igneous
Basalt with Iron Oxidation
Aphanitic Mafic Igneous Rock (composed primarily of Plagioclase and Pyroxene)
Hardness: 5-6 (Mohs scale); Color: Dark grey to black interior, dull reddish-brown (limonitic) weathered exterior; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (microcrystalline); Specific Gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs scale)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black interior, dull reddish-brown (limonitic) weathered exterior; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (microcrystalline); Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The visible reddish crust is a result of chemical weathering where iron minerals oxidize in the presence of water and oxygen.
Uses & applications
Used extensively in construction as aggregate (crushed stone), road base, and railroad ballast. Dense varieties are used for high-quality floor tiles and statues.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars, forming the dark 'maria' plains visible from Earth.
Field identification & locations
Identify in the field by its high density relative to other dark stones and its lack of visible large crystals (aphanitic). Look for the characteristic orange-brown 'rind' of rust on the exterior. Common worldwide in volcanic seafloors and continental flood basalt provinces.
