Phosphorite,phosphate rock orrock phosphate is a non-detritalsedimentary rock that contains high amounts ofphosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4%[1] 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.[1] By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%.[2]
The phosphate is present asfluorapatite Ca5(PO4)3F typically incryptocrystalline masses (grain sizes < 1 μm) referred to ascollophane-sedimentary apatite deposits of uncertain origin.[2] It is also present ashydroxyapatite Ca5(PO4)3OH or Ca10(PO4)6(OH)2, which is often dissolved from vertebrate bones and teeth. In contrast, fluorapatite can originate fromhydrothermal veins. Other sources also include chemically dissolved phosphate minerals fromigneous andmetamorphic rocks. Phosphorite deposits often occur in extensive layers, which cumulatively cover tens of thousands of square kilometres of theEarth's crust.[3]
Limestones andmudstones are common phosphate-bearing rocks.[4] 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 withshales,cherts, limestone,dolomites and sometimessandstone.[4] These layers contain the same textures and structures as fine-grained limestones. They may representdiagenetic replacements ofcarbonate minerals by phosphates.[2] They also can be composed of peloids, ooids, fossils, and clasts that are made up of apatite. Some phosphorites are very small and have no distinctive granular textures. This means that their textures are similar to that of collophane, or finemicrite-like texture. Phosphatic grains may be accompanied byorganic matter,clay minerals,silt-sized detrital grains, andpyrite. Peloidal or pelletal phosphorites occur normally, whereasoolitic phosphorites are not common.[4]
Phosphorites are known fromProterozoicbanded iron formations inAustralia, but are more common fromPaleozoic andCenozoic sediments. ThePermianPhosphoria Formation of the westernUnited States represents some 15 million years of sedimentation. It reaches a thickness of 420 metres and covers an area of 350,000 km2.[2] Commercially mined phosphorites occur inFrance,Belgium,Spain,Morocco,Tunisia,Saudi Arabia[5] andAlgeria. In the United States phosphorites have been mined inFlorida,Tennessee,Wyoming,Utah,Idaho andKansas.[6]
(1)Pristine: Phosphates that are in pristine conditions have not undergonebioturbation. In other words, the word pristine is used when phosphatic sediment, phosphatizedstromatolites and phosphatehardgrounds have not been disturbed.[7]
(2)Condensed: Phosphatic particles, laminae and beds are considered condensed when they have been concentrated. The extraction and reworking processes of phosphatic particles, as well as bioturbation, help this process.[7]
(3)Allochthonous: Phosphatic particles that were moved by turbulent or gravity-driven flows and deposited by these flows.[7]
The heaviest accumulation of phosphorus is mainly on theocean floor. Phosphorus accumulation occurs from atmosphericprecipitation, dust, glacial runoff, cosmic activity, underground hydrothermal volcanic activity, and deposition of organic material. The primary source of dissolved phosphorus is from continental weathering, which is carried by rivers to the ocean.[8] 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 themarine sediment.[9]
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.[9]
Phosphates are known to be deposited in a wide range ofdepositional environments. Commonly, phosphates are deposited in very shallow, near-shore marine or low-energy environments. This includes environments such as supratidal zones, littoral or intertidal zones, and most importantly, estuarine.[9] Currently, areas of oceanic upwelling cause the formation of phosphates. This is due to the constant influx of phosphate from the large, deep ocean reservoir (see below). This cycle allows the continuous growth of organisms.[7]
Supratidal zones: Supratidal environments are part of the tidal flat system where the presence of vigorous wave activity is non-existent. Tidal flat systems are formed along open coasts and in relatively low-wave-energy environments. They can also develop on high-energy coasts behind barrier islands, where they are sheltered from the high-energy wave action. Within the tidal flat system, the supratidal zone lies at a very high tide level. However, it can be flooded by extreme tides and cut across by tidal channels. This is also subaerially exposed, but is flooded twice a month by spring tides.[10]
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.[10]
Estuarine environments: Estuarine environments, or estuaries, are located at the lower parts of rivers that flow into the open sea. Since they are in the seaward section of the drowned valley system, they receive sediment from both marine and fluvial sources. These contain facies that are affected by tide and wave fluvial processes. An estuary is considered to stretch from the landward limit of tidal facies to the seaward limit of coastal facies. Phosphorites are often deposited in fjords within estuarine environments. These are estuaries with shallow sill constrictions. During the Holocene, sea-level rise led to the formation of fjord estuaries through the drowning of glacially eroded U-shaped valleys.[10]
The most common occurrence of phosphorites is related to strong marine upwelling of sediments. Upwelling is caused by deep water currents that are brought to the surface near the coast, where a large deposition of phosphorites may occur. This type of environment is the main reason why phosphorites are commonly associated with silica and chert. Estuaries are also known as a phosphorus “trap”. This is because coastal estuaries contain a high productivity of phosphorus from marsh grass and benthic algae, which allows an equilibrium exchange between living and dead organisms.[11]
Deposits which contain phosphate in quantity and concentration which are economic tomine asore for their phosphate content are not particularly common. The two primary sources for phosphate areguano, formed frombird orbat droppings, and rocks containing concentrations of the calcium phosphate mineral,apatite.
