Serpentinite from the Precambrian of Michigan, USA. (3.6 centimeters across at its widest)

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Serpentinite forms by metamorphism of olivine-rich peridotites (dunites - ultramafic, phaneritic, intrusive igneous rocks). Metamorphism of olivine in the presence of water results in the formation of the mineral serpentine (Mg3Si2O5(OH)4). A metamorphic rock composed principally of serpentine is thus a serpentinite.

Serpentinite has a mottled greenish color, has the look & feel of hard candle wax, and ranges in texture from crystalline to “foliated”. Many serpentinites have a foliated look to them, but it’s really not due to an alignment of crystals. The appearance of “foliated” serpentinites is really the result of extensive development of slickenlined surfaces.

Many serpentinites also have a small component of magnetite that is usually significant enough to feel a slight tug when a magnet is placed next to the rock.

Many Precambrian greenstone belts have significant occurrences of serpentinites. Slices of dunitic mantle caught up in orogenic belts by obduction (= ophiolites) are often serpentinized. Sometimes, mantle peridotite masses that were caught up in rising magmas have been serpentinized (for example, in kimberlites & lamproites).

The serpentinite shown above is composed of picrolite antigorite serpentine. It comes from a picrolite vein in metamorphosed Precambrian peridotite.

Geologic unit & age: Deer Lake Peridotite, late Neoarchean