Where is Amygdaloidal Basalt found?
Basalt with amygdules (secondary mineral fillings)
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Open the appAmygdaloidal Basalt occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Amygdaloidal Basalt
Deccan Traps, India
Extensive flood basalt province, rich in amygdaloidal basalts containing a wide variety of zeolites, agates, and other secondary minerals.
Columbia River Basalt Group, USA
Large igneous province in the Pacific Northwest, known for its basalt flows, some of which are amygdaloidal.
Keweenaw Peninsula, Michigan, USA
Part of the Midcontinent Rift System, famous for copper mineralization within amygdaloidal basalts, often associated with quartz, calcite, and zeolites.
Iceland
Active volcanic island with numerous basaltic lava flows, many exhibiting vesicular and amygdaloidal textures.
Brazil (Rio Grande do Sul)
Known for large amethyst geodes found within amygdaloidal basalt flows.
Collecting tips
- Look for weathered outcrops. Weathering can sometimes highlight the amygdules, making them more visible against the darker basalt matrix.
- Examine lava flow tops and bottoms. Vesicles (and thus amygdules) are often concentrated in the upper and lower portions of lava flows due to gas accumulation and cooling dynamics.
- Check for associated hydrothermal alteration. The presence of other alteration minerals or veins in the surrounding rock can indicate past hydrothermal activity, which is responsible for amygdule formation.
- Break open samples. Sometimes the most attractive amygdules are not visible on the surface and require breaking the rock to expose them.
Quick Check
- Color:
- Dark gray to black matrix with variable colored infillings.
- Luster:
- Dull to sub-vitreous matrix; amygdules vary by mineral.
- Streak:
- White to grayish-white (for basalt matrix, if fine enough to streak).
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