chloritisaatio

Chloritisation, or chloritisaatio, is a geological alteration process in which rocks are chemically and mineralogically altered to form chlorite-group minerals. This alteration commonly occurs during low-grade metamorphism or hydrothermal alteration and can affect a wide range of rock types, from pelitic to mafic rocks. Chlorite-group minerals include clinochlore, chamosite, and other members of the chlorite family.

The process generally develops under low to moderate metamorphic grades, such as greenschist facies, and is closely associated with hydrothermal fluid activity. Fluids introduce chlorine, hydroxyl, and aluminum into the rock, promoting replacement reactions that transform primary minerals into chlorite and related minerals while preserving some of the original texture in a pseudomorphic or partially recrystallized form. The result is often a greenish, chlorite-rich fabric that can define foliation or fracture networks.

Texturally, chloritisation commonly replaces existing minerals along foliation planes or in alteration halos surrounding fractures and ore bodies. It can modify the color, porosity, and mechanical properties of the rock and may occur as discrete chlorite rims, disseminated chlorite grains, or integrated chlorite in the matrix.

Significance in geology lies in its utility as an indicator of metamorphic grade and alteration history. Chloritisation helps map alteration zones, infer fluid flow paths, and constrain the thermal and chemical evolution of a rock during tectonic or hydrothermal processes. Identification typically relies on petrographic microscopy, X-ray diffraction, and electron microscopy, often supported by whole-rock chemistry and isotopic data to interpret the conditions of alteration.