metamorphic
Serpentinite (serpentinized ultramafic rock)
Serpentinite; chiefly lizardite, chrysotile, and antigorite, commonly with magnetite and minor chromite or carbonate
AI-generated identification · may be incorrectTypically dark green to black, mottled, waxy to greasy luster, and massive with little visible crystal structure; Mohs hardness about 2.5–4, generally lacking obvious cleavage. The green matrix with rusty weathering and scattered pale patches is consistent with serpentinization, though a photo alone cannot confirm the…
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Physical properties
Typically dark green to black, mottled, waxy to greasy luster, and massive with little visible crystal structure; Mohs hardness about 2.5–4, generally lacking obvious cleavage. The green matrix with rusty weathering and scattered pale patches is consistent with serpentinization, though a photo alone cannot confirm the diagnosis.
Formation & geological history
Forms when peridotite or other mantle ultramafic rocks react with water during seafloor alteration or tectonic uplift in subduction zones. It commonly represents ancient oceanic mantle and may range from Paleozoic to Mesozoic or younger depending on locality.
Uses & applications
Used as decorative and carving stone, aggregate, and historically as a source of asbestos-bearing material; attractive pieces are collected, but fibrous varieties should not be cut or powdered.
Geological facts
Serpentinite can contain magnetite, making some specimens weakly magnetic, and commonly develops a distinctive green, mottled “snake-skin” appearance. Chrysotile-bearing material may pose inhalation hazards if disturbed.
Field identification & locations
Look for green mottling, waxy surfaces, low hardness, and possible magnetism; common occurrences include California, Oregon, Italy, New Zealand, and many convergent-margin belts. Acid testing and thin-section or X-ray analysis are needed to distinguish it from chlorite-rich rocks or altered basalt.
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