igneous
Porphyritic volcanic rock, likely porphyritic andesite
Porphyritic andesite; predominantly plagioclase feldspar phenocrysts in a fine volcanic groundmass, possibly with pyroxene or amphibole
AI-generated identification · may be incorrectDark gray-brown, dull to slightly vitreous, with abundant pale tabular crystals or fragments; hardness about 6–7, no obvious cleavage as a whole, and typical specific gravity near 2.6–2.8. The irregular pale grains suggest phenocrysts, although the image cannot confirm the exact mineral identity.
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Physical properties
Dark gray-brown, dull to slightly vitreous, with abundant pale tabular crystals or fragments; hardness about 6–7, no obvious cleavage as a whole, and typical specific gravity near 2.6–2.8. The irregular pale grains suggest phenocrysts, although the image cannot confirm the exact mineral identity.
Formation & geological history
Formed when intermediate-composition magma cooled rapidly at or near a volcano, preserving larger crystals in a fine matrix. Most andesite formed in volcanic arcs during the Cenozoic or older convergent-plate episodes.
Uses & applications
Used locally as crushed aggregate, road stone, riprap, and dimension stone; attractive polished pieces may be collected, but it is not generally a jewelry material.
Geological facts
The contrasting large crystals and fine matrix create a porphyritic texture, recording two cooling stages. Similar-looking rocks can include porphyritic basalt, dacite, or crystal-rich volcanic breccia.
Field identification & locations
Look for a hard, dense volcanic rock with pale feldspar crystals scattered through a dark fine-grained matrix; a streak test, fresh fracture, and hand lens help distinguish it from concrete or breccia. Common in volcanic mountain belts worldwide.
