igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic Extrusive volcanic rock)
Hardness: 6-7 (Mohs/Basalt matrix), Color: Dark grey to reddish-brown groundmass with white/clear mineral fillings, Luster: Dull/Earthy matrix with vitrous/silky amygdules, Texture: Porphyritic or fine-grained with almond-shaped gas cavities (vesicles) filled by secondary minerals.
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Physical properties
Hardness: 6-7 (Mohs/Basalt matrix), Color: Dark grey to reddish-brown groundmass with white/clear mineral fillings, Luster: Dull/Earthy matrix with vitrous/silky amygdules, Texture: Porphyritic or fine-grained with almond-shaped gas cavities (vesicles) filled by secondary minerals.
Formation & geological history
Formed from the rapid cooling of magnesium and iron-rich lava. As the lava cools, gas bubbles are trapped (vesicles). Over geological time, groundwater Richter in dissolved minerals flows through the rock, depositing minerals like quartz, calcite, or zeolites into the bubbles, turning them into 'amygdules.'
Uses & applications
Used primarily in construction as crushed stone or aggregate. High-quality specimens with colorful mineral fillings (like agates) are popular for rock collecting and lapidary work.
Geological facts
The word 'amygdule' comes from the Latin word for 'almond,' referring to the shape of the mineral-filled gas pockets. The Lake Superior region is famous for amygdaloidal basalts that contain native copper.
Field identification & locations
Identify in the field by look for a heavy, dark-colored rock with distinct 'spots' or 'pockmarks' that are filled with a different, lighter mineral. Common in volcanic fields and oceanic crust exposures.
