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Chlorite

A generally green or black secondary mineral, (Mg,Fe,Al)₆(Si,Al)₄O₁₀(OH)₈, often formed by metamorphic alteration of primary dark rock minerals, that appears as a spot of green and resembles mica. Usually of secondary origin, derived from the alteration of garnets, pyroxenes, amphiboles and biotite. Also commonly found in schists and partially altered basalt.

Chlorite is a common accessory mineral in low- to medium-grade regional metamorphic rocks and is the dominant mineral in chlorite schist. It can form by alteration of ferromagnesian minerals in igneous rocks and is found occasionally in pegmatites and vein deposits. It is a common constituent of altered basic rocks and of alteration zones around metallic ore bodies. Chlorite also can form by diagenetic processes in sedimentary rocks.

Widespread group of layer silicate minerals composed of hydrous aluminum silicates, usually of magnesium and iron. The name, from the Greek for "green," refers to chlorite's typical colour. Chlorites have a silicate layer structure similar to that in micas. They characteristically occur as alteration products of other higher temperature minerals and are most common in sedimentary and igneous rocks and in some metamorphic rocks. Although often thought of as being the green-colored equivalent of biotite, it does not have the same atomic arrangement as other micas. Its chemistry is quite variable. It is often associated with such minerals as epidote and actinolite. It is a major component of slates, phyllites, greenstone and greenschists. Some varieties form during the primary crystallization of hydrous magmas. Chlorite may also form in soils and persist as a detrital mineral in sedimentary rocks.

Chlorite is identified by its greenish color and similarities to the micas in its appearance. Individual chlorite sheets will be flexible, those of Muscovite and Biotite are brittle. Hardness will seem to be less than 2.5 due to slippage on cleavage planes. Most commonly the individual grains are too small to easily see the sheet structure. In many greenish rocks, the chlorite grains are too small to see.

Chemically, the chlorites are distinguished from the micas by the presence of a considerable amount of water (about 13%) and by not containing alkalis; from the soft, scaly, mineral talc they differ in containing aluminium (about 20%) as an essential constituent. The magnesia (up to 36%) is often in part replaced by ferrous oxide (up to 30%), and the alumina to a lesser extent by ferric oxide; alumina may also be partly replaced by chromic oxide, as in the rose-red varieties kammererite and kotschubeite.

The name Chlorite derives from the Greek word meaning green.

Chlorite is a group name. Due to the extensive ionic substitution, a number of mineral species have been identified within the group. Most common are penninite and clinochlore. The chlorites bend plastically. Chemical analysis is required to distinguish members of this group from each other (such as powder diffraction).

The others include: Chamosite, Cookeite, Gonyerite, Manandonite, Nimite, Orthochamosite, Sudoite.

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