Chlorites


Also found in: Dictionary, Thesaurus, Medical.

Chlorites

 

a group of widely occurring minerals; the hydrous meta-aluminumosilicates of Mg and Fe, with a layered, mica-like crystal structure. The chemical composition of chlorites is (Mg, Fe2+) · [AlSi3O10(OH)2] · 3(Mg, Fe)(OH)2. Al isomorphically substitutes for Si within the limits Si7Al═Si4Al4 and for Mg within the limits Mg11 Al═Mg4Al4. Mg2+ may be entirely replaced by iron Fe2+ and Fe3+ and also partially by Mn2+, Cr, Ni, Ti, Li, or other elements. Scientists distinguish trioctahedral chlorites, dioctahedral chlorites, and chlorites with partially or completely disordered crystal structures. The layered crystal structure of chlorites is responsible for the great abundance of polymorphic modifications, or polytypes. Mixed layered formations of the chlorite-montmorillonite and chlorite-vermiculite (corrensite) types frequently occur. A distinction is also made between orthochlorites (unoxidized, containing not more than 4 percent Fe2O3) and leptochlorites (oxidized, rich in Fe2O3) according to the Fe2+/Fe3+ ratio.

Orthochlorites constitute a large group of minerals, which differ in the total iron content, that is, the magnitude of the Fe/(Fe + Mg) ratio of the octahedral layers, and in the Si/Al ratio in the tetrahedrons. The following orthochlorites are distinguished: (1) magnesium orthochlorites (in order of increasing Si content), which include corundophilite, sheridanite, clinochlore, penninite, and talc chlorite; (2) iron-magnesium orthochlorites, which include ripidolite, pycnochlorite, and diabantite; and (3) iron orthochlorites, which include pseudothuringite, daphnite, and brunsvigite. Leptochlorites include thuringite, chamosite, and delessite. There are also manganese chlorites (pennanite and gonyerite), chromium chlorites (kámmererite and kotschubeite), lithium chlorites (cookeite), and other types of chlorites.

A precise identification of chlorites is possible using X-ray diffraction analysis, electrographic methods, and thermal analysis. Chlorites crystallize in the monoclinic or triclinic system. They have a mica-like, lamellar pseudohexagonal crystal habit and exhibit perfect cleavage. The hardness on Mohs’ scale varies from 1.5 to 2.5. Chlorite lamellae are flexible but not elastic. The density of the minerals range from 2,600 to 3,300 kg/m3. Chlorites occur in the form of lamellar, plumose, globular, or cryptocrystalline oolitic aggregates. Their color usually ranges from light green to dark green, although white, yellow (low-iron), pink, red violet (containing Cr and Mn), and black (iron chlorite) varieties are also known.

Orthochlorites are important rock-forming minerals of the greenschists, which are rocks of the initial stages of regional metamorphism. They are characteristic of altered rocks lying near to ores in hydrothermal deposits and of altered lavas of volcanic regions. The processes of chloritization are widespread in nature and occur at relatively low temperatures. Chlorites often occur as alteration products of higher temperature iron-magnesium silicates, such as biotite and the amphiboles; they also replace scapolites, plagioclases, garnets, vesuvianite, staurolite, and many other minerals, with the formation of pseudomorphs. Chlorites appear in large quantities, together with talc and serpentine, during the hydrothermal transformation of ultrabasic rocks, volcanic tuffs, shales, and sometimes even dolomites. They are often present in ore-bearing quartz veins and aureoles around veins. Lithium chlorites are found in rare metal pegmatites, chromium chlorites occur in chromite deposits, and nickel chlorites form during the alteration of certain basic igneous rocks. The leptochlorites thuringite and chamosite are primarily sedimentary in origin. They sometimes form large bodies of industrial importance, such as the iron ores in the Urals, Thuringia, and the Lorraine.

REFERENCES

Serdiuchenko, D. P. Khlority, ikh khimicheskaia konstitutsiia i klassifikatsiia. Moscow, 1953. (Tr. In-ta geologich. nauk AN SSSR, fasc. 140.)
Kepezhinskas, K. B. Statisticheskii analiz khloritov i ikh parageneticheskietipy. Moscow, 1965.
Deer, W. A., R. A. Howie, and J. Zussman. Porodoobrazuiushchie mineraly, vol. 3: Listovye silikaty. Moscow, 1966. (Translated from English.)
Rostov, I. Mineralogiia. Moscow, 1971. (Translated from English.)
Godovikov, A. A. Mineralogiia. Moscow, 1975.

A. M. PORTNOV and L. G. FEL’DMAN

References in periodicals archive ?
Experimental alteration of a chlorite into regularly interstratified chlorite-vermiculite by chemical oxidation.
Relative intensities of chlorite 004 peaks for burn vs.
Chlorites Control in European Drinking Water Treatment Plants, in WEFTEC 07 Proceedings, San Diego, USA, 14-19 October, S014.
2] application is the formation of undesired and regulated inorganic by-products: chlorite (Cl[O.
Chlorite and chlorate were measured according to the U.
Crystallization began after chlorite had formed, but was already in progress before the oldest calcites began to form, and continued after formation of the youngest crystals of Calcite-III.
Probably the growth of chlorite was fairly continuous over a long period: chlorite is found as inclusions in Calcite-I, Calcite-II and Quartz-I, and it is found under, as well as grown over, crystals of these minerals.
1992) Etude mineralogique et geochimique du gisement de talc et chlorite de Trimouns (Pyrenees, France).
1993) Fluid-mineral interactions at the origin of the Trimouns talc and chlorite deposit (Pyrenees, France) In Hach-Ali, Ruiz and Gervilla, Eds.
1989) Comparative mineralogy, geochemistry and conditions of formation of two metasomatic talc and chlorite deposits: Trimouns (Pyrenees, France) and Rabenwald (Eastern Alps, Austria).
Based on the above data the loess-like substrate of Central Yakutia is enriched in chlorite in comparison with the loess-like parent material of European Russia, where a "hydromica-smectite" composition has been reported (Gradusov, 2000).
In the Cambic Turbic Cryosol (Reductaquic) (Pit 1-07) there is a decrease in the smectitic phase in the more acidic upper horizons, as well as a decrease in the chlorite content.