igneous
Gabbro
Phaneritic mafic intrusive igneous rock
Hardness: 6-7 Mohs; Color: dark grey to black with green/white specks; Luster: crystalline/sub-vitreous; Crystal structure: phaneritic (coarsegrained); Cleavage: distinctive pyroxene cleavage; Specific gravity: 2.7–3.3
- Hardness
- 6-7 Mohs
- Color
- dark grey to black with green/white specks
- Luster
- crystalline/sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: dark grey to black with green/white specks; Luster: crystalline/sub-vitreous; Crystal structure: phaneritic (coarsegrained); Cleavage: distinctive pyroxene cleavage; Specific gravity: 2.7–3.3
Formation & geological history
Formed by the slow cooling of magnesiumrich and ironrich magma deep beneath the Earth's surface (plutonic formation), often at mid-ocean ridges or in ancient continental crust.
Uses & applications
Extensively used as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality varieties are sold as 'Black Granite' for kitchen countertops and floor tiles.
Geological facts
Gabbro is the chemical equivalent of basalt; the main difference is that gabbro cooled slowly underground while basalt cooled quickly on the surface. Much of the Earth's oceanic crust is composed of gabbro.
Field identification & locations
Identify in the field by its dark, heavy nature and visible individual crystals (unlike the fine-grained basalt). Look for it in ophiolite complexes or deep-seated mountain roots.
