igneous
Amygdaloidal Basalt with Quartz/Calcite Inclusions
Amygdaloidal Basalt (SiO2 / CaCO3 inclusions)
Hardness: 5-6 (matrix); Color: Dark brown to reddish-tan weathered exterior (limonite staining); Luster: Dull/Earthy matrix with Vitreous inclusions; Crystal structure: Afanitic matrix with hexagonal (quartz) or rhombohedral (calcite) amygdules.
- Hardness
- 5-6 (matrix)
- Luster
- Dull/Earthy matrix with Vitreous inclusions
Identified More igneous →
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Physical properties
Hardness: 5-6 (matrix); Color: Dark brown to reddish-tan weathered exterior (limonite staining); Luster: Dull/Earthy matrix with Vitreous inclusions; Crystal structure: Afanitic matrix with hexagonal (quartz) or rhombohedral (calcite) amygdules.
Formation & geological history
Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped in the cooling lava were later filled with secondary minerals precipitated from hydrothermal fluids passing through the rock.
Uses & applications
Primary use is in construction as road aggregate or railroad ballast; amygdaloidal specimens are popular among amateur collectors and for educational purposes.
Geological facts
The term 'amygdule' comes from the Latin word for 'almond,' describing the shape of the mineral fillings. These rocks are often associated with the Lake Superior region's famous copper deposits.
Field identification & locations
Identify by looking for 'pockmarks' or holes (vesicles) that are filled with lighter-colored minerals against a dark, dense rock background. Commonly found in volcanic terrains like the Pacific Northwest or the Deccan Traps.
