igneous
Gabbro
Basaltic Plutonic Rock (Gabbro)
Phaneric texture, Hardness: 6-7 (Mohs scale), Color: Dark grey to black with possible green or white patches, Luster: Vitreous to dull, Crystal structure: Orthorhombic or Monoclinic minerals, Specific gravity: 2.7-3.3
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Physical properties
Phaneric texture, Hardness: 6-7 (Mohs scale), Color: Dark grey to black with possible green or white patches, Luster: Vitreous to dull, Crystal structure: Orthorhombic or Monoclinic minerals, 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. It is the intrusive equivalent of basalt and is often found in the lower oceanic crust and large intrusive bodies.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as high-durability dimension stone for kitchen countertops and floor tiles (often sold under the trade name 'black granite').
Geological facts
Gabbro is an essential component of the oceanic crust. Larger crystals indicate the magma cooled very slowly deep underground, allowing minerals like plagioclase feldspar and pyroxene to grow to visible sizes.
Field identification & locations
Identified in the field by its dark (mafic) color and coarse-grained coarse texture containing visible dark minerals. Common in mountainous regions with exposed deep-crustal rocks or ophiolites.
