igneous
Porphyritic Andesite
Andesite (Porphyry)
Hardness: 6-7 Mohs; Color: Dark grey to greenish groundmass with light pinkish or white phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Distinct in phenocrysts; Specific Gravity: 2.5-2.8
- Hardness
- 6-7 Mohs
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to greenish groundmass with light pinkish or white phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Distinct in phenocrysts; Specific Gravity: 2.5-2.8
Formation & geological history
Formed through the cooling of intermediate lava or shallow magma. The large phenocrysts (likely plagioclase feldspar) formed slowly at depth, while the fine-grained matrix formed rapidly during eruption or shallow intrusion. Common in volcanic arcs above subduction zones.
Uses & applications
Used primarily in construction for crushed stone, aggregate, and road base. Occasionally used for architectural paving or as landscape accents. Generally lacks jewelry value but is a standard teaching specimen for petrology.
Geological facts
Andesite is named after the Andes Mountains, where it is very common. The 'porphyritic' texture specifically tells a story of two different cooling rates in the magma's history.
Field identification & locations
Identify by looking for 'spots' (phenocrysts) set in a solid, fine-grained background. It is commonly found in the Cascades, the Andes, and Japan. Collectors should look for sharp crystal faces in the phenocrysts.
