igneous
Gabbro
Phaneritic mafic intrusive igneous rock
Hardness: 6-7 (Mohs); Color: Dark gray to black, often with greenish or brownish weathering; Luster: Dull to vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (augite/pyroxene)
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray to black, often with greenish or brownish weathering; Luster: Dull to vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (augite/pyroxene)
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (plutonic). It is the intrusive equivalent of basalt and is often found in oceanic crust and large layered intrusions.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as 'black granite' for counter tops, floor tiles, and cemetery markers in the dimension stone industry.
Geological facts
Gabbro is the most abundant rock in the deep oceanic crust. It is the chemical equivalent of basalt, but because it cools slowly underground, it develops larger crystals visible to the naked eye.
Field identification & locations
Identify by its dark color, heavy weight, and coarse-grained interlocking crystals. It lacks quartz but is rich in plagioclase feldspar and pyroxene. Common in mountain ranges and near ancient rift zones.
