klorit

Klorit is the local name used for a group of phyllosilicate minerals in the chlorite family. In English-language mineralogy, they are referred to as chlorites. These green to greenish-black minerals form a widespread group characterized by a distinctive three-layer structural arrangement and variable chemical composition, mainly involving magnesium, iron and aluminum.

Structure and composition: Chlorites have a 2:1:1 layered structure, consisting of a brucite-like octahedral sheet sandwiched between two 2:1 silicate sheets. This arrangement gives the minerals a characteristic platy habit and perfect basal cleavage. The chemical composition varies along a continuum of Mg, Fe and Al substitutions, leading to different members within the group. Commonly mentioned end-members include clinochlore (Mg-rich) and chamosite (Fe-rich).

Occurrence and formation: Klorit forms under low to medium-grade metamorphic conditions, particularly in greenschist facies, and also by hydrothermal alteration of mafic or ultramafic rocks. It is a common constituent of schists, phyllites, marbles and altered rocks, often occurring with other hydrous minerals such as talc, serpentine and mica.

Physical properties: Chlorite minerals are typically green and have a vitreous to pearly luster. They exhibit perfect basal cleavage and relatively soft hardness, generally in the range of about 2 to 4 on the Mohs scale. Specific gravity is commonly between 2.6 and 3.5. Thin flakes may be translucent and show a characteristic green color due to iron and magnesium content.

Geological significance: Chlorite acts as an index mineral for the chlorite zone in metamorphism, indicating relatively low to intermediate temperatures and pressures during rock formation. It is widely used in petrographic studies to infer metamorphic conditions.

Name and distribution: The name derives from Greek chloros, meaning "green," and lithos, meaning "stone." Klorit occurs worldwide in metamorphic and hydrothermally altered rocks.