metamorphic
Serpentinite
Serpentinite, dominated by serpentine-group minerals (chrysotile, lizardite, antigorite), commonly with magnetite and chlorite
AI-generated identification · may be incorrectTypically dark green to black, mottled, waxy to greasy luster, and massive or finely fibrous; Mohs hardness is about 2.5–5.5, with poor cleavage and specific gravity near 2.5–2.6. The specimen’s green-black color and mottled texture are consistent with serpentinite, though photo identification is not definitive.
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Physical properties
Typically dark green to black, mottled, waxy to greasy luster, and massive or finely fibrous; Mohs hardness is about 2.5–5.5, with poor cleavage and specific gravity near 2.5–2.6. The specimen’s green-black color and mottled texture are consistent with serpentinite, though photo identification is not definitive.
Formation & geological history
Forms when ultramafic mantle rocks such as peridotite are hydrated during hydrothermal alteration, commonly along faults and subduction zones. It can form from Precambrian through modern geological settings.
Uses & applications
Used as decorative stone, carvings, countertops, and ornamental specimens; historically used as a source of asbestos-bearing material. Avoid cutting or grinding unknown samples because some serpentinite contains fibrous chrysotile asbestos.
Geological facts
Serpentinite is California’s state rock and commonly contains magnetite, giving some pieces a magnetic response. It is often associated with chromite, talc, and nickel-bearing minerals.
Field identification & locations
Field clues include a green-black mottled appearance, relatively soft surface, waxy feel, and possible magnetism; a streak, hardness, and asbestos-safe laboratory test help confirm it. Common localities include California, the Alps, New Zealand, Japan, and ophiolite belts worldwide.
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