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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.
It is also a common diagenetic mineral in shales and sandstones, where it sometimes forms concretions, which can encase three-dimensionally preserved fossils. [6] In sedimentary rocks, siderite commonly forms at shallow burial depths and its elemental composition is often related to the depositional environment of the enclosing sediments. [7]
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]
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.
Similar to the marine nodules, concretion layers are defined based on iron and manganese content as well as their combination. [2] High iron content nodules appear a red or brown color, while high manganese content appears black or grey. [2] The dominant metal oxide is related to the elements enriched in the nodule.
An enterolith is a mineral concretion or calculus formed anywhere in the gastrointestinal system. Enteroliths are uncommon and usually incidental findings but, once found, they require at a minimum watchful waiting. If there is evidence of complications, they must be removed.
The most striking aspect of the boulders is their unusually large size and spherical shape, with a distinct bimodal size distribution. Approximately one-third of the boulders range in size from about 0.5 to 1.0 metre (1.6 to 3.3 ft) in diameter, the other two-thirds from 1.5 to 2.2 metres (4.9 to 7.2 ft).
Plinthite concretions are coherent enough to be separated readily from the surrounding soil. Plinthite commonly occurs within and above reticulately mottled horizons. The part of the iron-rich body that is not plinthite normally stains the fingers when rubbed while wet, but the plinthite center does not.