Phosphorite

thumb|Peloidal phosphorite, [[Phosphoria Formation, Simplot Mine, Idaho. 4.6 cm wide.]]

thumb|Fossiliferous peloidal phosphorite, (4.7 cm across), [[Yunnan, China. ]]

Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxide (P₂O₅). Marketed phosphate rock is enriched ("beneficiated") to at least 28%, often more than 30% P₂O₅. This occurs through washing, screening, deliming, magnetic separation or flotation.

The phosphate is present as fluorapatite Ca₅(PO₄)₃F typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane-sedimentary apatite deposits of uncertain origin.

Limestones and mudstones are common phosphate-bearing rocks. Phosphate-rich sedimentary rocks can occur in dark brown to black beds, ranging from centimeter-sized laminae to beds that are several meters thick. Although these thick beds can exist, they are rarely composed only of phosphatic sedimentary rocks. Phosphatic sedimentary rocks are commonly accompanied by or interbedded with shales, cherts, limestone, dolomites and sometimes sandstone. and Algeria. In the United States phosphorites have been mined in Florida, Tennessee, Wyoming, Utah, Idaho and Kansas.

Classification of phosphatic sedimentary rocks

(1) Pristine: Phosphates that are in pristine conditions have not undergone bioturbation. In other words, the word pristine is used when phosphatic sediment, phosphatized stromatolites and phosphate hardgrounds have not been disturbed. It is then processed by both micro- and macro-organisms. Diatomaceous plankton, phytoplankton, and zooplankton process and dissolve phosphorus in the water. The bones and teeth of certain fish (e.g. anchovies) absorb phosphorus and are later deposited and buried in the marine sediment.

Depending on the pH and salinity levels of the ocean water, organic matter will decay, releasing phosphorus from sediment in shallow basins. Bacteria and enzymes dissolve organic matter on the water–bottom interface, thus returning phosphorus to the beginning of its biogenic cycle. Mineralization of organic matter can also cause the release of phosphorus back into the ocean water.

Littoral environments/intertidal zones: Intertidal zones are also part of the tidal flat system. The intertidal zone is located within the mean high and low tide levels. It is subject to tidal shifts, meaning it is subaerially exposed once or twice a day. It is not exposed long enough to support the growth of most vegetation. The zone contains both suspension sedimentation and bed load.

Types of phosphorite deposition

• Phosphate nodules: These are spherical concentrations that are randomly distributed along the floor of continental shelves. Most phosphorite grains are sand size, although particles greater than 2&nbsp;mm may be present. These larger grains, referred to as nodules, can range up to several tens of centimeters in size. Phosphate nodules are known to occur in significant quantities offshore northern Chile.

thumb|right|Shelly phosphorite from Estonia

• Bioclastic phosphates or bone beds: Bone beds are bedded phosphate deposits that contain concentrations of small skeletal particles and coprolites.
• Continental margin phosphorites: Convergent, passive, upwelling, non-upwelling. This environment accumulates phosphorites in the form of hardgrounds, nodules and granular beds. , the world's total economic demonstrated resource of rock phosphate is 70 gigatonnes, which occurs principally as sedimentary marine phosphorites. It is thought that in India there are almost 260 million tons of rock phosphate. Other countries with significant production include Brazil, Russia, Jordan and Tunisia. Historically, large amounts of phosphates were obtained from deposits on small islands such as Christmas Island and Nauru, but these sources are now largely depleted.

Phosphate ore is mined and beneficiated into rock phosphate. Beneficiation of phosphate ore is a process which includes washing, flotation and calcining.

For use in the chemical fertilizer industry, beneficiated rock phosphate must be concentrated to levels of at least 28% phosphorus pentoxide (P₂O₅). However, most marketed grades of phosphate rock are 30% or more.

See also

• Phosphate mining in the United States

References

External links

• US Geological Survey
• USGS Statistics
• PIRSA website
• International Fertilizer Industry Association
• Science Direct