igneous
Basalt
Basalt (Aphanitic Mafic Igneous)
Hardness: 6 (Mohs scale); Color: Dark grey to black, turning reddish-brown when weathered; Luster: Dull to sub-metallic; Structure: Fine-grained/aphanitic; Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, turning reddish-brown when weathered
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, turning reddish-brown when weathered; Luster: Dull to sub-metallic; Structure: Fine-grained/aphanitic; Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava at or very near the Earth's surface. Common in oceanic crust and large volcanic provinces.
Uses & applications
Extensively used in construction as aggregate, road base, concrete, and railroad ballast. Also used for high-quality floor tiles and monuments.
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 on the Moon, Mars, and Venus.
Field identification & locations
Look for a heavy, dark, fine-grained rock with small vesicles (gas bubbles) or a dense, uniform texture. It is found in volcanic regions worldwide, including the Pacific Northwest, Iceland, and Hawaii.
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Dark Igneous Rock (Likely Basalt or Diabase)
Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.
Igneous
Magnetite
Iron(II,III) oxide (Fe3O4)
mineral
Blue Apatite
Apatite (Group), Ca5(PO4)3(F,Cl,OH)
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Sodalite
Sodalite - Na8(Al6Si6O24)Cl2
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Magnetite
Magnetite (Fe3O4)
mineral