igneous
Porphyry (Porphyritic Basalt / Andesite)
Porphyritic Igneous Rock
Mohs hardness 6-7, dull to sub-vitreous luster, dark greenish-gray fine-grained groundmass (aphanitic) embedded with distinct reddish-orange to pinkish plagioclase feldspar phenocrysts.
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Physical properties
Mohs hardness 6-7, dull to sub-vitreous luster, dark greenish-gray fine-grained groundmass (aphanitic) embedded with distinct reddish-orange to pinkish plagioclase feldspar phenocrysts.
Formation & geological history
Formed through a two-stage cooling process. Large phenocrysts form slowly deep within the magma chamber, after which the remaining melt erupts or moves closer to the surface, cooling rapidly to form the fine-grained matrix.
Uses & applications
Historically used as an ornamental stone in ancient Rome and Greece, and today used for dimension stone, road aggregate, landscape features, and lapidary art.
Geological facts
Porphyritic texture is unique because it preserves a record of two distinct cooling rates in a single magma body.
Field identification & locations
Identified by contrasting large mineral crystals (phenocrysts) scattered throughout a very fine-grained background rock. Commonly found in volcanic and sub-volcanic terranes, beach cobbles, and glacial deposits globally.
