In
sedimentology and
geology, a
nodule is small, irregularly rounded knot, mass, or lump of a
mineral or mineral aggregate that typically has a contrasting composition, such as a
pyrite nodule in
coal, a
chert nodule in
limestone, or a
phosphorite nodule in
marine shale, from the enclosing
sediment or
sedimentary rock. Normally, a nodule has a warty or knobby surface and exists as a discrete mass within the host
strata. In general, they lack any internal structure except for the preserved remnants of original
bedding or
fossils. Nodules are closely related to
concretions and sometimes these terms are used interchangeably. Minerals that typically form nodules include
calcite, chert,
apatite (phosphorite),
anhydrite, and
pyrite.