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 ?
The absorption features of chlorites located between 2250 and 2340 nm (4440 and 4270 [cm.sup.-1]) can be explicitly separated into their constituent absorption bands using the Gaussian-Lorentz model [18], and the mathematical shape description of the individual absorptions is accurate [19-24].
The paragenetic sequence includes compaction, early carbonate cementation, chlorite film, illite-smectite mixed-layer clays, feldspar dissolution, precipitation of kaolinite and quartz, montmorillonite illitization, and weak carbonate cementation [45].
The clay minerals are represented by illite, kaolinite, chlorite, smectite and relatively rare mixed-layer structure of chlorite-smectite (corrensite), which is identified for the first time in soils of the Czech Republic.
The 1.4-nm peak in HIV is often overlapped by peaks of other clay minerals such as vermiculite, chlorite, illite-smectite mixed clays and hydroxy-interlayered smectite (HIS) in X-ray diffraction, which makes the identification of HIV difficult in samples of polymineralic composition (Y in et al.
Shaw (1980) postulated that in the presence of alkaline pore water, the smectites are progressively transformed into illite through dehydration, absorption and alkaline cations, and lattice rearrangements via illite/smectite mixed layers (or the promotion of chlorite formation via chlorite/smectite mixed-layer phases depending on whether the pore waters are potassium or magnesium rich).
Chlorite is common in S-stage, which occurs either in molybdenite-bearing quartz veins (Type I chlorite, Figures 3(e) and 3(f)) or as replacement of hornblende (Type II chlorite, Figure 3(d)).
The minerals of the chlorite group are widely represented in the zone close to the footwall of the talc-chlorite structure.
Chlorite comprised the majority of the clay mineralogy of the soil (Fig.
Of all of the minerals in the assemblage, chlorite crystallized over the longest time period: it began to form simultaneously with the first generation of quartz, and finished during the second generation of quartz.
Mixed-layer clays with minor proportions of smectite or vermiculite layers are recognized by the asymmetries of the 10.0 [Angstrom] illite and 14 [Angstrom] chlorite peaks on XRD patterns of Mg saturated and heated samples respectively and suggest the presence of random mixed layer structures (Figs.