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.