igneous
Basalt
Extrusive mafic igneous rock; primarily (Ca,Na)(Mg,Fe,Al,Si)2O6 with (Ca,Na)(Al,Si)4O8
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of a terrestrial planet or moon. Most basalt on Earth is less than 200 million years old due to tectonic recycling.
Uses & applications
Used extensively in construction as an aggregate for road base, railroad ballast, and concrete. It is also used as a dimension stone, for stone wool insulation, and occasionally in jewelry as 'lava stone' beads.
Geological facts
Basalt is the most common rock type in the Earth’s crust and makes up most of the ocean floor. Large parts of the Moon's surface (the maria) and Mars are also composed of basalt.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and lack of visible large crystals. Common in volcanic regions and oceanic islands like Hawaii or Iceland. Collectors look for vesicles or 'pockets' that may contain secondary minerals like zeolites or olivine.
