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Concretion


Concretion A concretion is a volume of sedimentary rock in which a mineral cement fills the spaces between the sediment grains. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum. They form subsequent to burial during diagenesis. They quite often form by the precipitation of a considerable amount of cementing material around a nucleus, often organic, such as a leaf, tooth, and piece of shell or fossil. They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or an iron oxide or hydroxide such as goethite and hematite. They can also be composed of other minerals that include dolomite, ankerite, siderite, pyrite, marcasite, barite and gypsum. Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth. In concentric growth, the concretion grows as successive layers of mineral accrete to its surface. This process results in the radius of the concretion growing with time. In case of pervasive growth, cementation of the host sediments, by infilling of its pore space by precipitated minerals, occurs simultaneously throughout the volume of the area, which in time becomes a concretion. Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several thousand pounds. Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones. Concretionary sandstones are documented at Chittagong and chittagong hill tracts region in Bangladesh. They mostly belong to the Surma and Tipam Group of sediments. Extensive hardness and resistance to weathering render them suitable for using in construction purposes. [Sanzida Murshed]