Sericitisering

Sericitisering is a geological hydrothermal alteration process in which feldspar is chemically and texturally altered to sericite, a fine-grained white mica. The term is used in petrology and economic geology to describe the development of micaceous replacement zones within rocks that have been exposed to circulating fluids. In many languages the process may be written as sericitisation or sericitisation; sericitisering reflects a diachronic form or regional spelling. The end product is the appearance of pale, silky micaceous laminae replacing original feldspar grains.

Mechanism and conditions: Sericitisation occurs as hydrothermal fluids deliver potassium, silica, water and other ions that alter feldspar minerals. Over time, feldspar reacts to produce sericite along with secondary minerals such as quartz, chlorite, and illite. The process is typically associated with low to moderate temperatures and pressures, such as those found in low-grade metamorphism, fault zones, and ore-forming systems. It often creates opaque, dull surfaces and a distinctive silky luster on hand specimen; microtextures show platy sericite replacing euhedral feldspar crystals along fractures and pore spaces.

Significance and identification: Sericitisation is used as an indicator of fluid-rock interaction and can mark alteration halos around hydrothermal ore deposits. It affects rock strength and porosity, sometimes facilitating further mineralization. Petrologists identify sericitization via petrographic microscopy and X-ray diffraction, while field indicators include pervasive fine-grained mica replacement and the presence of sericite as a dominant fine-grained mica in the altered zone. The process is distinguished from chloritization and illitization by the composition of the secondary mica and associated minerals.