igneous
Granodiorite
Phaneritic-textured Intrusive Granodiorite
Hardness: 6-7 on Mohs scale; Color: Salt-and-pepper (mottled grey, white, and black); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained crystals visible to the naked eye); Specific gravity: 2.6 to 2.73.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt-and-pepper (mottled grey, white, and black)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt-and-pepper (mottled grey, white, and black); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained crystals visible to the naked eye); Specific gravity: 2.6 to 2.73.
Formation & geological history
Formed through the slow cooling of silica-rich magma deep within the Earth's crust (plutonic). It is common in continental crust and often found in mountain ranges formed by volcanic arcs. Geological age varies from Precambrian to Cenozoic depending on the specific batholith.
Uses & applications
Used widely as crushed stone for road construction and aggregate. High-quality slabs are used for countertops, building facades, and paving stones (often sold commercially as 'grey granite').
Geological facts
Granodiorite is the rock that makes up the famous Rosetta Stone and much of the Plymouth Rock. It is intermediate in composition between granite and diorite, containing more plagioclase feldspar than orthoclase feldspar.
Field identification & locations
Identify in the field by looking for a coarse grain size where white/grey plagioclase feldspar outnumbers pinkish orthoclase, mixed with dark biotite or hornblende. It is found in abundance in the Sierra Nevada mountains and other subduction zone cordilleras.
