igneous
Porphyritic igneous rock, likely porphyritic basalt or andesite
Plagioclase-phyric volcanic rock; exact identification requires closer examination
AI-generated identification · may be incorrectDark gray, fine-grained groundmass with conspicuous pale-gray, rounded to blocky crystals, probably plagioclase phenocrysts. Typical hardness is about 5–6 on Mohs scale; crystals are too indistinct in the image to assess cleavage or luster reliably.
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Physical properties
Dark gray, fine-grained groundmass with conspicuous pale-gray, rounded to blocky crystals, probably plagioclase phenocrysts. Typical hardness is about 5–6 on Mohs scale; crystals are too indistinct in the image to assess cleavage or luster reliably.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly before the remaining melt cooled rapidly at or near the surface. Its geological age and precise setting cannot be determined from the photograph.
Uses & applications
Commonly used as crushed aggregate or building stone; this small specimen is mainly of educational or collecting interest.
Geological facts
The contrasting large crystals and fine background texture are called porphyritic texture and record a change in cooling conditions.
Field identification & locations
Look for prominent light crystals embedded in a much finer, dark volcanic matrix; a fresh surface and hand lens can help distinguish plagioclase from other minerals. Volcanic rocks with this appearance occur worldwide.
