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Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as geological curiosities.The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 3 m (9.8 ft) in diameter.Spheroidal concretions, as large as 9 m (30 ft) in diameter, have been found eroding out of the Qasr El Sagha Formation within the Faiyum depression of Egypt.They typically form when a mineral precipitates and cements sediment around a nucleus, which is often organic, such as a leaf, tooth, piece of shell or fossil.For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found.Occasionally, concretions contain a fossil, either as its nucleus or as a component that was incorporated during its growth but concretions are not fossils themselves.
(1995), are World War II military shells, bombs, and shrapnel, which are found inside siderite concretions found in an English coastal salt marsh.
Because of the variety of unusual shapes, sizes and compositions, concretions have been interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extraterrestrial debris or human artifacts.
Detailed studies (i.e., Boles et al., 1985; Thyne and Boles, 1989; Scotchman, 1991; Mozley and Burns, 1993; Mc Bride et al., 2003; Chan et al., 2005; Mozley and Davis, 2005) published in peer-reviewed journals have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified during diagenesis.
Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.
Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones.Concretions are often exposed at the surface by subsequent erosion that removes the weaker, uncemented material.