igneous
Gabbro
Gabbro (mafic intrusive igneous rock)
Hardness: 6-7 (Mohs scale); Color: Dark green, gray, or black with light plagioclase mottling; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to pyroxene/feldspar); SG: 2.7-3.3.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark green, gray, or black with light plagioclase mottling
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark green, gray, or black with light plagioclase mottling; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to pyroxene/feldspar); SG: 2.7-3.3.
Formation & geological history
Formed by the slow cooling and crystallization of magnesium-rich and iron-rich magma deep beneath the Earth's surface (plutonic). Found in mid-ocean ridges and ancient mountain belts.
Uses & applications
Used in construction as 'black granite' for countertops, floor tiles, and grave markers; crushed stone for road aggregate and railroad ballast.
Geological facts
Gabbro is the chemical equivalent of basalt, but while basalt forms on the surface, gabbro forms underground. It is a major component of the lower oceanic crust.
Field identification & locations
Identify by its dark, coarse-grained appearance and the presence of visible glistening pyroxene and plagioclase crystals. Often found in ophiolite complexes.
