nodule

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concretion

concretion, mass or nodule of mineral matter, usually oval or nearly spherical in shape, and occurring in sedimentary rock. It is formed by the accumulation of mineral matter in the pore spaces of the sediment, usually around a fossil or fossil fragment acting as a nucleus. Most concretions are very dense and compact, and are usually composed of calcite, silica, or iron oxide. The material making up the concretion is believed to come from the surrounding rock, being redeposited around the nucleus. Concretions range in diameter from a fraction of an inch to many feet, although most are but a few inches in diameter. Perhaps the best known are the flint nodules found in chalk deposits such as those at Dover, England. Concretions having radiating cracks filled with mineral matter are called turtle stones, or septaria.
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nodule

[′näj·ül]
(anatomy)
A small node.
A small aggregation of cells.
(botany)
A bulbous enlargement found on roots of legumes and certain other plants, whose formation is stimulated by symbiotic, nitrogen-fixing bacteria that colonize the roots.
(geology)
A small, hard mass or lump of a mineral or mineral aggregate characterized by a contrasting composition from and a greater hardness than the surrounding sediment or rock matrix in which it is embedded.
(medicine)
A primary skin lesion, seen as a circumscribed solid elevation.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

nodule

1. any of the knoblike outgrowths on the roots of clover and many other legumes: contain bacteria involved in nitrogen fixation
2. Anatomy any small node or knoblike protuberance
3. a small rounded lump of rock or mineral substance, esp in a matrix of different rock material
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005