igneous
Gabbro
Gabbro (Mg, Fe)2Si2O6 / (Ca, Na)(Al, Si)4O8
Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees; Specific gravity: 2.7-3.3
- Hardness
- 6-7 Mohs
- Color
- Dark grey, black, or dark green
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees; Specific gravity: 2.7-3.3
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma inside the Earth's crust (intrusive). They are common in the oceanic crust and often associated with mid-ocean ridges.
Uses & applications
Used extensively in the construction industry as crushed stone for road base, railroad ballast, and concrete aggregate. If polished, it is sold as 'Black Granite' for countertops and cemetery markers.
Geological facts
Gabbro is the intrusive equivalent of basalt. Large parts of the Earth's oceanic crust are made of gabbro, and it has also been identified on the surface of the Moon.
Field identification & locations
Identified in the field by its dark color and lacks quartz. It has large, interlocking crystals indicative of underground cooling. Common in areas of ancient volcanic activity and magmatic intrusions.
