igneous
Basalt with Large Augite Phenocrysts
Porphyritic Basalt (containing Ca(Mg,Fe,Al)(Al,Si)2O6)
Hardness: 5.5-6.0; Color: Dark black crystals in a tan/yellowish-brown weathered matrix; Luster: Vitreous to dull; Crystal Structure: Monoclinic (Augite); Specific Gravity: 2.9-3.4
- Hardness
- 5
- Luster
- Vitreous to dull
Identified More igneous →
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Physical properties
Hardness: 5.5-6.0; Color: Dark black crystals in a tan/yellowish-brown weathered matrix; Luster: Vitreous to dull; Crystal Structure: Monoclinic (Augite); Specific Gravity: 2.9-3.4
Formation & geological history
Formed from the rapid cooling of magnesium and iron-rich lava at the Earth's surface. The large black Augite crystals (phenocrysts) formed slowly at depth before the eruption of the surrounding fine-grained matrix.
Uses & applications
Study of volcanic processes, educational specimens, and a decorative curiosity for mineral collectors.
Geological facts
This specimen is a classic example of 'porphyritic texture', which tells a two-stage cooling history of the magma. The large black crystals of Augite are specific minerals belonging to the pyroxene group.
Field identification & locations
Identify in the field by the distinct, blocky black crystals embedded in a lighter, spesso vesicular (holed) or weathered volcanic groundmass. Common in volcanic regions like Hawaii, Iceland, and the American Southwest.
