Chloriet

Chlorite is a group of green, micaceous sheet silicate minerals in the phyllosilicate family. They form mainly as alteration products of mafic rocks and as low- to medium-grade metamorphic and hydrothermal products. Chlorite minerals are typically green, with a pearly to vitreous luster, and show perfect basal cleavage, giving a foliated or scaly appearance. They are relatively soft, with a Mohs hardness of about 2 to 2.5.

Chemically, chlorites have variable compositions dominated by magnesium, iron, and aluminum, with silicon and hydroxide groups. They share a characteristic layered structure consisting of a brucite-like octahedral sheet sandwiched between two tetrahedral silicate sheets, forming a 2:1:1 configuration that underpins their micaceous habit and cleavage. End-member varieties include clinochlore (Mg-rich) and chamosite (Fe-rich).

Geologic occurrence and formation: Chlorite is common in greenschist- to lower metamorphic grade rocks and in hydrothermally altered rocks derived from mafic precursors. Its presence often indicates relatively low-temperature metamorphism. In metamorphic petrology, chlorite concentration and chemistry are used to estimate conditions of metamorphism, and the mineral defines a chlorite zone or index.

Etymology and usage: The name derives from the Greek chloros, meaning green. While not a primary ore mineral, chlorite is an important indicator mineral in petrography and mineralogy, and it contributes to the coloration and texture of many metamorphosed and altered rocks.