igneous
Granodiorite
Granodiorite (Quartz + Plagioclase + Alkali Feldspar + Biotite/Hornblende)
Hardness: 6-7 Mohs; Color: Mottled black and white ('salt and pepper'); Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained, visible crystals); Specific gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Color
- Mottled black and white ('salt and pepper')
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled black and white ('salt and pepper'); Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained, visible crystals); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the slow cooling of silica-rich magma deep within the Earth's crust (plutonic). It is common in batholiths associated with mountain-building subduction zones, often dating from the Archean to the Cenozoic eras.
Uses & applications
Used widely in heavy construction, road surfacing, and as ornamental stone (granite-style countertops and monuments). It is also a primary host rock for porphyry copper deposits.
Geological facts
Famous structures made of granodiorite include the Rosetta Stone and the base of the Statue of Liberty. It is intermediate in composition between granite and diorite, containing more plagioclase than orthoclase.
Field identification & locations
Identify in the field by its coarse 'salt and pepper' appearance and visible quartz grains. It can be distinguished from granite by its lower potassium feldspar content (less pink/orange) and higher dark mineral content. It is extremely common in the Sierra Nevada mountains and the Andes.
