metamorphic
Serpentinite
Serpentinite (Mg, Fe, Ni, Al, Zn, Mn)2-3(Si, Al, Fe)2O5(OH)4
Hardness: 3-6 (variable); Color: Green to greenish-black with waxy or silky luster; Crystal structure: Monoclinic; Cleavage: Poor; Texture: Greasy or waxy to the touch, sometimes fibrous.
- Hardness
- 3-6 (variable)
- Color
- Green to greenish-black with waxy or silky luster
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Physical properties
Hardness: 3-6 (variable); Color: Green to greenish-black with waxy or silky luster; Crystal structure: Monoclinic; Cleavage: Poor; Texture: Greasy or waxy to the touch, sometimes fibrous.
Formation & geological history
Formed through 'serpentinization,' a low-temperature metamorphic process where ultramafic rocks (like peridotite) from the Earth’s mantle are hydrated by water as they are pushed toward the surface, often at tectonic plate boundaries.
Uses & applications
Used as decorative stone, in construction (terrazzo), as a source of magnesium, and historically as a source of asbestos. High-quality varieties like 'Williamsite' are used in jewelry.
Geological facts
Serpentinite is the official state rock of California. It often creates 'serpentine barrens,' where the unique soil chemistry limits plant growth to only specialized species.
Field identification & locations
Identify by its characteristic green color, waxy/slick feel (often called 'slickensides'), and presence in fault zones or ophiolite complexes. It is soft enough to be scratched by a steel knife in most cases.
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