Un nódulo es una concreción contenida en algunas rocas o que se ha formado en el fondo del mar. Por lo general tiene forma globular muy pequeña, donde en el seno de una roca se ha concentrado en alto grado uno de los componentes de ésta. Suele tener un tamaño de varios centímetros. Los pisolitos miden unos milímetros y los oolitos sólo un milímetro. Las rocas que los contienen más abundantemente son las margas y las calizas. En las rocas magmáticas se encuentran unos de color oscuro a los cuales se da el nombre de gabarros. La composición química de los nódulos puede ser compleja, aunque siempre predomina un mineral: caliza, dolomita, limonita, sílex, fosfatos. En el seno de los océanos se hallan diseminados millones de nódulos polimetálicos.

Leer el artículo completo en Wikipedia · CC-BY-SA

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. In sedimentology and geology, nodular is used to describe a sediment or sedimentary rock composed of scattered to loosely packed nodules in 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. Nodule is also used for widely scattered concretionary lumps of manganese, cobalt, iron, and nickel found on the floors of the world's oceans. This is especially true of manganese nodules. Manganese and phosphorite nodules form on the seafloor and are syndepositional in origin. Thus, technically speaking, they are concretions instead of nodules. Chert and flint nodules are often found in beds of limestone and chalk. They form from the redeposition of amorphous silica arising from the dissolution of siliceous spicules of sponges, or debris from radiolaria and the postdepositional replacement of either the enclosing limestone or chalk by this silica.

Leer el artículo completo en Wikipedia · CC-BY-SA