igneous
Porphyritic igneous rock, possibly basaltic-andesite
Porphyritic andesite/basalt; likely plagioclase phenocrysts in a fine mafic groundmass
AI-generated identification · may be incorrectDark gray-brown, fine-grained matrix with abundant pale, elongated plagioclase crystals; likely hardness 5–6, dull to weakly subvitreous luster, and no obvious cleavage at specimen scale. Exact identification requires a fresh surface, hand lens, and possibly thin-section or chemical testing.
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Physical properties
Dark gray-brown, fine-grained matrix with abundant pale, elongated plagioclase crystals; likely hardness 5–6, dull to weakly subvitreous luster, and no obvious cleavage at specimen scale. Exact identification requires a fresh surface, hand lens, and possibly thin-section or chemical testing.
Formation & geological history
Formed when magma cooled relatively quickly after earlier crystal growth, producing large plagioclase phenocrysts in a finer groundmass; commonly associated with volcanic arcs and relatively young volcanic terrains, generally Cenozoic or younger.
Uses & applications
Used locally as aggregate, roadstone, riprap, and decorative landscape rock; specimens have modest collector value unless exceptionally fresh or from a notable locality.
Geological facts
The contrasting crystal sizes are called porphyritic texture and record a two-stage cooling history. The pale crystals are probably plagioclase feldspar, but weathering and image quality prevent a definitive species-level identification.
Field identification & locations
Look for a dark volcanic matrix containing conspicuous white-to-gray feldspar laths; a streak test, magnet test, acid test, and close inspection can help distinguish it from porphyritic basalt, andesite, or diabase. Common in volcanic regions worldwide.