As of 2006[update], the US is the world's leading producer and exporter of phosphate fertilizers, accounting for about 37% of world P2O5 exports.[13] As of December 2018[update], the world's total economic demonstrated resource of rock phosphate is 70gigatonnes,[14] which occurs principally assedimentary marine phosphorites.[15]
As of 2012[update],China, theUnited States andMorocco are the world's largest miners of phosphate rock, with a production of 77megatonnes, 29.4 Mt and 26.8 Mt (including 2.5 Mt in theSahara of Morocco) respectively in 2012 while global production reached 195 Mt.[16] It is thought that in India there are almost 260 million tons of rock phosphate.[17] Other countries with significant production includeBrazil,Russia,Jordan andTunisia. Historically, large amounts of phosphates were obtained from deposits on small islands such asChristmas Island andNauru, 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.[1]Froth flotation is used to concentrate the mined ore to rock phosphate. The mined ore is crushed and washed, creating a slurry. This ore slurry is then treated withfatty acids to cause calcium phosphate to becomehydrophobic.
This rock phosphate is then either solubilized to produce wet-processphosphoric acid, or smelted to produce elementalphosphorus. Phosphoric acid is reacted with phosphate rock to make the fertilizer triplesuperphosphate or withanhydrous ammonia to produce theammonium phosphate fertilizers. Elemental phosphorus is the base for furnace-grade phosphoric acid, phosphorus pentasulfide,phosphorus pentoxide, andphosphorus trichloride.[citation needed]
Approximately 90% of rock phosphate production is used forfertilizer and animal feed supplements, and the balance for industrial chemicals.[1] In addition, phosphorus from rock phosphate is also used in food preservatives, baking flour, pharmaceuticals, anticorrosion agents, cosmetics, fungicides, insecticides, detergents, ceramics, water treatment and metallurgy.[15]
For use in thechemical 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.[1]
It must also have reasonable amounts of calcium carbonate (5%), and <4% combinediron andaluminium oxides.[citation needed] Worldwide, the resources of high-grade ore are declining, and use of lower grade ore may become more attractive.[1]
Beneficiated rock phosphate is also marketed and accepted as an"organic" alternative to "chemical" phosphate fertilizer, which has been further concentrated from it, because it is perceived as being more "natural". According to a report for the FAO, it can be moresustainable to apply rock phosphate as a fertilizer in certain soil types and countries, although it has many drawbacks. According to the report, it may have higher sustainability compared to more concentrated fertilizers because of reduced manufacturing costs and the possibility of local procurement of the refined ore.[1]
Rare earth elements are being found within phosphorites. With the increasing demand for rare earth elements driven by modern technology, a new method of sourcing these elements, independent of China, is becoming increasingly important. With yields greater than those from deposits in China, phosphorites offer a new resource located within the U.S. that would likely lead to independence from the influence of countries outside of the U.S.[18]
