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Serpentinite is a metamorphic rock composed predominantly of one or more serpentine group minerals (antigorite, lizardite, chrysotile). It typically exhibits a distinctive greasy or waxy luster, a smooth or fibrous texture, and a range of green, yellowish-green, or brownish-green colors, often with mottled or veined patterns. Its formation is a key process in subduction zones and mid-ocean ridges, where oceanic crust and mantle rocks are hydrated. Serpentinite is often associated with ophiolite complexes, which represent fragments of oceanic lithosphere obducted onto continental margins.
How to Identify
- Color
- Typically various shades of green (light to dark green, yellowish-green, bluish-green), often mottled or veined with black, white, or reddish-brown.
- Luster
- Greasy, waxy, silky (especially fibrous varieties like chrysotile), or dull.
- Texture
- Smooth, slippery, fibrous, or granular. Often feels soapy to the touch. Can be massive, foliated, or schistose.
- Crystal Form
- Serpentine minerals typically form microscopic to macroscopic aggregates. Antigorite and lizardite are platy or massive, while chrysotile is fibrous.
- Cleavage
- Serpentine minerals generally have good to perfect cleavage in one direction (basal), but in massive serpentinite, it may not be apparent. Fibrous chrysotile exhibits excellent fibrous parting.
- Geological Environment
- Found in areas of oceanic crust and upper mantle that have undergone hydration, such as mid-ocean ridges, subduction zones, and obducted ophiolite complexes. Often associated with fault zones and shear zones.
Key Facts
- Hardness: 2.5-4 (Mohs scale)
- Specific Gravity: 2.5-2.9 g/cm
- Crystal System: Monoclinic (antigorite, lizardite), Orthorhombic (chrysotile)
- Color: Green, yellowish-green, brownish-green, blackish-green
- Luster: Greasy, waxy, silky, dull
- Transparency: Translucent to opaque
- Fracture: Conchoidal to splintery (fibrous varieties)
- Cleavage: Good to perfect basal cleavage in individual serpentine minerals, but often not visible in massive rock. Fibrous chrysotile exhibits excellent fibrous parting.
- Composition: Hydrous magnesium silicate, primarily composed of serpentine group minerals (Mg3Si2O5(OH)4)
Quick Check
- Color: Green (various shades), often mottled
- Luster: Greasy, waxy, silky
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, platy, fibrous, lamellar, or granular aggregates.
- Cleavage Type: Good to perfect basal cleavage (001) in individual serpentine minerals, but often obscured in rock masses.
- Fracture Type: Conchoidal, splintery (fibrous varieties), or uneven.
- Tenacity: Flexible, sectile (chrysotile), or brittle.
- Luster Type: Greasy, waxy, silky, dull.
Formation
Serpentinite forms through the process of serpentinization, a low-temperature hydrothermal alteration of ultramafic rocks (peridotite, dunite, pyroxenite) rich in olivine and pyroxene. This process involves the reaction of these primary minerals with water (H2O) at temperatures typically between 200-500 C, often in the presence of dissolved CO2. The hydration reactions convert olivine and pyroxene into serpentine group minerals (antigorite, lizardite, chrysotile), magnetite, and brucite. The volume increase during serpentinization can lead to significant deformation and fracturing of the rock.
Usage
Historically, serpentinite has been used as a building and ornamental stone due to its attractive green colors and ability to take a polish. It is also used for carving and sculptures. In some regions, it has been used as a source of magnesium and as a soil amendment. Chrysotile asbestos, a fibrous variety of serpentine, was extensively used in insulation, brake linings, and other industrial products, but its use has been largely phased out due to health concerns.
Age Distribution
Ranges from Precambrian to Cenozoic, depending on the age of the protolith and the metamorphic event.
Where to Find
California, USA
Extensive serpentinite belts are found throughout California, particularly in the Coast Ranges and Sierra Nevada foothills, where they are associated with ophiolite complexes and fault zones. It is the state rock of California.
Appalachian Mountains, USA/Canada
Serpentinite bodies are present in the Appalachian orogen, representing remnants of ancient oceanic crust and mantle.
Oman Ophiolite
One of the best-exposed and studied ophiolite complexes globally, with vast outcrops of serpentinized peridotite.
Cyprus Ophiolite (Troodos Massif)
Another classic ophiolite complex with significant serpentinite occurrences.
Alps, Europe
Numerous serpentinite bodies are found within the Alpine orogenic belt, often associated with tectonic mélanges.
Urals, Russia
Extensive serpentinite occurrences are found in the Ural Mountains, related to ancient oceanic crust.
Finding Tips
Look for Ultramafic Associations
Serpentinite forms from ultramafic rocks, so look for areas known to have peridotite, dunite, or pyroxenite. These are often found in ophiolite sequences, which are fragments of oceanic crust and upper mantle obducted onto continents.
Identify Green, Soapy Rocks
Serpentinite is typically green and often has a distinctive greasy or waxy feel. It can be easily scratched with a knife (Mohs hardness 2.5-4).
Check for Veining and Shearing
Serpentinization often involves significant volume increase and deformation, leading to characteristic veining (e.g., magnesite, talc, chrysotile) and sheared textures.
Be Aware of Associated Minerals
Serpentinite can be associated with chromite, magnetite, talc, magnesite, and sometimes native metals like nickel or platinum group elements.
Safety Precautions for Asbestos
WARNING: Some serpentinite rocks contain chrysotile asbestos, a fibrous form of serpentine. Inhaling asbestos fibers can cause serious lung diseases, including asbestosis, lung cancer, and mesothelioma. When collecting or handling serpentinite, especially if it appears fibrous or friable, avoid creating dust. Do not cut, grind, or aggressively break samples without proper respiratory protection (e.g., P100 respirator) and ventilation. Wetting the rock can help suppress dust. If unsure, treat all fibrous serpentinite with extreme caution. It is advisable to avoid collecting or disturbing fibrous varieties unless you are a trained professional with appropriate safety equipment.
Similar Rocks
Soapstone
Talc-schist
Also known as: Steatite
Chlorite Schist
Chlorite-rich schist
Also known as: Greenstone
Jade
Nephrite (amphibole), Jadeite (pyroxene)
Also known as: Nephrite, Jadeite
Scientific Classification
- Mineral Class
- Phyllosilicates
- Group
- Serpentine Group
- Crystal System
- Monoclinic (antigorite, lizardite), Orthorhombic (chrysotile)
- Chemical Formula
- Mg3Si2O5(OH)4 (general formula for serpentine minerals)
- Composition
- Hydrous magnesium silicate, often with minor iron, nickel, aluminum, and chromium substitutions.
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