amygdule

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Englisch - Englisch
A rounded mass of secondary mineral found in volcanic rock
Secondary deposit of minerals found in a rounded, elongated, or almond-shaped cavity in volcanic rock. The cavities (vesicles) were created by the expansion of gas bubbles or steam within lava. Because gas bubbles tend to rise through the lava, amygdules are most common near the tops of flows. Many minerals have been found as amygdules, including some spectacular museum specimens of zeolites