Serpentinite

Serpentinite is a rock formed by the hydration and metamorphism of ultramafic rocks, typically peridotite or dunite, through a process called serpentinization. The dominant minerals are serpentine group minerals—antigorite, lizardite, and chrysotile—with accessory minerals such as magnetite, talc, chlorite, carbonate minerals, and brucite. The presence of chrysotile means that some serpentinites may contain asbestos fibers, a consideration for health and safety in handling and use.

Formation and setting: Serpentinite forms at low to moderate temperatures during hydrothermal alteration of ultramafic rocks at tectonic plate boundaries, especially in subduction zones and along ophiolite belts. It is commonly associated with ultramafic and metamorphosed terrains and is widespread in ophiolite complexes around the world, often marking ancient oceanic lithosphere that has been tectonically emplaced.

Properties and occurrence: Serpentinite typically appears green to olive, with a waxy or silky luster and a soft feel (Mohs hardness around 3–4). It can have a relatively low density and may fracture along serpentine planes. The rock occurs in ophiolite sequences and in metamorphic belts where ultramafic rocks have been tectonically exposed. Its occurrence is of interest in geology for studying hydration processes in the mantle and the tectonic history of convergent margins.

Uses and safety: Serpentinite is used as a decorative or dimension stone in some contexts and has been a source of talc or magnesium in certain varieties. The potential asbestos content in chrysotile-bearing serpentinite means that some varieties are subject to regulatory and health considerations.