Chlorite group

Chlorite group

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.

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 comprised 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

Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxene, amphibole, and biotite. Chlorite is a common mineral associated with hydrothermal ore deposits and commonly occurs with epidote, sericite, adularia and sulfide minerals. 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 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 magmas are generated.

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.

The name chlorite is from the Greek chloros, meaning "green", in reference to its color.

See also

References

  • Cornelius S. Hurlbut and Cornelis Klein, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
  • Timothy L. Grove, Nilanjan Chatterjee, Stephen W. Parman, and Etienne Medard, 2006, The influence of H2O on mantle wedge melting. Earth and Planetary Science Letters 249, p. 74-89.
  • Mineral Galleries
  • Mindat.org
  • Chlorite - Maricopa edu

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