igneous
Porphyritic Andesite
Andesite (Porphyry)
Hardness: 6-7 Mohs; Color: Brownish-purple to dark gray matrix with white/light gray phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture common in matrix).
- Hardness
- 6-7 Mohs
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Brownish-purple to dark gray matrix with white/light gray phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture common in matrix).
Formation & geological history
Formed through the rapid cooling of intermediate lava near or at the Earth's surface, where larger crystals (phenocrysts like plagioclase) formed early in a magma chamber before the remaining liquid solidified quickly. Most common in volcanic arcs and subduction zones.
Uses & applications
Commonly used as crushed stone for road construction, aggregate in concrete, and architectural decorative stone. In some regions, it is used for building tiles and monuments due to its durability.
Geological facts
Andesite is the second most common volcanic rock in Earth's crust after basalt. It is named after the Andes Mountains, where it is found in great abundance along the subduction zone.
Field identification & locations
Identify by looking for 'polka-dot' crystals set in a fine, dark, earthy-toned background. Found globally in volcanic regions like the American Northwest (Cascades), the Andes, and Japan. Collectors look for high contrast between crystals and matrix.
