinic andnot orthorhombic.

2 Occurrence

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

Chlorite is commonly found in igneous rocks as an al-teration product of mac minerals such as pyroxene,amphibole, and biotite. In this environment chloritemay be a retrograde metamorphic alteration mineral ofexisting ferromagnesian minerals, or it may be presentas a metasomatism product via addition of Fe, Mg, orother compounds into the rock mass. Chlorite is a com-mon mineral associated with hydrothermal ore depositsand commonly occurs with epidote, sericite, adularia andsulde minerals. Chlorite is also a common metamorphicmineral, usually indicative of low-grade metamorphism.It is the diagnostic species of the zeolite facies and oflower greenschist facies. It occurs in the quartz, albite,sericite, chlorite, garnet assemblage of pelitic schist.Within ultramac rocks, metamorphism can also pro-duce predominantly clinochlore chlorite in associationwith talc.Experiments indicate that chlorite can be stable in peri-

1

2 7 EXTERNAL LINKS

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

dotite of the Earths mantle above the ocean lithospherecarried down by subduction, and chlorite may even bepresent in the mantle volume from which island arcmagmas are generated.Chlorite occurs naturally in a variety of locations andforms. For example, chlorite is found naturally in cer-tain parts of Wales in mineral schists.[1] Chlorite is foundin large boulders scattered on the ground surface on RingMountain in Marin County, California.[2]

3 Members of the chlorite group

Chlorite schist

Clinoclore, pennantite, and chamosite are the most com-

mon varieties. Several other sub-varieties have beendescribed. A massive compact variety of clinochloreused as a decorative carving stone is referred to by thetrade name seraphinite. It occurs in the Korshunovskoyeiron skarn deposit in the Irkutskaya Oblast of EasternSiberia.[3]

4 Distinguishing from other min-erals

Chlorite is so soft that it can be scratched by a nger nail.The powder generated by scratching is green. It feels oilywhen rubbed between the ngers. The plates are exible,but not elastic like mica.Talc is much softer and feels soapy between ngers. Thepowder generated by scratching is white.Mica plates are elastic whereas chlorite plates are exiblewithout bending back.

5 See also List of minerals Thuringite

6 References[1] Greenly, E. (1902). The Origin and Associations

of the Jaspers of South-eastern Anglesey. Quar-terly Journal of the Geological Society 58: 425440.doi:10.1144/GSL.JGS.1902.058.01-04.29.

[2] C. Michael Hogan (2008) Ring Mountain, TheMegalithicPortal, ed A. Burnham

[3] Seraphinite on Mindat

Cornelius S. Hurlbut and Cornelis Klein, 1985,Manual of Mineralogy, 20th ed., John Wiley andSons, New York ISBN 0-471-80580-7

Grove, T; Chatterjee, N; Parman, S; Medard,E (2006). The inuence of H2O on mantlewedge melting. Earth and Planetary Science Let-ters 249: 7489. Bibcode:2006E&PSL.249...74G.doi:10.1016/j.epsl.2006.06.043.

Mineral Galleries Mindat.org Chlorite Maricopa edu

7 External links

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Chlorite structureOccurrenceMembers of the chlorite group Distinguishing from other mineralsSee alsoReferences External links Text and image sources, contributors, and licensesTextImagesContent license