Ingeology and particularly insedimentology, anodule is a small, irregularly rounded knot, mass, or lump of amineral or mineralaggregate that typically has a contrasting composition from the enclosingsediment orsedimentary rock. Examples includepyrite nodules incoal, achert nodule inlimestone, or aphosphorite nodule inmarineshale. Normally, a nodule has a warty or knobby surface and exists as a discrete mass within the hoststrata. In general, they lack any internal structure except for the preserved remnants of originalbedding orfossils. Nodules are closely related toconcretions and sometimes these terms are used interchangeably. Minerals that typically form nodules includecalcite, chert,apatite (phosphorite),anhydrite, and pyrite.[1][2]
Nodular is used to describe a sediment or sedimentary rock composed of scattered to loosely packed nodules in a matrix of like or unlike character. It is also used to describe mineral aggregates that occur in the form of nodules, e.g. colloform mineral aggregate with a bulbed surface.[1]
Nodule is also used for widely scattered concretionary lumps of manganese,cobalt,iron, andnickel found on the floors of the world'soceans. This is especially true ofmanganese nodules. Manganese and phosphorite nodules form on the seafloor and are syndepositional in origin. Thus, technically speaking, they are concretions instead of nodules.[1][2]
Chert andflint nodules are often found in beds of limestone andchalk. They form from the redeposition ofamorphoussilica arising from the dissolution ofsiliceousspicules ofsponges, or debris fromradiolaria and the postdepositional replacement of either the enclosing limestone or chalk by this silica.[2]
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