Chloritisation

Chloritisation is a metamorphic process where minerals in rocks undergo chemical alteration, primarily through the addition of magnesium and iron, and the removal of silicon and aluminum. This process is most commonly associated with low- to medium-grade metamorphism, particularly in the presence of hydrothermal fluids rich in magnesium and iron. Chloritisation is a key reaction in the formation of chlorite, a group of phyllosilicate minerals characterized by their green color and sheet-like structure.

The process typically occurs in mafic and ultramafic rocks, such as basalt and gabbro, as well as in sedimentary rocks containing clay minerals. During chloritisation, unstable minerals like pyroxene, amphibole, or biotite are transformed into chlorite, often accompanied by the formation of other secondary minerals such as epidote, quartz, and carbonate. This alteration is driven by changes in temperature, pressure, and fluid composition, typically within the greenschist facies of metamorphism, though it can also occur in lower-grade environments under specific conditions.

Chloritisation plays a significant role in the stabilization of rocks, contributing to the formation of more stable mineral assemblages under metamorphic conditions. It is also important in economic geology, as it can affect the weathering and alteration of ore deposits, influencing their stability and potential for exploitation. Additionally, chloritisation can impact the physical properties of rocks, such as their strength and permeability, which are relevant in engineering and environmental studies.

The process is often studied in conjunction with other metamorphic reactions, such as serpentinisation and albitisation, to understand the broader evolution of rock compositions during metamorphism. Chloritisation is particularly notable in ophiolite sequences, where it helps define the transition zones between different rock types in oceanic crust. Overall, this metamorphic alteration is a fundamental mechanism in the transformation of rock materials under geological conditions.