igneous
Gabbro
Gabbro (mafic intrusive igneous rock)
Hardness: 6-7 Mohs; Color: Dark green, gray, or black; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.7-3.3. It is composed primarily of calcium-rich plagioclase feldspar and pyroxene.
- Hardness
- 6-7 Mohs
- Color
- Dark green, gray, or black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Dark green, gray, or black; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.7-3.3. It is composed primarily of calcium-rich plagioclase feldspar and pyroxene.
Formation & geological history
Formed by the slow crystallization of magnesium-rich and iron-rich magma beneath the Earth's surface (intrusive). It is the plutonic equivalent of basalt and is typically found in the lower oceanic crust.
Uses & applications
Used as 'black granite' in construction for countertops, floor tiles, and facing stone; also used as crushed stone for road base and railroad ballast.
Geological facts
Gabbro is the most abundant rock in the deep oceanic crust. It is chemically equivalent to basalt, but while basalt cools quickly on the surface, gabbro cools slowly underground, allowing large crystals to grow.
Field identification & locations
Identify by its dark color, heavy weight, and coarse-grained phaneritic texture. It lacks the quartz found in granite. Common in ophiolite complexes and large layered intrusions like the Duluth Complex.
