What is a nodule in mineralogy ?

Nodule : definition

A nodule is a roughly rounded mass with a generally concreted texture, centimeter to decimetric size, rarely more. Nodules are found especially in a sedimentary environment where they result from phenomena of chemical precipitation or organic activity (siliceous nodule, phosphates, calcarous, etc...).

The polymetallic nodules of the ocean floor are mineral concretions resulting from the succession of crusts of metallic oxides (dominant manganese) around a nucleus. They would be a distal manifestation of oceanic hydrothermal vents, black smokers.