igneous
Gabbro
Phaneritic Basic Igneous Rock
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained) holocrystalline; Cleavage: Good in two directions (due to pyroxene/plagioclase); Specific Gravity: 2.7-3.3
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black, sometimes greenish
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained) holocrystalline; Cleavage: Good in two directions (due to pyroxene/plagioclase); Specific Gravity: 2.7-3.3
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (intrusive). They are often found in oceanic crust and large plutonic bodies. Geological age varies globally from Archean to Cenozoic.
Uses & applications
Used as crushed stone for road construction and railroad ballast; polished varieties are used as 'black granite' for kitchen countertops, floor tiles, and cemetery markers.
Geological facts
Gabbro is the intrusive equivalent of basalt. Large parts of the Earth's lower oceanic crust are composed of gabbro. It frequently contains valuable amounts of chromium, nickel, and platinum group elements.
Field identification & locations
Identify by its dark color and visible, interlocking coarse crystals of dark minerals (pyroxene) and greyish plagioclase. Found in mountainous regions or areas with exposed oceanic crust (ophiolites). Collectors look for specimens with large phaneritic crystals.
