chloritisering

Chloritisering, also spelled chloritisation, is the geological alteration process in which chlorite-group minerals crystallize during metamorphism or hydrothermal alteration, replacing existing minerals in rocks. The result is a chlorite-rich assemblage that can accompany other secondary minerals such as sericite, epidote, or carbonates.

Occurrence and setting: Chloritisering is common in pelitic and mafic rocks and is characteristic of low- to intermediate-grade metamorphism, particularly within greenschist facies. It can occur as pervasive alteration or as discrete halos around intrusions, faults, or fluid pathways. The intensity and mineralogy of the chlorite depend on the rock type and fluid chemistry.

Mechanisms: Fluids carrying chlorine, magnesium, iron, aluminum, and water interact with primary minerals (feldspars, pyroxenes, biotite, etc.), driving their replacement by chlorite and related phyllosilicates. The chlorite composition ranges from magnesium-rich to iron-rich varieties (for example, clinochlore to ripidolite), reflecting the Fe/Mg ratio of the fluid-rock system. Textures may show stepwise replacement, preserved grain outlines, or intergrowths with other alteration minerals.

Diagnostics and significance: Chloritisering is identifiable petrographically by greenish chlorite sheets and characteristic X-ray or microprobe signatures. It informs interpretations of metamorphic grade, fluid history, and alteration halos. In economic geology, chlorite-rich zones can mark hydrothermal pathways and are considered in exploration models for related mineralization.

See also: chlorite, chloritisation, sericitisation, greenschist facies, hydrothermal alteration.