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Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth. [11] [12] In concentric growth, the concretion grows as successive layers of mineral precipitate around a central core. This process results in roughly spherical concretions that grow with time.
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.
The iron forming these concretions came from the breakdown of iron-bearing silicate minerals by weathering to form iron oxide coatings on other grains. During later diagenesis of the Navajo Sandstone while deeply buried, reducing fluids , likely hydrocarbons , dissolved these coatings.
Iron is stored in many organisms in the form of ferritin, which is a ferrous oxide encased in a solubilizing protein sheath. [ 10 ] Species of bacteria , including Shewanella oneidensis , Geobacter sulfurreducens and Geobacter metallireducens , use iron oxides as terminal electron acceptors .
Although channel iron deposits are typically low-grade at 53% to 57% Fe in-situ, they are composed of goethite-limonite which are hydrated iron oxide species. Ore typically contains around 8% to 12% water, and <5% SiO 2, and <3% Al 2 O 3. The hydrous iron oxides can be calcined, and the CID ore on a volatile-free basis is around 63% Fe or more.
Calcite cement in an ooid-rich limestone; Carmel Formation, Jurassic of Utah. Minerals bond grains of sediment together by growing around them. This process is called cementation and is a part of the rock cycle. Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains ...
The term can also refer to the process by which waterborne minerals, such as calcium carbonate , iron oxide (hematite or limonite) or silica , replace organic material within the body of an organism that has died and was buried by sediments. [2] Mineralization may also refer to the product resulting from the process of mineralization.
Banded iron formation from the Barberton Greenstone Belt, South Africa. A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe 3 O 4) or hematite (Fe 2 O 3), alternating with bands of iron-poor chert, often red in color, of similar thickness.