igneous
Porphyry
Porphyritic Basalt / Porphyritic Andesite
Mohs hardness of 6 to 7. Displays a porphyritic texture with light-colored feldspar phenocrysts embedded in a fine-grained dark grayish-green volcanic groundmass. Luster is dull to sub-vitreous.
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Physical properties
Mohs hardness of 6 to 7. Displays a porphyritic texture with light-colored feldspar phenocrysts embedded in a fine-grained dark grayish-green volcanic groundmass. Luster is dull to sub-vitreous.
Formation & geological history
Formed through a two-stage cooling process of igneous magma: large feldspar crystals formed slowly underground before a volcanic eruption rapidly cooled the remaining fine-grained matrix surrounding them.
Uses & applications
Used historically as decorative stone, monuments, and sculpture. Common modern uses include aggregate, decorative landscaping, and lapidary art/polishing for collectors.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple. The famous 'Imperial Porphyry' was highly prized by Roman emperors for royal monuments and sarcophagi due to its rich color and durability.
Field identification & locations
Look for distinct, larger, blocky lighter crystals (phenocrysts) set in a uniform, fine-grained dark rock matrix. Commonly found along glaciated regions, volcanic arcs, and gravel beds near ancient volcanic activity.
