igneous
Diorite
Phaneritic intermediate plutonic rock
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (mottled black and white/gray); Luster: Sub-vitreous; Crystal structure: Phaneritic (coarse-grained coarse crystals); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt and pepper (mottled black and white/gray)
- Luster
- Sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (mottled black and white/gray); Luster: Sub-vitreous; Crystal structure: Phaneritic (coarse-grained coarse crystals); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed by the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain building zones (subduction zones). It is intermediate in composition between granite and gabbro.
Uses & applications
Commonly used in construction as crushed stone for roads, as a base material, and occasionally as polished 'black granite' for countertops and headstones.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures and tablets because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.
Field identification & locations
Identify in the field by its distinctive 'salt and pepper' appearance with roughly equal parts light plagioclase feldspar and dark minerals like hornblende or biotite. It lacks the visible quartz found in granite.
