igneous
Porphyritic basaltic-andesite
Porphyritic intermediate volcanic rock; chiefly plagioclase and pyroxene/amphibole, with altered mafic minerals and iron oxides
AI-generated identification · may be incorrectDark green-gray, fine-grained groundmass with conspicuous pale feldspar phenocrysts and rusty alteration; generally massive, nonfoliated, with dull to slightly vitreous luster. Hardness is approximately 5–6, with poor or absent cleavage as a rock and specific gravity around 2.7–3.0.
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Physical properties
Dark green-gray, fine-grained groundmass with conspicuous pale feldspar phenocrysts and rusty alteration; generally massive, nonfoliated, with dull to slightly vitreous luster. Hardness is approximately 5–6, with poor or absent cleavage as a rock and specific gravity around 2.7–3.0.
Formation & geological history
Formed when intermediate-to-mafic lava cooled relatively quickly at or near Earth’s surface, allowing larger crystals to grow before rapid final cooling; commonly Cenozoic to older volcanic material. The green and brown colors likely reflect chlorite, epidote, and weathering.
Uses & applications
Used locally as crushed aggregate, riprap, road stone, and building stone; attractive polished pieces may be collected, but it is not usually a gemstone.
Geological facts
Its porphyritic texture records two-stage cooling: slow crystallization underground followed by eruption. Pale areas may be feldspar crystals or weathered mineral-filled cavities rather than true inclusions.
Field identification & locations
Identify by its fine dark matrix, scattered light feldspar crystals, and high density; it should not fizz with dilute hydrochloric acid unless carbonate veins are present. Common in volcanic belts worldwide, including the Andes, Cascades, Japan, and Mediterranean regions.
