igneous
Gabbro
Phaneritic Basic Igneous Rock (Gabbro)
Hardness: 6-7 Mohs; Color: dark gray, black, or dark green; Luster: dull to vitreous; Crystal Structure: phaneritic (large, visible crystals); Specific Gravity: 2.7-3.3; Composition: Plagioclase feldspar, pyroxene, and minor olivine.
- Hardness
- 6-7 Mohs
- Color
- dark gray, black, or dark green
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: dark gray, black, or dark green; Luster: dull to vitreous; Crystal Structure: phaneritic (large, visible crystals); Specific Gravity: 2.7-3.3; Composition: Plagioclase feldspar, pyroxene, and minor olivine.
Formation & geological history
Formed from the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (plutonic). It is equivalent to basalt but cooled slowly enough to form visible crystals.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as a decorative stone known in the trade as 'Black Granite' for countertops and flooring.
Geological facts
Gabbro makes up a large part of the Earth's lower oceanic crust. It is rarer on the surface than its volcanic counterpart, basalt, which makes it a favorite for studying intrusive magmatic processes.
Field identification & locations
Identified in the field by its dark color and coarse-grained texture. It lacks the quartz found in granite. Common in oceanic crust exposures and mountain ranges like the Adirondacks or the Scottish Highlands.
