igneous
Granodiorite
Granodiorite (Quartz-Feldspar-Biotite Plutonic Rock)
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (gray to white with dark flecks). Luster: Phaneritic (crystalline), dull to pearly. Structure: Phaneritic, coarse-grained. Specific gravity: 2.6-2.7. Primarily composed of plagioclase feldspar, quartz, and biotite mica or hornblende.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt-and-pepper (gray to white with dark flecks)
- Luster
- Phaneritic (crystalline), dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (gray to white with dark flecks). Luster: Phaneritic (crystalline), dull to pearly. Structure: Phaneritic, coarse-grained. Specific gravity: 2.6-2.7. Primarily composed of plagioclase feldspar, quartz, and biotite mica or hornblende.
Formation & geological history
Formed by the slow cooling of silica-rich magma deep within the Earth's crust (plutonic). It is intermediate between granite and diorite. Geological age varies greatly from Precambrian to Cenozoic depending on the tectonic setting, often associated with subduction zones and mountain building (orogeny).
Uses & applications
Extensively used in construction as crushed stone for road base, concrete, and railroad ballast. When polished, it is sold commercially as 'granite' for countertops, floor tiles, and dimensional stone in monuments.
Geological facts
The Rosetta Stone, one of the most famous archaeological finds in history, was carved from a slab of granodiorite. It is the most common rock type in the Sierra Nevada batholith of California.
Field identification & locations
Identify in the field by its coarse-grained texture and high percentage of white/gray plagioclase compared to the pinkish orthoclase found in true granite. Found in massive batholiths and mountain ranges. Collectors look for large crystals of biotite or accessory minerals like sphene.
