Gabbro

Igneous, Intrusive

Gabbro

Also known as: Black Granite (in commercial use, though geologically incorrect)

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Description

Gabbro is a dark-colored, coarse-grained intrusive igneous rock. It is the intrusive equivalent of basalt, meaning they have the same chemical composition but differ in grain size due to different cooling rates. Gabbro is primarily composed of calcic plagioclase feldspar (typically labradorite or bytownite) and pyroxene (augite), with lesser amounts of olivine, hornblende, and iron-titanium oxides (magnetite, ilmenite). Its texture is phaneritic, meaning individual mineral grains are visible to the naked eye. The color is typically dark green to black, often with a speckled appearance due to the presence of lighter plagioclase and darker mafic minerals.

How to Identify

Color: Typically dark green to black, often with a speckled appearance due to the presence of lighter plagioclase and darker mafic minerals.
Luster: Vitreous to dull, depending on the mineral components and weathering.
Texture: Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye, typically 1 mm to 5 mm in size, but can be larger in pegmatitic varieties.
Crystal Form: Anhedral to subhedral interlocking crystals of plagioclase and pyroxene.
Cleavage: Minerals within gabbro (plagioclase, pyroxene) exhibit distinct cleavage planes. Plagioclase has two good cleavages at nearly 90 degrees, and pyroxene has two good cleavages at approximately 90 degrees.
Geological Environment: Forms in the lower oceanic crust, within large layered mafic intrusions, and as sills and dikes in continental settings. It is a fundamental component of ophiolite sequences, representing ancient oceanic crust.

Key Facts

Hardness: 6-7 on Mohs scale (due to constituent minerals like plagioclase and pyroxene)
Specific Gravity: 2.9-3.2 g/cm3
Crystal System: Not applicable for a rock; constituent minerals are typically monoclinic (pyroxene, hornblende) or triclinic (plagioclase).
Color: Dark green to black, often speckled.
Luster: Vitreous to dull.
Transparency: Opaque (as a rock); individual mineral grains can be translucent to opaque.
Fracture: Conchoidal to uneven (for individual minerals); rock typically exhibits irregular fracture.
Cleavage: Present in constituent minerals (e.g., plagioclase: 2 directions at ~900; pyroxene: 2 directions at ~900).
Composition: Primarily calcic plagioclase feldspar (35-65%) and pyroxene (20-50%), with minor olivine, hornblende, and iron-titanium oxides. Quartz is typically absent or present in very minor amounts (<5%).

Quick Check

Color: Dark (black, dark green, dark gray)
Luster: Vitreous to dull
Streak: White to very light gray (from plagioclase), or no streak (from mafic minerals)

Physical Characteristics

Crystal Habit: Anhedral to subhedral interlocking granular texture (phaneritic).
Cleavage Type: Good to perfect in constituent minerals (e.g., plagioclase, pyroxene).
Fracture Type: Irregular to subconchoidal.
Tenacity: Brittle.
Luster Type: Vitreous to dull.

Formation

Gabbro forms from the slow crystallization of mafic magma deep beneath the Earth's surface. This slow cooling allows for the growth of large, interlocking mineral crystals, giving it a phaneritic (coarse-grained) texture. It is a primary component of the lower oceanic crust and is also found in large intrusive bodies (plutons, sills, dikes) within continental crust.

Usage

Gabbro is widely used as crushed stone for road construction, railway ballast, and aggregate in concrete. Due to its durability, dark color, and ability to take a high polish, it is also extensively used as dimension stone for countertops, floor tiles, facing stone, and monuments, often marketed commercially as 'black granite'. Some varieties, particularly those rich in plagioclase feldspar, can exhibit labradorescence and are used as ornamental stones.

Age Distribution

Found throughout Earth's history, from Archean to Cenozoic, wherever oceanic crust or large mafic intrusions have formed.

Where to Find

Oceanic Crust

Gabbro is a major constituent of the lower oceanic crust, forming beneath mid-ocean ridges. It can be found exposed in ophiolite complexes, which are fragments of oceanic crust and upper mantle thrust onto continental margins (e.g., Oman, Cyprus, Newfoundland).

Layered Intrusions

Large, economically significant layered mafic intrusions worldwide contain vast quantities of gabbro. Examples include the Bushveld Complex (South Africa), Stillwater Complex (USA), and Skaergaard Intrusion (Greenland).

Continental Rifts and Hotspots

Gabbroic intrusions are associated with continental rifting environments and large igneous provinces (LIPs) formed by mantle plumes (e.g., Duluth Complex, USA; Siberian Traps, Russia).

Mountain Ranges

Gabbro can be found in eroded mountain ranges where deep-seated igneous intrusions have been uplifted and exposed (e.g., parts of the Appalachian Mountains, USA; Scottish Highlands, UK).

Finding Tips

Look for Dark, Coarse-Grained Rocks

Gabbro is distinctly dark and its individual mineral grains are visible without magnification. This distinguishes it from fine-grained basalt.

Check for Mineral Composition

Identify the presence of dark pyroxene and lighter, often iridescent, plagioclase feldspar. A hand lens can be very useful for this.

Consider the Geological Context

Gabbro is typically found in areas associated with ancient oceanic crust (ophiolites), large intrusive complexes, or areas of significant mafic magmatism.

Examine Weathered Surfaces

While fresh gabbro is dark, weathered surfaces can sometimes appear brownish or greenish due to alteration of mafic minerals.

Similar Rocks

Basalt

Also known as: Volcanic Rock

Diorite

Also known as: Intermediate Plutonic Rock

Peridotite

Also known as: Ultramafic Rock

Norite

Also known as: Orthopyroxene Gabbro

Scientific Classification

Mineral Class: Igneous Rock
Group: Plutonic (Intrusive) Mafic Rock
Crystal System: Not applicable for a rock; refers to the crystal systems of its constituent minerals.
Chemical Formula: No single chemical formula for a rock; composed of various silicate minerals.
Composition: Mafic, with high content of ferromagnesian minerals and calcic plagioclase. SiO2 content typically 45-52 wt%.

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