metamorphic
Serpentinite
Serpentinite (Mg, Fe, Ni)₃Si₂O―(OH)₄
Hardness: 3-6 on Mohs scale; Color: Dark green to black, often with mottled white veins; Luster: Greasy, waxy, or silky; Crystal structure: Monoclinic; Cleavage: Poor; Specific gravity: 2.5-3.2. It frequently has a 'soapy' or slippery feel.
- Hardness
- 3-6 on Mohs scale
- Color
- Dark green to black, often with mottled white veins
- Luster
- Greasy, waxy, or silky
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Physical properties
Hardness: 3-6 on Mohs scale; Color: Dark green to black, often with mottled white veins; Luster: Greasy, waxy, or silky; Crystal structure: Monoclinic; Cleavage: Poor; Specific gravity: 2.5-3.2. It frequently has a 'soapy' or slippery feel.
Formation & geological history
Formed through the hydration of ultramafic rocks (like peridotite) from the Earth's mantle. This process, called serpentinization, typically occurs at tectonic plate boundaries (subduction zones) at low temperatures and high pressures.
Uses & applications
Used as a decorative architectural stone, in floor tiles, counter tops, and as a source for magnesium. Historically used for carving and as a heat-resistant material. Some varieties (asbestos-bearing) were used for insulation, though this is now restricted for health reasons.
Geological facts
Serpentinite is the official state rock of California. It is unique because it contains high levels of heavy metals like chromium and nickel, making its soil ('serpentine soil') toxic to many plants but host to rare, endemic species.
Field identification & locations
Identify in the field by its dark green color and 'snake-skin' appearance with waxy, polished surfaces (slickensides) caused by tectonic movement. Commonly found in ophiolite complexes, the Coast Ranges of California, and the Alps. Collectors should look for fibrous or translucent varieties.
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