igneous
Weathered porphyritic volcanic rock (likely basalt/andesite)
Fine-grained mafic to intermediate volcanic rock; probably porphyritic basalt or andesite, composed mainly of plagioclase with pyroxene and iron oxides
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dull earthy brown from weathering, with scattered dark mineral grains and possible small phenocrysts; massive, fine-grained texture and no obvious cleavage. Specific gravity roughly 2.7–3.0.
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Physical properties
Hardness about 5.5–6.5 Mohs; dull earthy brown from weathering, with scattered dark mineral grains and possible small phenocrysts; massive, fine-grained texture and no obvious cleavage. Specific gravity roughly 2.7–3.0.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, then chemically weathered to a brown oxidation rind. Its exact geological age cannot be determined from the photograph; volcanic rocks are widespread around southeastern Brazil.
Uses & applications
Commonly used as aggregate, roadstone, riprap, and sometimes building stone; this ordinary weathered specimen has little commercial or jewelry value.
Geological facts
The dark spots likely represent pyroxene, magnetite, or altered mafic minerals, while the brown color reflects oxidation of iron-bearing minerals. It could also be a weathered volcanic cobble transported by stream or soil processes.
Field identification & locations
The rounded shape suggests natural abrasion. A fresh broken surface, streak, acid test, and hand-lens examination would help distinguish basalt from andesite; basalt is typically darker and denser, while andesite is commonly gray to brown. The Rio de Janeiro region can contain weathered volcanic and mafic rocks, though the image alone is not diagnostic.
