Serpentinite with Magnetite Veins

Metamorphic Rock (Serpentinite) with accessory mineral (Magnetite)

Serpentinite (Mg,Fe)3Si2O5(OH)4 with Magnetite (Fe3O4)

Also known as: Serpentine rock with Magnetite, Ophiolite (when part of an ophiolite sequence)

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Description

Serpentinite with Magnetite Veins is a metamorphic rock primarily composed of serpentine group minerals, characterized by its typically green, greenish-black, or mottled appearance, often with a greasy or waxy luster. The presence of magnetite veins manifests as distinct, often dark gray to black, metallic streaks or bands cutting through the serpentinite matrix. These veins can range from hairline fractures to several centimeters in thickness. The magnetite itself is opaque, strongly magnetic, and has a metallic luster. The overall texture of the rock can be massive, foliated, or fibrous, depending on the dominant serpentine polymorph and the degree of deformation. The magnetite veins provide a stark contrast in color and luster to the surrounding serpentinite.

How to Identify

Color: Serpentinite matrix: typically shades of green (light to dark), greenish-black, yellowish-green, or mottled. Magnetite veins: dark gray to black.
Luster: Serpentinite matrix: greasy, waxy, silky (fibrous varieties), or dull. Magnetite veins: metallic.
Texture: Serpentinite matrix: often massive, fine-grained, sometimes fibrous (chrysotile), or platy (antigorite). Can feel smooth or slightly soapy. Magnetite veins: granular to massive, often sharp and distinct within the serpentinite.
Crystal Form: Serpentine minerals are typically anhedral to subhedral, forming interlocking masses or fibrous aggregates. Magnetite forms euhedral to anhedral crystals, often granular within veins.
Cleavage: Serpentine minerals: variable, often poor or absent in massive varieties, good basal cleavage in antigorite, fibrous cleavage in chrysotile. Magnetite: no true cleavage, but exhibits octahedral parting.
Geological Environment: Associated with ophiolite complexes, subduction zones, and suture zones where oceanic crust and upper mantle rocks have been obducted or tectonically emplaced. Also found in metamorphic terrains where ultramafic intrusions have undergone hydration.

Key Facts

Hardness: Serpentinite: 2.5-4 (Mohs). Magnetite: 5.5-6.5 (Mohs).
Specific Gravity: Serpentinite: 2.5-2.8 g/cm³. Magnetite: 5.17-5.18 g/cm³.
Crystal System: Serpentine minerals: Monoclinic (antigorite, lizardite) or Orthorhombic (chrysotile). Magnetite: Isometric.
Color: Serpentinite: Green, yellowish-green, blackish-green, mottled. Magnetite: Iron-black to dark gray.
Luster: Serpentinite: Greasy, waxy, silky, dull. Magnetite: Metallic to submetallic.
Transparency: Serpentinite: Translucent to opaque. Magnetite: Opaque.
Fracture: Serpentinite: Conchoidal to splintery. Magnetite: Uneven to subconchoidal.
Cleavage: Serpentine minerals: Variable, often poor or absent; fibrous in chrysotile. Magnetite: None, but octahedral parting is common.
Composition: Serpentinite: Hydrous magnesium iron silicate. Magnetite: Iron oxide.

Quick Check

Color: Green to black (serpentinite) with dark gray to black metallic veins (magnetite).
Luster: Greasy/waxy/silky (serpentinite) and metallic (magnetite).
Streak: White to pale green (serpentinite) and black (magnetite).

Physical Characteristics

Crystal Habit: Serpentine: Massive, platy, fibrous, lamellar. Magnetite: Octahedral, granular, massive.
Cleavage Type: Serpentine: Variable, often poor or absent; fibrous in chrysotile. Magnetite: None, but octahedral parting.
Fracture Type: Serpentine: Conchoidal to splintery. Magnetite: Uneven to subconchoidal.
Tenacity: Serpentine: Tough, sometimes brittle or flexible (chrysotile). Magnetite: Brittle.
Luster Type: Serpentine: Greasy, waxy, silky, dull. Magnetite: Metallic to submetallic.

Formation

Serpentinite forms through the hydration and metamorphic alteration of ultramafic rocks (peridotite, dunite) at low to moderate temperatures (200-600 °C) and pressures. This process, known as serpentinization, involves the reaction of olivine and pyroxene with water, producing serpentine minerals (chrysotile, antigorite, lizardite) and often magnetite as a byproduct. Magnetite veins typically form during or after the main serpentinization event, either as a direct product of the alteration of iron-bearing silicates or through later hydrothermal fluid circulation concentrating iron oxides.

Usage

Serpentinite itself has been used as a decorative stone (verde antique marble), for carving, and as a source of asbestos (chrysotile variety). Magnetite, when present in sufficient quantities, can be mined as an iron ore. The presence of magnetite veins can sometimes indicate areas of economic interest for iron, or other associated metals like nickel and chromium, which are often found in ultramafic complexes. Historically, magnetite-rich serpentinites were used as natural compasses due to their magnetic properties.

Age Distribution

Variable, from Precambrian to Cenozoic, depending on the age of the protolith and the metamorphic event.

Where to Find

Ophiolite Complexes Worldwide

Prominent examples include the Troodos Ophiolite (Cyprus), Semail Ophiolite (Oman), Coast Range Ophiolite (California, USA), and various occurrences in the Alps, Himalayas, and Appalachian Mountains. These are major sites for serpentinite formation and associated magnetite.

Ultramafic Intrusions in Metamorphic Terrains

Regions with ancient cratons and orogenic belts, such as parts of Canada (e.g., Quebec, Ontario), Russia (Urals), Australia, and Scandinavia, where ultramafic rocks have been metamorphosed.

Subduction Zones and Accretionary Wedges

Areas where oceanic crust is being subducted, leading to serpentinization of mantle peridotite, often exposed in mountain ranges formed by continental collision.

Finding Tips

Look for Green Rocks in Ophiolites

Serpentinite is a key component of ophiolite sequences. When exploring these areas, look for dark green to black, often slick-feeling rocks. The magnetite veins will appear as darker, metallic streaks.

Use a Magnet

Magnetite is strongly magnetic. A simple hand magnet can be used to test the dark veins or inclusions within the serpentinite. If it attracts the magnet, it's likely magnetite.

Observe Luster and Hardness

Serpentinite typically has a greasy or waxy luster and is relatively soft (Mohs 2.5-4). Magnetite has a metallic luster and is harder (Mohs 5.5-6.5). This contrast can help differentiate the two components.

Check for Associated Minerals

Serpentinite often occurs with chromite, talc, magnesite, and sometimes relict olivine or pyroxene. The presence of these minerals can confirm the ultramafic origin.

Similar Rocks

Chlorite Schist

Chlorite Schist (primarily chlorite, (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6)

Also known as: Green Schist

Talc Schist

Talc Schist (primarily talc, Mg3Si4O10(OH)2)

Also known as: Soapstone (massive variety)

Gabbro

Gabbro (plagioclase feldspar, pyroxene, olivine)

Also known as: Black Granite (commercial term)

Scientific Classification

Mineral Class: Serpentine: Phyllosilicate. Magnetite: Oxide.
Group: Serpentine Group. Spinel Group (for Magnetite).
Crystal System: Serpentine: Monoclinic or Orthorhombic. Magnetite: Isometric.
Chemical Formula: Serpentinite: (Mg,Fe)3Si2O5(OH)4. Magnetite: Fe3O4.
Composition: Serpentinite: Magnesium, iron, silicon, oxygen, hydrogen. Magnetite: Iron, oxygen.

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