igneous
Amygdaloidal Basalt with Epidote
Amygdaloidal Basalt (Ca2(Al,Fe)3(SiO4)3(OH))
Hardness: 6-7 (epidote) and 5-6 (basalt matrix); Color: Dark grey to black matrix with yellowish-green inclusions; Luster: Dull to sub-vitreous; Structure: Vesicular/amygdaloidal; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 (epidote) and 5-6 (basalt matrix)
- Color
- Dark grey to black matrix with yellowish-green inclusions
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (epidote) and 5-6 (basalt matrix); Color: Dark grey to black matrix with yellowish-green inclusions; Luster: Dull to sub-vitreous; Structure: Vesicular/amygdaloidal; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the cooling of mafic lava flows. The holes (vesicles) created by gas bubbles during cooling were later filled with secondary minerals like epidote through hydrothermal activity, often during the Proterozoic or Paleozoic eras.
Uses & applications
Commonly used as crushed stone for aggregate in construction and road building. Attractive specimens are used for lapidary work, jewelry, and educational geological collections.
Geological facts
The green spots are epidote, a mineral that frequently forms in volcanic rocks undergoing low-grade metamorphism. This specific look is a hallmark of the Keweenawan volcanic sequence in the Great Lakes region.
Field identification & locations
Identify in the field by the dark, fine-grained volcanic matrix containing rounded green mineral 'pockets'. Commonly found on beaches of the Great Lakes (especially Lake Superior) and other ancient volcanic terrains.
