igneous
Porphyry
Porphyritic Andesite/Rhyolite
Hardness: 6-7 (Mohs scale), Color: Dark purple to reddish-brown with lighter inclusions, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic groundmass with phenocrysts, Cleavage: None
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark purple to reddish-brown with lighter inclusions, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic groundmass with phenocrysts, Cleavage: None
Formation & geological history
Formed during the cooling of silica-rich magma where a slow cooling phase (forming large crystals) is followed by a rapid cooling phase (forming the fine-grained matrix).
Uses & applications
Used historically as a decorative stone in imperial architecture (Imperial Porphyry), more commonly today as crushed stone for road construction and high-durability countertops.
Geological facts
The term 'Porphyry' comes from the Ancient Greek word for purple. Ancient Romans prized the deep purple variety from Mons Porphyrites in Egypt, identifying it with royalty.
Field identification & locations
Identify in the field by looking for a fine-grained, dark-colored matrix containing distinct, visible mineral crystals (phenocrysts). Commonly found in volcanic arcs.
