Chlorite group
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 group | |
|---|---|
| General | |
| Category | Phyllosilicates |
| Formula (repeating unit) | (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 |
| IMA symbol | Chl |
| Crystal system | Monoclinic 2/m; with some triclinic polymorphs. |
| Identification | |
| Color | Various shades of green; rarely yellow, red, or white. |
| Crystal habit | Foliated masses, scaley aggregates, disseminated flakes. |
| Cleavage | Perfect 001 |
| Fracture | Lamellar |
| Mohs scale hardness | 2–2.5 |
| Luster | Vitreous, pearly, dull |
| Streak | Pale green to grey |
| Specific gravity | 2.6–3.3 |
| Refractive index | 1.57–1.67 |
| Other characteristics | Folia flexible – not elastic |
| References | |
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.