igneous
Porphyritic Igneous Rock with Iron Oxide Weathering
Weathered Porphyry (containing Quartz, Feldspar, Amphibole/Biotite, and Hematite/Limonite)
Hardness: 5.5 to 7 on Mohs scale depending on specific mineral grains; Color: Yellowish-brown to rusty brown matrix with dark blackish phenocryst inclusions; Luster: Earthy, dull, to vitreous on fresh mineral faces; Cleavage: Uneven to subconchoidal fracture; Specific Gravity: Approximately 2.6 - 2.8.
- Hardness
- 5
- Luster
- Earthy, dull, to vitreous on fresh mineral faces
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Physical properties
Hardness: 5.5 to 7 on Mohs scale depending on specific mineral grains; Color: Yellowish-brown to rusty brown matrix with dark blackish phenocryst inclusions; Luster: Earthy, dull, to vitreous on fresh mineral faces; Cleavage: Uneven to subconchoidal fracture; Specific Gravity: Approximately 2.6 - 2.8.
Formation & geological history
Formed through the cooling and crystallization of magma beneath or at the Earth's surface in two stages, producing large phenocrysts embedded in a fine-grained matrix. Subsequent exposure to air and water caused oxidation of iron-bearing minerals, imparting the rusty brown limonite/hematite patina.
Uses & applications
Commonly collected as a geological sample; coarser or unweathered porphyry types are used as crushed stone, dimension stone, or decorative aggregate.
Geological facts
The distinct reddish-brown coating is caused by the chemical weathering of iron-rich silicate minerals (like amphibole, pyroxene, or biotite) into secondary iron oxide oxides like limonite and hematite.
Field identification & locations
Identified in the field by its dark phenocrysts (dark mineral crystals) matrixed within a heavily oxidized, brownish groundmass. Found worldwide in volcanic and plutonic regions. Collectors often test for hardness or break off a small corner to observe unweathered internal minerals.
