konkretiot

Konkretiot are hard, compact mineral masses that form within sedimentary rocks or soils as minerals precipitate around a central nucleus, producing nodules that differ from the surrounding rock. They are not fossils themselves, though they may enclose fossils or organic matter and may influence the erosion of the host rock.

They form through diagenetic processes when mineral-rich groundwater circulates through sediments and precipitates minerals such as calcite, silica, hematite or other iron oxides, pyrite, or dolomite around a nucleus. The nucleus can be a fossil fragment, a grain of sand, or a piece of organic material. Growth typically occurs over long timescales, and concretion shapes range from near-spherical to ellipsoidal, sometimes showing concentric layering or internal cracks that later fill with minerals (septarian concretions).

Concretions occur widely in marine and lacustrine sedimentary sequences and can vary in size from a few centimeters to over a meter. They are common in shale, sandstone, and chalk formations. Notable examples include iron oxide concretions known as Moqui marbles in the southwestern United States and carbonate concretions in the Chalk of England.

In geology, concretions are used as indicators of past groundwater chemistry and diagenetic conditions. They are often mistaken for fossils or geodes, but at their core they are mineral precipitates that grew around a central core rather than hollow mineral geodes. See also: concretion, nodules, septarian nodule.