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The Serpentine Group refers to a group of common rock-forming minerals, primarily antigorite, chrysotile, and lizardite, which are hydrous magnesium iron phyllosilicates. The rock composed predominantly of these minerals is called serpentinite. Serpentine minerals are characterized by their distinctive greasy or waxy luster, often mottled green colors resembling a snake's skin (hence the name), and a relatively soft, often fibrous or platy habit. They are key indicators of past hydrothermal activity and tectonic processes involving oceanic crust.
How to Identify
- Color
- Typically shades of green (light to dark, olive, yellowish-green, bluish-green), but can also be yellowish, brownish, or black. Often mottled or veined.
- Luster
- Greasy, waxy, silky (especially fibrous varieties like chrysotile), or dull.
- Texture
- Massive, platy, fibrous, or granular. Often feels smooth or slightly soapy to the touch.
- Crystal Form
- Usually massive, fine-grained, or platy aggregates. Fibrous forms (chrysotile) are common. Individual crystals are rare and microscopic.
- Cleavage
- Poor to distinct in one direction (basal), but often not observed in massive varieties. Fibrous varieties show cleavage parallel to the fibers.
- Geological Environment
- Ultramafic igneous rocks (peridotite, dunite) that have undergone hydrothermal alteration (serpentinization) in oceanic crust, ophiolites, subduction zones, and metamorphic terrains.
Key Facts
- Hardness: 2.5 - 4 (Mohs scale)
- Specific Gravity: 2.5 - 2.9 g/cm
- Crystal System: Monoclinic or Orthorhombic (depending on the specific mineral within the group)
- Color: Green (various shades), yellowish, brownish, black; often mottled
- Luster: Greasy, waxy, silky, dull
- Transparency: Translucent to opaque
- Fracture: Conchoidal, splintery, or uneven
- Cleavage: Poor to distinct basal cleavage (001), often not observed in massive forms. Fibrous varieties show cleavage parallel to fibers.
- Composition: Hydrous magnesium iron phyllosilicate
Quick Check
- Color: Green (various shades), yellowish, brownish, black; often mottled
- Luster: Greasy, waxy, silky, dull
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, platy, fibrous (asbestiform), lamellar, granular
- Cleavage Type: Basal (001) - poor to distinct
- Fracture Type: Conchoidal, splintery, or uneven
- Tenacity: Brittle to tough (fibrous varieties)
- Luster Type: Greasy, waxy, silky, dull
Formation
Serpentine minerals form through the serpentinization of ultramafic rocks (peridotite, dunite) at low to moderate temperatures (200-500 C) and pressures, typically in the presence of water. This hydrothermal alteration process involves the hydration and metamorphic transformation of olivine and pyroxene minerals.
Usage
Historically used for carving, ornamental stone, architectural facings, and as a source of asbestos (chrysotile). Modern uses are more limited due to asbestos concerns, but it is still used as a decorative stone, for sculptures, and in some industrial applications where non-fibrous varieties are utilized.
Age Distribution
Found in rocks ranging from Precambrian to Cenozoic, often associated with ancient oceanic crust and subduction zones.
Where to Find
California, USA
Extensive serpentinite belts, particularly in the Coast Ranges and Sierra Nevada foothills, often associated with ophiolite complexes. California's state rock is serpentinite.
Appalachian Mountains, USA/Canada
Numerous occurrences of serpentinite and serpentine minerals, particularly in Newfoundland, Quebec, Vermont, and North Carolina.
Oman
Part of the Semail Ophiolite, one of the best-preserved ophiolite sequences globally, with vast exposures of serpentinized peridotite.
Italy
Significant occurrences in the Apennine Mountains and Alps, often used historically for architectural and decorative purposes.
Russia
Ural Mountains contain large serpentinite bodies, historically important for chrysotile asbestos mining.
Greece
Ophiolite complexes in various regions, including the Pindus Mountains.
Finding Tips
Look for Ultramafic Rocks
Serpentine is almost exclusively found in altered ultramafic rocks. Look for dark, heavy, often weathered outcrops in areas known for ophiolites or ancient oceanic crust.
Check for Greasy Luster and Soapy Feel
The characteristic greasy or waxy luster and smooth, sometimes soapy feel are good indicators. Use a knife to test hardness (2.5-4 on Mohs scale).
Observe Color and Mottling
The distinctive green colors, often with darker mottling or veining, are highly characteristic. Look for variations in green hues.
Beware of Asbestos
Fibrous varieties (chrysotile) are asbestos. If you encounter fibrous material, do not disturb it, collect it, or inhale any dust. Treat all fibrous serpentine with extreme caution. Non-fibrous varieties are generally safe to handle.
Geological Maps
Consult geological maps for areas marked with 'ultramafic rocks,' 'serpentinite,' or 'ophiolite' occurrences.
Similar Rocks
Talc
Mg3Si4O10(OH)2
Also known as: Steatite, Soapstone
Chlorite
(Mg,Fe)3(Si,Al)4O10(OH)2 (Mg,Fe)3(OH)6
Also known as: Clinochlore, Chamosite
Jade
Ca2(Mg,Fe)5Si8O22(OH)2 (Nephrite); Na(Al,Fe3+)Si2O6 (Jadeite)
Also known as: Nephrite, Jadeite
Scientific Classification
- Mineral Class
- Phyllosilicates
- Group
- Serpentine Group
- Crystal System
- Monoclinic (lizardite, chrysotile) or Orthorhombic (antigorite)
- Chemical Formula
- Mg3Si2O5(OH)4 (general formula for the group)
- Composition
- Hydrous magnesium iron phyllosilicate, with varying amounts of Fe, Al, Ni, and other trace elements substituting for Mg.
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