igneous
Diorite
Phaneritic Intermediate Plutonic Rock
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium to coarse-grained); Cleavage: Distinct in two directions at 60/120 degrees for hornblende; Specific gravity: 2.8-3.0
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (black and white)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium to coarse-grained); Cleavage: Distinct in two directions at 60/120 degrees for hornblende; Specific gravity: 2.8-3.0
Formation & geological history
Formed by the slow cooling of intermediate-composition magma deep beneath the Earth's surface (plutonic). Found in volcanic arcs and mountain-building regions where oceanic crust subducts under continental crust.
Uses & applications
Used primarily as crushed stone for road construction and as dimension stone for building facades, monuments, and occasionally kitchen countertops under the trade name 'black granite'.
Geological facts
Diorite is often called 'salt and pepper rock' due to its contrasting mineral colors. The Code of Hammurabi, one of the oldest sets of laws in history, was inscribed on a massive basaltic-diorite stele.
Field identification & locations
Identify by its coarse grains and equal mix of light (plagioclase feldspar) and dark (hornblende or biotite) minerals. Unlike granite, it contains little to no visible quartz. Common in batholiths and mountain ranges like the Andes.
