chlorit

Chlorit, commonly spelled chlorite in English, is a group of phyllosilicate minerals that form a distinct mineral family within the mica-like chlorite group. These minerals are characterized by a green to olive color and a micaceous, foliated habit. The chlorite group is defined by a three-layer structure in which a brucite-like octahedral sheet is sandwiched between two tetrahedral sheets, giving a 2:1:1 stacking arrangement. The composition varies with magnesium, iron, and aluminum, with silicon and hydroxide completing the framework, which produces a range of green shades.

End-members and varieties: The chlorite group includes several minerals with different Mg-Fe-Al compositions. Common end-members are clinochlore (Mg-rich) and chamosite (Fe-rich). Other derivatives and related minerals exist, reflecting substitutions that occur during geological processes.

Formation and occurrence: Chlorite forms during low-grade regional metamorphism and hydrothermal alteration of mafic to ultramafic rocks. It is a common constituent in greenschist and lower-grade schists and can occur in alteration halos around ore veins. In hand specimens, chlorite appears as flaky, plate-like crystals or masses and is frequently intergrown with muscovite, epidote, quartz, serpentine, and talc.

Physical properties: Chlorite minerals are typically green to dark green, with a vitreous to pearly luster. They have a Mohs hardness around 2–3 and exhibit perfect basal cleavage, producing platy, micaceous grains. The streak is usually white to pale green, and specific gravity ranges roughly from 2.6 to 3.3.

Uses and significance: Chlorite is primarily of interest as a mineralogical indicator of metamorphic conditions and as a minor rock-forming constituent. It is not a major ore of any element.