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Chlorite Group

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2 - 2½
A group of mostly monoclinic (also triclinic or orthorhombic) micaceous phyllosilicate minerals with a structure consisting of T-O-T layers with two layers having their silicate tetrahedral apices pointing towards each other, separated by an interlayer that may be simple octahedrally coordinated cations or which may be a brucite-*like* layer of two sheets of closely packed OH groups with the interstices between sheets providing the octahedral cordination site; the T-O-T layers and interlayer are bonded by electrostatic and hydrogen bonding forces; as the "a" and "b" directions of the T-O-T layer may be oriented to the interlayer "a" and "b" directions in twelve different stacking sequences, resulting in twelve different polytype possibilities (not all of which have been found in Nature yet for each species).

The general formula may be stated A5-6T4Z18, where A = Al, Fe2+, Fe3+, Li, Mg, Mn, or Ni, while T = Al, Fe3+, Si, or a combination of them, and Z = O and/or OH.

The most common species in the chlorite group are clinochlore and chamosite.

The Árkai index (ÁI; Árkai, 1991) is an index for the estimation of very low metamorphic grade in rocks, based on the evolution of chlorite. It is often used to complement the Kübler index (KI), earlier called the 'illite crystallinity' for the characterisation of pelites, and can also be applied in metabasites.

Classification of Chlorite Group

Physical Properties of Chlorite Group

Hardness (Mohs):
2 - 2½
2.6 - 3.3 g/cm3 (Measured)    

Crystallography of Chlorite Group

Crystal System:

Relationship of Chlorite Group to other Species

Group Members:
Baileychlore (Zn,Fe2+,Al,Mg)6(Si,Al)4O10(OH)8
Borocookeite Li1+3xAl4-x(BSi3O10)(OH)8
Chamosite (Fe2+,Mg,Al,Fe3+)6(Si,Al)4O10(OH,O)8
Clinochlore Mg5Al(AlSi3O10)(OH)8
Cookeite (Al2Li)Al2(AlSi3O10)(OH)8
Donbassite Al4.33(AlSi3O10)(OH)8
Franklinfurnaceite Ca2Fe3+Mn32+Mn3+(Zn2Si2O10)(OH)8
Glagolevite Na(Mg,Al)6(AlSi3O10)(OH,O)8
Gonyerite (Mn2+,Mg)5Fe3+(Fe3+Si3O10)(OH)8
Nimite (Ni,Mg,Al)6((Si,Al)4O10)(OH)8
Orthochamosite (Fe2+,Mg,Fe3+)5Al(AlSi3O10)(OH,O)8
Pennantite Mn52+Al(AlSi3O10)(OH)8
Sudoite Mg2Al3(Si3Al)O10)(OH)8

Other Names for Chlorite Group

Name in Other Languages:
Simplified Chinese:绿泥石族

Other Information

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Chlorite Group

Reference List:
Dodge, F.C.W. (1973): Chlorites from the granitic rocks of the central Sierra Nevada batholith. Mineralogical Magazine, 39, 58-64.

Post, J.L. and Janke, N.C. (1974): Properties of “swelling chlorite” in some Mesozoic formations of California. Clays and Clay Minerals, 22, 67-77.

Hazen, R.M. and Finger, L.W. (1978): The crystal structures and compressibilities of layer minerals at high pressure. II. Phlogopite and chlorite. American Mineralogist, 63, 293-296.

S. Durovic, K. Dornberger-Schiff and Z. Weiss (1983): Chlorite polytypism. I. OD interpretation and polytype symbolism of chlorite structures. Acta Crystallogr. B39, 547-552.

Spinnler, G.E. (1985): HRTEM study of antigorite, pyroxene-serpentine reactions and chlorite, 248 p. PhD Thesis, Arizona State University, Tempe, Arizona.

Schreyer, W., Fransolet, A.M., and Abraham, K. (1986): A miscibility gap in trioctahedral Mn-Mg-Fe chlorites: evidence from the Lienne Valley manganese deposit, Ardennes, Belgium. Contributions to Mineralogy and Petrology, 94, 333-342.

Bailey, S. W. (1988): Chlorites: Structures and crystal chemistry. Reviews in Mineralogy 19, 347-403.

Wiewiora, A. & Weiss, Z. (1990): Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: II. The chlorite group. Clay Minerals 25, 83-92.

Árkai, P. (1991): Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary. J. Metamorphic Geology 9, 723-734.

Hillier, S. and Velde, B. (1991): Octahedral occupancy and the chemical composition of disgenetic (low-temperature) chlorite. Clay Minerals, 26, 149-168.

De Caritat, P., Hutcheon, I. & Walshe, J.E. (1993): Chlorite geothermometry: a review. Clays and Clay Minerals, 41, 219-239.

Árkai, P. and Ghabrial, D. (1997): Chlorite crystallinity as an indicator of metamorphic grade of low-temperature meta-igneous rocks; a case study from the Bükk Mountains, Northeast Hungary. Clay Minerals, 32, 205-222.

Li, G., Peacor, D.R., and Essene, E.J. (1998) The formation of sulfides during alteration of biotite to chlorite-corrensite. Clays and Clay Minerals, 46, 649-657.

Lougear, A., M. Grodzicki, C. Bertoldi, A.X. Trautwein, K. Steiner, and G. Amthauer (2000): Mössbauer and molecular orbital study of chlorites. Physica and Chemistry of Minerals, 27, 258-269.

Bertoldi, C., Benisek, A., Cemic, L., and Dachs, E. (2001): The heat capacity of two natural chlorite group minerals derived from differential scanning calorimetry. Physics and Chemistry of Minerals, 28, 332-336.

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Localities for Chlorite Group

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