igneous
Granodiorite
Granodiorite (phaneritic intrusive igneous rock)
Hardness: 6-7 Mohs; Color: Grayish-white with dark specks (mottled appearance); Luster: Vitreous to dull; Crystal structure: Phaneritic (coarsegrained), containing plagioclase feldspar, quartz, and phlogopite/biotite; Specific Gravity: 2.6-2.7.
- Hardness
- 6-7 Mohs
- Color
- Grayish-white with dark specks (mottled appearance)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Grayish-white with dark specks (mottled appearance); Luster: Vitreous to dull; Crystal structure: Phaneritic (coarsegrained), containing plagioclase feldspar, quartz, and phlogopite/biotite; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the slow cooling of silica-rich magma beneath the Earth's surface (plutonic). It is typically found in large batholiths associated with mountain-building events and subduction zones throughout various geological ages from the Archean to the Cenozoic.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. When polished, it is used as ornamental 'granite' for countertops, floor tiles, and grave markers.
Geological facts
The famous Rosetta Stone, which was key to deciphering Egyptian hieroglyphs, was long thought to be basalt but was later correctly identified as granodiorite. It is intermediate in composition between granite and diorite.
Field identification & locations
Identify in the field by its interlocking coarse crystals. Unlike true granite, it contains more plagioclase feldspar than orthoclase (potassium) feldspar, giving it a salt-and-pepper look without the pinkish hues often seen in granite. Common in the Sierra Nevada and Andes ranges.
