igneous
Gabbro
Phaneritic Basic Igneous Rock (composed primarily of pyroxene and calcium-rich plagioclase)
Hardness: 6-7 (Mohs scale); Color: Dark gray, black, or dark greenish-gray; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.7–3.3
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark gray, black, or dark greenish-gray
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark gray, black, or dark greenish-gray; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.7–3.3
Formation & geological history
Formed from the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface. It is the intrusive equivalent of basalt and is typically found in oceanic crust and large plutonic bodies.
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, floor tiles, and cemetery markers.
Geological facts
Gabbro is the most abundant rock in the deep oceanic crust. It is chemically identical to basalt, but it has much larger crystals because it cooled slowly underground instead of erupting on the surface.
Field identification & locations
Identify in the field by its dark color and coarse, interlocking crystals that are visible to the naked eye. It lacks the quartz found in granite. Common in the Adirondack Mountains, Lake Superior region, and oceanic ophiolites.
