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A concretion is a hard and compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. [1] Concretions are often ovoid or spherical in shape, although irregular shapes also occur.
They commonly occur as concretions in mudstones and siltstones. Pyrite and marcasite (FeS 2) are sulfide minerals and favor reducing conditions. They are the most common in fine-grained, dark colored mudstones. Hematite (Fe 2 O 3) is usually the pigment in red beds and requires oxidizing conditions.
The second paper studying spherule size extended the study area 2–3 km further south on the plains to Victoria Crater. [4] This paper reported similar observations to the first but went further to suggest the observed size variation might be due to sampling different sediment stratigraphic levels at different locations.
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains). The image shows a Sandstone Concretion in the wall at the beach of Año Nuevo State Reserve. This concretion was too high in the wall to place a ruler, but I estimate it to be about 400 mm long.
These concretions occur in the black shale in the lower part of the Kettle Point Formation. Because the concretions are so much harder than the enclosing weakly indurated shale, they readily weather out of the shale along the shoreline and are incorporated into the rubble mantling the lake bottom adjacent to the outcrop.
In the prostate gland, where they are also known as prostatic concretions, corpora amylacea are rich in aggregated protein that has many of the features of amyloid, whereas those in the central nervous system are generally smaller and do not contain amyloid. [3]
A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab.Coal balls were formed in Carboniferous Period swamps and mires, when peat was prevented from being turned into coal by the high amount of calcite surrounding the peat; the calcite caused it to be turned into stone instead.
Concretions are sometimes thought to be fossils, and occasionally one contains a fossil, but are generally not fossils themselves. Chert or flint nodules in limestone can often take forms that resemble fossils.