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nodule - (mineralogy) a small rounded lump of mineral substance (usually harder than the surrounding rock or sediment) mineralogy - the branch of geology that studies minerals: their structure and properties and the ways of distinguishing them.
Manganese nodule. Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization.