igneous
Porphyritic Andesite
Andesite (Intermediate Extrusive Volcanic Rock)
Hardness: 6-7 (Mohs scale); Color: Dark reddish-brown groundmass with white plagioclase phenocrysts; Luster: Dull/Earthy groundmass with vitreous crystals; Crystal structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: Distinct in phenocrysts; Specific gravity: 2.4-2.8.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull/Earthy groundmass with vitreous crystals
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark reddish-brown groundmass with white plagioclase phenocrysts; Luster: Dull/Earthy groundmass with vitreous crystals; Crystal structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: Distinct in phenocrysts; Specific gravity: 2.4-2.8.
Formation & geological history
Formed via the rapid cooling of intermediate lava on the Earth's surface, typically at subduction zones above convergent plate boundaries. The larger crystals (phenocrysts) formed slowly underground before being erupted. These rocks are common in volcanic arcs like the Andes or Cascades.
Uses & applications
Used primarily in construction for crushed stone, road base, and aggregate. Some attractive porphyritic specimens are used for decorative landscaping or minor carving.
Geological facts
The name 'Andesite' is derived from the Andes Mountains in South America. The reddish hue is often due to the oxidation of iron-bearing minerals (hematite) within the rock matrix.
Field identification & locations
Identified by the 'chocolate chip cookie' appearance of light-colored crystals embedded in a darker, fine-grained matrix. Found in volcanic regions globally. Collectors look for high contrast between the phenocrysts and the groundmass.
