igneous
Porphyritic Granite (or Porphyry)
Porphyritic Igneous Rock
AI-generated identification · may be incorrectHardness: 6-7 on Mohs scale; Color: Pinkish-red groundmass with contrasting white or light-colored tabular feldspar phenocrysts and dark green/black mafic minerals; Luster: Vitreous to dull; Crystal structure: Porphyritic texture with large crystals (phenocrysts) embedded in a fine-grained groundmass;…
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-red groundmass with contrasting white or light-colored tabular feldspar phenocrysts and dark green/black mafic minerals; Luster: Vitreous to dull; Crystal structure: Porphyritic texture with large crystals (phenocrysts) embedded in a fine-grained groundmass; Specific gravity: 2.6-2.7
Formation & geological history
Formed through a two-stage cooling process in a magmatic environment. Initially, magma cools slowly deep underground, allowing large crystals (phenocrysts like the white feldspar laths) to form. Subsequently, the magma ascends closer to the surface or erupts, causing the remaining melt to cool rapidly into a fine-grained groundmass.
Uses & applications
Used as decorative architectural stone, dimension stone for buildings and monuments, and occasionally in landscaping or as a collector specimen.
Geological facts
The term porphyry derives from the ancient Greek word for purple, referring to the imperial purple stone quarried in Egypt, which was a type of porphyritic andesite. The distinct texture of contrasting large and small crystals makes it a classic teaching example of cooling history in geology.
Field identification & locations
Easily identified in the field by its distinct texture featuring large, blocky feldspar crystals (phenocrysts) floating in a finer matrix. Commonly found in glacial drift, stream gravels, and ancient intrusive volcanic provinces worldwide.
