Granodiorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Phaneritic (salt and pepper) texture, predominantly light gray to white with black minerals. Luster: Vitreous to dull. Crystal structure: Granular/Coarse-grained. Cleavage: Good in feldspar minerals. Specific gravity: 2.6 – 2.7.

Formation & Geological History

Formed from the slow cooling and crystallization of silica-rich magma deep within the Earth's crust (plutonism). Common in continental volcanic arcs and cordilleran mountain building phases. Age ranges from billions of years (Archean) to more recent tectonic events.

Uses & Applications

Used extensively in construction as crushed stone, road base, and dimension stone for architectural cladding and paving. Often polished for kitchen countertops and memorial monuments.

Geological Facts

Granodiorite is the rock type that makes up the famous Rosetta Stone. It is effectively the intermediate rock between granite and diorite, containing more plagioclase feldspar than orthoclase.

Field Identification & Locations

Identify in the field by its coarse-grained texture, high percentage of plagioclase (turning chalky white when weathered), and dark accessory minerals like biotite or hornblende. It is common in batholiths like the Sierra Nevada.

Identified on: 4/23/2026

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