igneous
Porphyritic Basalt or Andesite
Porphyritic Basalt or Porphyritic Andesite
AI-generated identification · may be incorrectDark gray to black fine-grained matrix (groundmass) with distinct, larger light-colored crystals (phenocrysts), likely plagioclase feldspar. Hardness is typically 5-6 on the Mohs scale.
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Physical properties
Dark gray to black fine-grained matrix (groundmass) with distinct, larger light-colored crystals (phenocrysts), likely plagioclase feldspar. Hardness is typically 5-6 on the Mohs scale.
Formation & geological history
Formed from cooling magma or lava where a two-stage cooling process occurred. The larger crystals cooled slowly deep underground, and the finer matrix cooled rapidly upon or near the surface.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth, tumbled pieces like this are often used for decorative purposes or collecting.
Geological facts
The term 'porphyritic' refers to the texture of igneous rocks with two distinct sizes of crystals, indicating a complex cooling history.
Field identification & locations
Identified by the contrast between the large phenocrysts and the fine-grained aphanitic groundmass. Found globally in volcanic regions.
