Serpentinite

Description

Serpentinite is a metamorphic rock composed predominantly of one or more serpentine group minerals. It typically has a distinctive greasy or waxy luster and a smooth, often mottled or variegated appearance, resembling a snake's skin, from which it derives its name. Its color ranges from dark green to yellowish-green, brownish-green, or black. It is generally soft, easily scratched, and can feel soapy or slippery to the touch. Serpentinite often contains relict minerals from its ultramafic protolith, such as chromite, magnetite, and sometimes olivine or pyroxene.

How to Identify

Color

Typically dark green, yellowish-green, olive green, brownish-green, or black; often mottled or variegated.

Luster

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

Texture

Massive, fibrous, platy, or granular. Often feels smooth or soapy to the touch. Can exhibit a 'snake-skin' like pattern.

Crystal Form

Serpentine minerals are typically cryptocrystalline or microcrystalline, forming aggregates. Fibrous varieties (chrysotile) can show delicate, flexible fibers. Platy varieties (antigorite, lizardite) form fine-grained masses.

Cleavage

Serpentine minerals generally have one perfect cleavage (basal), but in massive serpentinite, this is rarely observed as distinct planes due to the fine-grained, interlocking nature of the aggregates. Fibrous chrysotile shows excellent cleavage parallel to the fiber length.

Geological Environment

Associated with ophiolite complexes (fragments of oceanic crust and upper mantle obducted onto continental margins), subduction zones, and major fault zones where ultramafic rocks have been hydrated. Also found in metamorphic terrains where peridotites have undergone hydrothermal alteration.

Key Facts

• Hardness: 2.5-4 on the Mohs scale (for serpentine minerals).
• Specific Gravity: 2.5-2.8 g/cm³.
• Crystal System: Monoclinic (antigorite, chrysotile) or Orthorhombic (lizardite). Serpentinite as a rock is massive or aggregate, not a single crystal.
• Color: Dark green, yellowish-green, olive green, brownish-green, black; often mottled.
• Luster: Greasy, waxy, silky (fibrous varieties), dull.
• Transparency: Translucent to opaque.
• Fracture: Conchoidal to splintery or uneven.
• Cleavage: Perfect basal cleavage in individual serpentine minerals, but rarely observed in massive rock.
• Composition: Hydrous magnesium iron phyllosilicate (Mg,Fe)₃Si₂O₅(OH)₄, with varying proportions of antigorite, lizardite, and chrysotile.

Quick Check

• Color: Green (various shades), black, brownish-green, often mottled.
• Luster: Greasy, waxy, silky, dull.
• Streak: White.

Physical Characteristics

• Crystal Habit: Massive, platy, fibrous, or granular aggregates. Individual serpentine minerals are typically microcrystalline.
• Cleavage Type: Perfect basal cleavage (001) in individual serpentine minerals, but generally not visible in the rock.
• Fracture Type: Conchoidal, splintery, or uneven.
• Tenacity: Brittle to flexible (chrysotile asbestos).
• Luster Type: Greasy, waxy, silky, dull.

Formation

Serpentinite forms through the process of serpentinization, a low-temperature hydrothermal alteration of ultramafic rocks (peridotite, dunite) from the Earth's mantle. This process involves the reaction of olivine and pyroxene with water, leading to the formation of serpentine group minerals (antigorite, lizardite, chrysotile), brucite, and magnetite. It commonly occurs in oceanic crust at mid-ocean ridges, subduction zones, and within continental collision zones where mantle rocks are exposed.

Usage

Historically, serpentinite has been used as a building and ornamental stone (often called 'verde antique' or 'Connemara marble'). It is also a source of magnesium and nickel. Chrysotile asbestos, a fibrous variety of serpentine, was extensively used in insulation, brake linings, and other industrial products due to its heat resistance and tensile strength, though its use is now heavily restricted or banned due to health hazards. Some varieties are carved into sculptures and jewelry.

Age Distribution

Found in rocks ranging from Precambrian to Cenozoic, typically associated with ancient and active orogenic belts and oceanic crust.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Phyllosilicates (Sheet Silicates)

Group

Serpentine Group Minerals (Antigorite, Lizardite, Chrysotile)

Crystal System

Monoclinic (Antigorite, Chrysotile), Orthorhombic (Lizardite)

Chemical Formula

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

Composition

Hydrous magnesium iron phyllosilicate, often with minor aluminum, nickel, and chromium substitutions. The rock serpentinite is composed predominantly of these minerals.

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