Serpentinites

Serpentinites are a group of metamorphic rocks dominated by serpentinite minerals, formed by the hydration and metamorphism of ultramafic rocks such as peridotite and dunite. The defining minerals are from the serpentine group—primarily antigorite, lizardite, and chrysotile—often with accessory magnetite, talc, chlorite, brucite, and carbonate minerals. They typically appear green to green-gray and are relatively low in silica and aluminum, with a softness that renders them easily altered at the surface.

Formation and textures arise during serpentinzation, a low-temperature and high-water activity process that can occur in ophiolitic belts and subduction zones. Fluids introduced into ultramafic rocks at shallow to moderate depths alter the original minerals to serpentine, talc, and other secondary phases, often releasing hydrogen gas. Serpentinites can exhibit a range of textures from as-homogeneous masses to foliated, shear-enhanced belts that reflect deformation during tectonic processes.

Occurrences and significance: serpentinites are common in ophiolite complexes, subduction-zone mélanges, and exhumed ultramafic terrains worldwide, including several well-known ophiolites and continental exposures. They serve as important indicators of mantle hydration and tectonic hydration histories and can influence rheology and seismic properties in subduction zones.

Economically and environmentally, serpentinites may host talc deposits and, in some settings, chrysotile asbestos-bearing material, which carries health and regulatory considerations. They are also utilized as decorative stones in some cases.