igneous
Amygdaloidal Basalt
Vesicular Basalt with secondary mineral infilling (commonly Zeolite, Calcite, or Quartz)
Hardness: 6-7 (groundmass), secondary minerals vary; Color: Dark grey to black groundmass with white or light-colored spots (amygdules); Luster: Dull/earthy matrix with vitreous/pearly inclusions; Texture: Aphanitic with amygdaloidal structure.
- Hardness
- 6-7 (groundmass), secondary minerals vary
- Luster
- Dull/earthy matrix with vitreous/pearly inclusions
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Physical properties
Hardness: 6-7 (groundmass), secondary minerals vary; Color: Dark grey to black groundmass with white or light-colored spots (amygdules); Luster: Dull/earthy matrix with vitreous/pearly inclusions; Texture: Aphanitic with amygdaloidal structure.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. Gas bubbles (vesicles) trapped in the cooling lava were later filled with secondary minerals precipitated from hydrothermal fluids or groundwater over millions of years.
Uses & applications
Used primarily as crushed stone for construction, road base, and railroad ballast. Decorative specimens are popular among amateur geologists and rock collectors due to the contrasting 'polka-dot' appearance.
Geological facts
The term 'amygdaloidal' comes from the Greek word 'amygdalo', meaning almond, referring to the almond-like shape of the mineral-filled vesicles. This rock is a common host for native copper in regions like the Keweenaw Peninsula.
Field identification & locations
Identify in the field by looking for a heavy, dark, fine-grained volcanic rock containing distinct, rounded white or colorful mineral 'spots' that don't rub off. Commonly found in volcanic fields, oceanic crust outcrops, and areas like the Lake Superior region or basaltic trap rocks.
