igneous
Gabbro
Gabbro (mafic intrusive igneous rock)
Hardness: 6-7 (Mohs); Color: Dark gray, greenish-black, or black; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3
- Hardness
- 6-7 (Mohs)
- Color
- Dark gray, greenish-black, or black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray, greenish-black, or black; Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.7-3.3
Formation & geological history
Formed by the slow crystallization of magnesium-rich and iron-rich magma beneath the Earth's surface. It is the intrusive equivalent of basalt and often found in oceanic crust.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and concrete aggregate. High-quality varieties are polished and sold as 'black granite' for countertops and monuments.
Geological facts
Gabbro makes up a large part of the Earth's lower oceanic crust. Though chemically identical to basalt, its coarse grains show the magma cooled much more slowly.
Field identification & locations
Identify in the field by its dark color and visible interlocking crystals of plagioclase feldspar and pyroxene. It lacks the quartz typical of granite.
