Concretions

A concretion is an oval or spherical isolated form of sedimentary rock present in a layer of another type of sediment. The shape is isometric, elongated, partly angular till partly rounded and they look like an ellipse.

Examples

Gravel formation

Gravel formation can form due to a chemical reaction where the silicic acid penetrates the soil or rock where surrounding minerals and rocks are partly replaced by silicates. This happens due to weathering. This process can cause flint to form in calciumcarbonate rich environments.

Iron concretions

Iron concretions are sedimentary rocks which originated from the precipitation of iron iodes out of water. The concretions consist of 20-75% of iron mixed with sand, silt and clay. It can also contain some other materials like: water, phosphor, organic material and aluminum. The rock is soft with a rusty color.

Manganese concretions

Manganese concretions contain a high concentration of nickel, zinc and cupper. This makes this type of concretion a very valuable concretion. There can be biogenic material present and this causes some confusion and often are these concretions mistaken for fossils. They originate from the cleavage planes of rocks where they form due to crystallization. They always form concentric around a core. They can also be found at the bottom of a shallow sea or lake.

Phosphate concretions

This concretion can be found as an individual layer in cemented sedimentary rocks consisting of marl, chalk and chalky marl. This concretion is localized on a hardened substrate made of bioclastic materials. The color of this concretion varies from light grey with brown to black.

Phosphate patches

The presence of greenish yellow spots is an indication for phosphate patches. This patches originate from the oxidation of phosphate and iron connections in the subsoil. These connections can only exist when surplus of phosphate is present in the subsoil. This surplus doesn't exist in an area where regular fertilization takes place, but can take place at the storage of fertilizer, close to latrines ,at place where cattle stays for a long time and at spots where bone material is burried.

Rust patches

Rust patches arise at places with stagnant water above an aquitard layer in the soil profile. In this layer many oxidation and reduction processes take place. This causes the iron oxide to be visible in the layer and give a red brownish color to the soil.

Sand crusting

Sand crusting is a concretion. This concretion forms when mineral rich water flows through a sand layer. These minerals form bridges between the sand particles and causes cementation of the loose sand particles. When sand crusting continues one would get [sandstone]. Sand crusting can be recognized by spherical chunks of sand in a loose sand layer.

Vivianite

Vivianite is a mineral that consist of hydrated ironphosphate. Vivianite can arise close to the surface in an anaerobic environment. Iron concretions in the subsoil can be affected by phosphote rich water resulting in the forming of vivianite with a light blue color. If vivianite reaches the surface and is exposed to oxygen and sunlight it starts oxidize resulting in vivianite made up of phosphate and trivalent iron with a black color. Vivianite is a very crumbly material present on the top of iron concretions.