igneous
Porphyritic Andesite
Andesite (Porphyritic texture)
Hardness: 6-7 Mohs; Color: Gray to bluish-gray groundmass with white, cream, or pinkish phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine) background with coarse crystals; Cleavage: None (rock mass); Specific gravity: 2.4-2.8
- Hardness
- 6-7 Mohs
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray to bluish-gray groundmass with white, cream, or pinkish phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine) background with coarse crystals; Cleavage: None (rock mass); Specific gravity: 2.4-2.8
Formation & geological history
Formed through the rapid cooling of intermediate-composition lava at or near the Earth's surface, typically in volcanic arc environments above subduction zones. The larger phenocrysts grew slowly underground before the final eruption.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and aggregate in concrete. It is also used as masonry stone and occasionally polished for architectural trim.
Geological facts
The name 'Andesite' is derived from the Andes Mountains in South America. The 'polka dot' look of this specimen is caused by large feldspar crystals cooling at a different rate than the surrounding volcanic matrix.
Field identification & locations
Identify by looking for distinct, larger 'spots' (phenocrysts) embedded in a fine-grained, solid gray background. It is commonly found in the Pacific Northwest, the Andes, and glaciated areas where it has been transported as till.
