igneous
Granodiorite
Granodiorite (Phaneritic intrusive igneous rock containing Plagioclase, Quartz, and Biotite/Amphibole)
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled gray, white, and black). Luster: Dull to vitreous. Crystal structure: Phaneritic (visible coarse-grained crystals). Cleavage: Distinct in feldspar components. Specific gravity: 2.6-2.7.
- Hardness
- 6-7 on Mohs scale
- Color
- Salt-and-pepper (mottled gray, white, and black)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled gray, white, and black). Luster: Dull to vitreous. Crystal structure: Phaneritic (visible coarse-grained crystals). Cleavage: Distinct in feldspar components. Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the slow cooling of silica-rich magma deep underground in batholiths and plutons. It is typically associated with subduction zones and continental crust formation, with ages ranging from Precambrian to Cenozoic.
Uses & applications
Primary use is in construction as crushed stone or 'granite' road base. It is also used for building facades, paving stones, and memorial monuments due to its durability.
Geological facts
Granodiorite is the rock that makes up the famous Rosetta Stone. It is chemically intermediate between granite and diorite, containing more plagioclase feldspar than orthoclase (potassium) feldspar.
Field identification & locations
Identify in the field by its speckled appearance and lack of distinct pink 'potassium' color compared to true granite. Commonly found in large mountain ranges like the Sierra Nevada or the Scottish Highlands. Collectors look for polished slabs showing mineral variety.
