Igneous
Basalt
Mafic Igneous Rock (primarily Plagioclase Feldspar and Pyroxene)
Hardness: 6 (Mohs scale); Color: dark gray to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- dark gray to black
- Luster
- dull to earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: dark gray to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It can range in age from modern eruptions to billions of years old.
Uses & applications
Extensively used in construction for aggregate, road base, concrete, and railroad ballast. Also used as dimension stone for statues and architecture, and transformed into rock wool for insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also found on the Moon, Mars, and Venus, often forming spectacular columnar joints like the Giant's Causeway.
Field identification & locations
Identify by its heavy weight (high density), dark color, and lack of visible crystals to the naked eye. Commonly found in volcanic regions, oceanic island chains, and deep-sea ridges. Collectors often look for amygdaloidal basalt (containing mineral-filled gas bubbles).
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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.
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Magnetite
Iron(II,III) oxide (Fe3O4)
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Apatite (Group), Ca5(PO4)3(F,Cl,OH)
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Magnetite
Magnetite (Fe3O4)
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