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In geology, a rounded mineral concretion that is distinct from, and may be separated from, the formation in which it occurs. Nodules usually are elongated and have a knobby irregular surface; they are generally oriented parallel to the bedding. Chert and flint, clay ironstone, and phosphorites commonly occur as nodules. Manganese-rich nodules are found on the ocean floor.
This entry comes from Encyclopædia Britannica Concise. For the full entry on nodule, visit Britannica.com.