igneous
Porphyritic basalt
Basalt with plagioclase phenocrysts; chiefly Ca-rich plagioclase (labradorite–bytownite) and pyroxene
AI-generated identification · may be incorrectDark gray to black, fine-grained groundmass with abundant pale, blocky plagioclase crystals; typically vitreous to dull, hardness about 5–6, poor cleavage overall, and specific gravity roughly 2.8–3.1. The visible texture is porphyritic, indicating two-stage cooling.
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Physical properties
Dark gray to black, fine-grained groundmass with abundant pale, blocky plagioclase crystals; typically vitreous to dull, hardness about 5–6, poor cleavage overall, and specific gravity roughly 2.8–3.1. The visible texture is porphyritic, indicating two-stage cooling.
Formation & geological history
Formed when basaltic magma began crystallizing slowly underground, then erupted and cooled rapidly at or near the surface. Such rocks range from Precambrian to modern and commonly occur in volcanic flows, dikes, and oceanic crust.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally decorative stone; attractive specimens may be collected but are generally not jewelry-grade.
Geological facts
The pale crystals are likely plagioclase phenocrysts embedded in a darker aphanitic matrix. A fresh surface may show tiny black pyroxene grains and can be weakly magnetic if magnetite is present.
Field identification & locations
Identify it by its dark volcanic matrix and conspicuous pale feldspar crystals; distinguish it from diorite by the much finer groundmass. Common worldwide in volcanic provinces, including Iceland, Hawaii, India, and oceanic crust.
