igneous
Olivine-bearing Basalt / Scoria
Mafic Igneous Rock (primarily (Mg, Fe)2SiO4)
Hardness: 6-7 (Mohs); Color: Dark grey to black groundmass with prominent olive-green phenocrysts; Luster: Vitreous to dull; Crystal structure: Orthorhombic (olivine crystals); Cleavage: Poor; Specific Gravity: 2.7–3.3.
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to black groundmass with prominent olive-green phenocrysts; Luster: Vitreous to dull; Crystal structure: Orthorhombic (olivine crystals); Cleavage: Poor; Specific Gravity: 2.7–3.3.
Formation & geological history
Formed through the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The green olivine crystals often crystallize early in the cooling magma chamber before being erupted. This type of rock is common in volcanic hotspot regions like Hawaii and Iceland.
Uses & applications
Used primarily in construction as road base or aggregate. High-quality olivine crystals (Peridot) are used in jewelry. In industrial settings, it is used as a flux in steelmaking and as a casting sand.
Geological facts
Olivine is one of the first minerals to crystallize from magma and is the most common mineral in the Earth's upper mantle. Specimens found on the moon by Apollo astronauts were often high in olivine content.
Field identification & locations
Identify in the field by checking for a dark, often vesicular (holed) or fine-grained matrix containing translucent green 'glassy' grains. Commonly found in basaltic lava flows and volcanic fields. Collectors should look for large, clear green crystals known as 'bombs' or 'xenoliths'.
