igneous
Porphyritic Rhyolite or Andesite (Porphyry)
Porphyritic igneous rock (Composition depends on exact mineralogy, often Rhyolite or Andesite)
Hardness: 6-7 (Mohs scale depending on groundmass and phenocrysts); Color: reddish-brown or purple groundmass with white/light-colored phenocrysts (likely feldspar or quartz); Luster: Subvitreous to dull; Crystal structure: Porphyritic texture with distinct larger crystals in a fine-grained matrix.
- Hardness
- 6-7 (Mohs scale depending on groundmass and phenocrysts)
- Luster
- Subvitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale depending on groundmass and phenocrysts); Color: reddish-brown or purple groundmass with white/light-colored phenocrysts (likely feldspar or quartz); Luster: Subvitreous to dull; Crystal structure: Porphyritic texture with distinct larger crystals in a fine-grained matrix.
Formation & geological history
Formed from magma that cooled in two stages: an initial slow cooling deep underground forming the large crystals (phenocrysts), followed by a rapid cooling closer to or at the surface, forming the fine-grained matrix (groundmass). Geological age varies widely.
Uses & applications
Often used as aggregate in construction, road base, and historically for decorative stone work. Sometimes collected as a lapidary rough if it takes a good polish.
Geological facts
The word 'porphyry' comes from the Ancient Greek word for purple, referring to a specific type of purple-red rock valued in antiquity. Today, porphyritic refers to the texture rather than a specific chemical composition.
Field identification & locations
Easily identified by its distinctive 'chocolate chip cookie' appearance—larger crystals embedded in a finer matrix. Common in volcanic regions globally.
