igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to reddish-brown groundmass with green, white, or glassy secondary mineral inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull/earthy matrix with vitreous inclusions
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to reddish-brown groundmass with green, white, or glassy secondary mineral inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular/Amygdaloidal; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The 'amygdules' (the green/light spots) form when gas bubbles (vesicles) in the cooling lava are later filled with secondary minerals like quartz, calcite, chlorite, or zeolites through hydrothermal activity.
Uses & applications
Used primarily as crushed stone for aggregate in construction, road base, and concrete. Amygdaloidal varieties are popular among mineral collectors, especially when vesicles contain semi-precious stones like agates or zeolites.
Geological facts
The word 'amygdule' comes from the Greek word 'amygdala', meaning almond, referring to the almond-shaped mineral fillings. These rocks are primary sources for native copper in regions like the Keweenaw Peninsula of Michigan.
Field identification & locations
Identify in the field by looking for a dark, fine-grained volcanic matrix containing distinct, rounded 'pockets' filled with different-colored minerals. Commonly found in ancient volcanic rift zones and oceanic crust exposures.
