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Concept
Chlorite group
Summary
The chlorites are the group of phyllosilicate minerals common in low-grade metamorphic rocks and in altered igneous rocks. Greenschist, formed by metamorphism of basalt or other low-silica volcanic rock, typically contains significant amounts of chlorite.
Chlorite minerals show a wide variety of compositions, in which magnesium, iron, aluminium, and silicon substitute for each other in the crystal structure. A complete solid solution series exists between the two most common end members, magnesium-rich clinochlore and iron-rich chamosite. In addition, manganese, zinc, lithium, and calcium species are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and deeply buried sediments.
The name chlorite is from the Greek chloros (χλωρός), meaning "green", in reference to its color. Chlorite minerals do not contain the element chlorine, also named from the same Greek root.
Chlorite forms blue-green crystals resembling mica. However, while the plates are flexible, they are not elastic like mica, and are less easily pulled apart. Talc is much softer and feels soapy between the fingers.
The typical general formula for chlorite is . This formula emphasizes the structure of the group, which is described as TOT-O and consists of alternating TOT layers and O layers. The TOT layer (Tetrahedral-Octahedral-Tetrahedral = T-O-T) is often referred to as a talc layer, since talc is composed entirely of stacked TOT layers. The TOT layers of talc are electrically neutral and are bound only by relatively weak van der Waals forces. By contrast, the TOT layers of chlorite contain some aluminium in place of silicon, which gives the layers an overall negative charge.
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