Serpentine

Description

Serpentinite is a metamorphic rock consisting almost entirely of serpentine group minerals. It typically has a greasy or waxy luster, a distinctive green to greenish-black color, and often exhibits a mottled, slickensided, or fibrous texture. It is relatively soft and can be easily scratched. The rock's name derives from its resemblance to a serpent's skin. Serpentinite is often associated with tectonic boundaries and ophiolite sequences, representing altered oceanic crust and upper mantle material.

How to Identify

Color

Typically shades of green (light green, olive green, dark green, greenish-black), often mottled or variegated. Can also be yellowish-green, brownish-green, or rarely reddish.

Luster

Greasy, waxy, silky (especially fibrous varieties), or dull.

Texture

Massive, platy, fibrous, or granular. Often feels smooth or soapy to the touch. May exhibit slickensides (polished, striated surfaces) due to tectonic movement.

Crystal Form

Individual serpentine minerals are typically microscopic, forming aggregates. Macroscopically, it appears massive, platy, or fibrous. Chrysotile (a serpentine mineral) forms flexible, silky fibers.

Cleavage

Serpentine minerals have perfect cleavage in one direction (basal), but in massive serpentinite, this is rarely observed macroscopically. Instead, the rock tends to fracture irregularly or splinter.

Geological Environment

Commonly found in ophiolite complexes (remnants of oceanic crust and upper mantle obducted onto continents), suture zones, fault zones, and areas of regional metamorphism where ultramafic rocks have been hydrated. Often associated with peridotite, gabbro, and basalt.

Key Facts

• Hardness: 2.5 - 4 on Mohs scale (for serpentine minerals, the rock itself can vary)
• Specific Gravity: 2.5 - 2.8 g/cm³
• Crystal System: Monoclinic (for antigorite, lizardite, chrysotile)
• Color: Green, yellowish-green, brownish-green, greenish-black
• Luster: Greasy, waxy, silky, dull
• Transparency: Translucent to opaque
• Fracture: Conchoidal, splintery, or uneven
• Cleavage: Perfect basal cleavage in individual serpentine minerals, but rarely observed in massive rock; rock typically fractures irregularly.
• Composition: Hydrous magnesium iron phyllosilicate minerals (Mg,Fe)₃Si₂O₅(OH)₄

Quick Check

• Color: Green to greenish-black, often mottled
• Luster: Greasy, waxy, or silky
• Streak: White

Physical Characteristics

• Crystal Habit: Massive, platy, fibrous (chrysotile), lamellar (antigorite), microcrystalline (lizardite).
• Cleavage Type: Perfect {001} in individual serpentine minerals, but not typically observed in the rock.
• Fracture Type: Conchoidal, splintery, or uneven.
• Tenacity: Flexible (chrysotile), sectile, or brittle.
• Luster Type: Greasy, waxy, silky, dull.

Formation

Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals. It forms through the serpentinization of ultramafic rocks (peridotite, dunite, pyroxenite) or mafic rocks (gabbro, basalt) in the presence of water. This process involves hydration and hydrolysis of olivine and pyroxene, typically at low to moderate temperatures (200-500 °C) and pressures. It can occur in various tectonic settings, including subduction zones (forearc serpentinites), mid-ocean ridges (oceanic serpentinites), and continental rifts, often associated with ophiolite complexes.

Usage

Historically and currently used as a decorative stone (e.g., 'verde antique' marble), for carving, and as a building material. Due to its heat resistance, some varieties were used as fire-resistant materials. However, its use is now limited due to the potential presence of asbestos minerals. Some serpentinites are mined for nickel, chromium, and magnesium, which are often associated with ultramafic rocks.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, wherever suitable protoliths and metamorphic conditions occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Phyllosilicates

Group

Serpentine Group Minerals (Antigorite, Lizardite, Chrysotile)

Crystal System

Monoclinic (Antigorite, Lizardite, Chrysotile)

Chemical Formula

(Mg,Fe)₃Si₂O₅(OH)₄ (general formula for serpentine minerals)

Composition

Hydrous magnesium iron phyllosilicate. The rock serpentinite is composed predominantly of these minerals, often with minor amounts of magnetite, chromite, talc, brucite, and relict olivine or pyroxene.

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