The concretion starts at a crystallization center , mostly due to a different chemistry, and gradually grows further outwards. Examples of this are Cretaceous flint nodules in lime, geodes in clay and loess clusters . The latter, carbonate concretions up to the size of a ball, are also described in soil science . These are generally irregular, round, to nut-sized accumulations of iron and manganese oxides , which form when exposed to air, in the aerobic phase, in gley and pseudogley soils . They are dark brown to black and usually dissolve again when the anaerobic phase occurs again. But turf iron stone can also form.
Concretions up to three meters in size and often perfectly spherical can be found on Champ Island .
With the formation of cracks and the subsequent filling of the cracks with newly formed minerals, septaries arise .
- Walter David Keller: The Common Rocks and Minerals of Missouri . University of Missouri Press 1961, ISBN 9780826205858 , p. 66 ( limited online copy in Google Book Search - USA )
- Katz, B., 1998, Concretions Digital West Media, Inc.