Chlorite group
Green phyllosilicate minerals common in low-grade metamorphic rocks.
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The chlorite group is a collection of phyllosilicate minerals that are particularly common in low-grade metamorphic rocks and in igneous rocks that have undergone alteration. Despite its name, chlorite contains no chlorine; the name comes from the Greek word chloros, meaning "green," a reference to the mineral's characteristic color. This green hue is responsible for the color of many rocks, including greenschist, which forms from the metamorphism of basalt or other low-silica volcanic rocks and typically contains significant amounts of chlorite. Chlorite minerals exhibit a wide range of chemical compositions, with magnesium, iron, aluminium, and silicon readily substituting for one another in the crystal structure.
A complete solid solution series exists between the two most common end members: magnesium-rich clinochlore and iron-rich chamosite. Other varieties containing manganese, zinc, lithium, or calcium are also known. This compositional variability leads to considerable differences in physical, optical, and X-ray properties, and it allows chlorite minerals to remain stable across a broad range of temperature and pressure conditions. As a result, chlorite is ubiquitous in low- and medium-temperature metamorphic rocks, some igneous rocks, hydrothermal rocks, and deeply buried sediments.
Properties
The mineral forms blue-green crystals that resemble mica, but its plates are flexible rather than elastic and are less easily pulled apart. Structurally, chlorite is described as having a TOT-O arrangement, consisting of alternating talc-like layers and brucite-like layers. It is considered a clay mineral, but it is nonswelling and has a relatively low cation exchange capacity. Chlorite is a common secondary mineral in igneous rocks, often forming from the alteration of mafic minerals like biotite, hornblende, or pyroxene.
Occurrence
It is also a widespread weathering product and is found in clay-rich sedimentary rocks. In metamorphic geology, chlorite is the diagnostic mineral of the greenschist facies, and it defines the chlorite zone, the zone of mildest metamorphism identified by G.M. Barrow in the Scottish Highlands. At higher temperatures, chlorite is destroyed by reactions that produce biotite, muscovite, and quartz. Chlorite is also a common product of hydrothermal propylitic alteration, occurring with epidote, actinolite, and calcite.
Lore & Background
Chlorite minerals typically form blue-green crystals that resemble mica, but unlike mica, their plates are flexible rather than elastic and are less easily pulled apart. The name derives from the Greek word for green, referencing their common color, though they contain no chlorine. The group is defined by a TOT-O structure: alternating talc-like TOT layers (tetrahedral-octahedral-tetrahedral) and positively charged brucite-like O layers.
In chlorite, some aluminium substitutes for silicon in the TOT layers, giving them a negative charge, which is balanced by the O layers; this contrasts with talc, whose neutral TOT layers are bound only by weak forces, and with mica, whose charged TOT layers are bonded by individual cations. Chlorite is a nonswelling clay mineral with low cation exchange capacity, as water is not adsorbed between its layers. Composition varies widely, with magnesium, iron, aluminium, and silicon substituting for one another; a complete solid solution exists between magnesium-rich clinochlore and iron-rich chamosite.
Manganese, zinc, lithium, and calcium varieties also occur. This chemical range allows chlorite to form under many temperature and pressure conditions. It is common in low-grade metamorphic rocks and altered igneous rocks, often coloring slates, schists, and greenstones. Chlorite is a key mineral in the greenschist facies, formed near 450°C and 5 kbar, and is also produced by hydrothermal alteration and weathering.
It appears in clay, sedimentary rocks, and even in mantle peridotite above subducting oceanic lithosphere. Common varieties include clinochlore, chamosite, and pennantite; a massive clinochlore variety used for carving is called seraphinite. Chlorite has no major industrial uses, though chlorite schist has been employed as roofing granules and decorative stone.
Reader's Guide
Chlorite is a common mineral found in metamorphic, igneous, and sedimentary rocks. It is an important rock-forming mineral in low- to medium-grade metamorphic rocks formed from mafic or pelitic protoliths. In igneous rocks, it typically occurs as a secondary mineral from alteration of mafic minerals such as biotite, hornblende, pyroxene, or garnet. The green color of many igneous rocks, slates, and schists is due to disseminated chlorite.
G.M. At higher temperatures, chlorite is destroyed by reactions producing biotite, muscovite, and quartz. Chlorite also forms by hydrothermal propylitic alteration and may be stable in mantle peridotite above subducting ocean lithosphere. It has no major industrial uses but appears in decorative stone and roofing granules.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Chlorite group (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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