igneous
Granodiorite
Granodiorite (phaneritic intrusive igneous rock)
Hardness: 6-7 Mohs; Color: Salt-and-pepper (white-grey with black flecks); Luster: Dull to vitreous; Structure: Phaneritic (coarse-grained crystals); Cleavage: None (rock breaks irregularly, though component minerals have cleavage); Specific Gravity: 2.6-2.7.
- Hardness
- 6-7 Mohs
- Color
- Salt-and-pepper (white-grey with black flecks)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt-and-pepper (white-grey with black flecks); Luster: Dull to vitreous; Structure: Phaneritic (coarse-grained crystals); Cleavage: None (rock breaks irregularly, though component minerals have cleavage); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the slow cooling of silica-rich magma deep underground (plutonic). Found in large batholiths often associated with mountain-building events (orogenic belts). These can range from Precambrian to Cenozoic in age.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete, and railroad ballast. When polished, it is sold as 'Granite' for countertops, floor tiles, and grave markers.
Geological facts
The famous Plymouth Rock is a glacial erratic made of granodiorite. It is chemically intermediate between granite and diorite, containing more plagioclase feldspar than orthoclase.
Field identification & locations
Identify by its 'salt and pepper' look where white/grey grains (feldspar/quartz) are mixed with dark specks (biotite/hornblende). Common in the Sierra Nevada mountains and Pacific Northwest beaches as water-worn cobbles. Collectors look for large, well-defined mineral grains.
