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Serpentine is not a single mineral but a group of common rock-forming hydrous magnesium iron phyllosilicate minerals. The group is characterized by its distinctive greasy or waxy luster, often mottled green colors resembling a snake's skin (hence the name), and a generally soft, slippery feel. The three main polymorphs are antigorite, lizardite, and chrysotile, each with distinct crystal structures and habits. Serpentinite is the rock composed predominantly of serpentine group minerals.
How to Identify
- Color
- Typically shades of green (light to dark, olive, yellowish-green, bluish-green), but can also be yellowish, brownish, black, or white. Often mottled or veined.
- Luster
- Greasy, waxy, silky (especially chrysotile), or dull.
- Texture
- Smooth, soapy, or slippery to the touch. Massive varieties can be fine-grained to cryptocrystalline. Fibrous varieties (chrysotile) are soft and flexible.
- Crystal Form
- Rarely forms distinct crystals. Usually massive, platy (antigorite, lizardite), or fibrous (chrysotile). Antigorite often forms lamellar or platy aggregates. Lizardite is typically fine-grained and massive. Chrysotile forms flexible, silky fibers.
- Cleavage
- Perfect basal cleavage in antigorite and lizardite, though often not observed in massive forms. Chrysotile exhibits fibrous parting rather than true cleavage.
- Geological Environment
- Predominantly found in serpentinites, which are metamorphic rocks derived from the hydrothermal alteration of ultramafic igneous rocks (peridotites, dunites) in oceanic crust, subduction zones, and orogenic belts. Also occurs in contact metamorphic aureoles around igneous intrusions into dolomitic limestones.
Key Facts
- Hardness: 2.5-5 on the Mohs scale (varies by polymorph; chrysotile is ~2.5-3, antigorite ~3.5-4, lizardite ~2.5-3.5)
- Specific Gravity: 2.5-2.6 g/cm³
- Crystal System: Monoclinic (antigorite, chrysotile), Trigonal (lizardite)
- Color: Green (most common), yellow, brown, black, white
- Luster: Greasy, waxy, silky (chrysotile), dull
- Transparency: Translucent to opaque
- Fracture: Conchoidal to splintery (massive forms), fibrous (chrysotile)
- Cleavage: Perfect basal cleavage (001) in antigorite and lizardite, but often not visible in massive forms. Chrysotile exhibits fibrous parting.
- Composition: Hydrous magnesium iron phyllosilicate
Quick Check
- Color: Green (various shades), yellowish, brownish, black
- Luster: Greasy, waxy, silky, dull
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, platy, lamellar, fibrous (chrysotile), pseudomorphic after olivine or pyroxene.
- Cleavage Type: Perfect basal (001) in antigorite and lizardite, but often indistinct or not observed. Fibrous parting in chrysotile.
- Fracture Type: Conchoidal to splintery in massive varieties; fibrous in chrysotile.
- Tenacity: Brittle to flexible (chrysotile fibers are flexible and inelastic).
- Luster Type: Greasy, waxy, silky, dull.
Formation
Serpentine minerals form through the serpentinization of ultramafic rocks (e.g., peridotite, dunite) at low to moderate temperatures (typically 200-500 °C) and pressures, in the presence of water. This hydrothermal alteration process involves the hydration and chemical transformation of primary magnesium-rich silicate minerals like olivine and pyroxene. The general reaction for olivine serpentinization is: 2Mg2SiO4 (olivine) + 3H2O = Mg3Si2O5(OH)4 (serpentine) + Mg(OH)2 (brucite).
Usage
Historically, serpentine (especially chrysotile) was extensively used as asbestos due to its heat resistance and fibrous nature. Today, due to health concerns, its use is severely restricted. Massive serpentine varieties (serpentinite) are used as decorative building stone, dimension stone, and for carving. Some translucent varieties (e.g., bowenite) are used as gemstones or ornamental stones. It is also a source of magnesium.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, wherever ultramafic rocks have undergone hydrothermal alteration.
Where to Find
California, USA
Extensive serpentinite belts, particularly in the Coast Ranges and Sierra Nevada foothills, where it is the state rock. Associated with ophiolite complexes.
Appalachian Mountains, USA
Occurs in numerous localities from Georgia to Newfoundland, associated with ancient oceanic crust remnants.
The Alps, Europe
Well-known for serpentinite occurrences, particularly in Italy, Switzerland, and Austria, related to Alpine orogeny.
Urals Mountains, Russia
Significant deposits of serpentinite and associated minerals.
Quebec, Canada
Historically a major source of chrysotile asbestos, particularly in the Thetford Mines region.
Cyprus
Part of the Troodos Ophiolite, a classic example of oceanic crust and mantle exposed on land.
Finding Tips
Look for Ultramafic Rock Associations
Serpentine is almost exclusively found in areas where ultramafic rocks (dark, dense igneous rocks rich in magnesium and iron, like peridotite or dunite) have been altered. Look for outcrops of these rocks.
Identify Green, Greasy Rocks
Serpentine often has a distinctive olive-green to dark green color and a greasy or waxy luster. It feels smooth and sometimes soapy to the touch.
Check for Veins and Fractures
Serpentine often forms in veins or as massive bodies within altered ultramafic rocks. Look for areas with extensive fracturing and hydrothermal alteration.
Be Aware of Asbestos Varieties
If you encounter fibrous serpentine (chrysotile), exercise extreme caution. Do not disturb or collect it without proper safety equipment and knowledge of asbestos handling protocols. It is best to avoid collecting fibrous forms altogether.
Similar Rocks
Talc
Mg3Si4O10(OH)2
Also known as: Steatite (for massive variety)
Chlorite
(Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6
Also known as: None specific
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 (antigorite, chrysotile), Trigonal (lizardite)
- Chemical Formula
- Mg3Si2O5(OH)4 (idealized)
- Composition
- Magnesium, Silicon, Oxygen, Hydrogen (with minor iron and other substitutions)
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