Phosphorite


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phosphorite

[′fäs·fə‚rīt]
(petrology)
A sedimentary rock composed chiefly of phosphate minerals.

Phosphorite

 

(also phosphate rock), a sedimentary rock composed of more than 50 percent amorphous or microcrystalline minerals of the apatite group (or, on conversion to P2O5, more than 18 percent). In geological prospecting, rocks with satisfactory dressability consisting of 5 to 18 percent P2O5 are also considered phosphorites, especially in open-pit mining.

According to G. I. Bushinskii (1965), there are five types of apatites among the phosphates that compose phosphorites: fluorapatite, carbonate apatite, hydroxylapatite, francolite, and kurskite. According to A. V. Kazakov (1937), the phosphate matter of all phosphorites consists of highly dispersed fluorapatite, and the differences in chemical composition are explained by the presence of mineral impurities. Phosphorites always contain organic matter, carbonates of calcium, magnesium, and iron, clay minerals, pyrite, iron hydroxides, quartz, and chalcedony. They also often contain concentrations of uranium and lanthanides of the cerium group, as well as yttrium, lead, and strontium. Less frequently, they contain impurities of vanadium, scandium, zirconium, selenium, and beryllium. Distinctions are made between massive, nodular (concretional), granular, porous, scoriaceous, pebble, and conglomerate phosphorites on the basis of texture. On the basis of structure, a distinction is made between laminar and sinter phosphorites. Phosphorites are generally black or gray; sometimes they are white, green, red, or yellow.

On the basis of morphological and petrographic characteristics, a distinction is made between deposits of bedded (microgranular), granular, and nodular phosphorites, accumulations of phosphate shells and the skeletons of fish and other organisms, osseous breccia, guano phosphates (formed by the decomposition of the excrement of sea birds), phosphorite pebbles, and phosphatized limestones, marls, and chalk.

Bedded (geosyncline) phosphorites comprise dense uniform rocks with a conchoidal fracture. Composed of round phosphate granules and oolites cemented with phosphate, carbonate, or siliceous matter, they are found in large strata (more than 10 m thick) and are characterized by resistance to destruction and high quality (28–36 percent P2O5). Such deposits occur in the Cambrian formations at Karatau in the USSR, Khubsugul in the Mongolian People’s Republic, K’unyang in the People’s Republic of China, and the Georgina River in Australia, as well as in the Permian deposits of the Rocky Mountains in the United States.

Granular phosphorites comprise carbonate or terrigenous sedimentary rocks with numerous phosphate concretions and organic residues (phosphatized fragments of fish, reptiles, mollusks, and foraminifers) cemented with carbonate, siliceous, and clay matter. The productive strata are usually 2–3 m thick, although some reach 10 m; the deposits contain 22–30 percent P2O5. Granular phosphorites are found in Cretaceous and Paleogenic formations in the European part of the USSR and in North Africa (Algeria, Tunisia, and Morocco) and in the Miocene strata of the Sechura region in Peru.

A distinction is made between marine and continental accumulations on the basis of formation conditions. The origin of marine phosphorites is controversial. According to some scientists, including A. V. Kazakov, A. S. Sokolov, A. I. Smirnov, G. Mansfield, and V. McKelvey, deep ocean waters rich in dissolved phosphorus as a result of the decomposition of plankton were carried by currents to shoals and lost their carbon dioxide in the photosynthesis zone, leading to the chemical precipitation of P2O5. Rejecting the possibility of the chemogenic formation of phosphorites, other investigators, namely, G. I. Bushinskii and V. N. Kholodov, proposed that phosphorus at a certain moment in geological history entered the oceans in large amounts from the continents, was precipitated by plankton and other organisms near the shores (at the mouths of ancient rivers), and, as a result of diagenetic redistribution in the sediment, formed phosphorite deposits.

Most industrial reserves of phosphorus in the world occur in bedded and granular phosphorites; nodular and karst phosphorites and guano deposits are also important sources of phosphorus. Other types of phosphorites are only of theoretical interest.

Up to 90 percent of the phosphorite production is used for the manufacture of phosphate fertilizers, such as phosphorite meal, superphosphate, dicalcium phosphate dihydrate, basic (or Thomas) slag, and ammonium phosphate. In addition, a number of rare elements are extracted in commercial quantities as a byproduct from phosphorites.

REFERENCES

Kazakov, A. V. Khimicheskaia priroda fosfatnogo veshchestva fosforitov i ikh genezis. Leningrad, 1937.
Bushinskii, G. I. “Fosfaty kal’tsiia fosforitov.” In Voprosy geologii agronomicheskikh rud. Moscow, 1956.
Bushinskii, G. I. Drevnie fosfority Azii i ikh genezis. Moscow, 1966.
Gimmel’farb, B. M. Zakonomernosti razmeshcheniia mestorozhdenii fosforitov SSSR i ikh geneticheskaia klassifikatsiia. Moscow, 1965.
Shatskii, N. S. “Fosforitonosnye formatsii i klassifikatsiia fosforitovykh zalezhei.” In Doklady Soveshchaniia po osadochnym porodam, fasc. 2. Moscow, 1955.
Kholodov, V. N. O redkikh i radioaktivnykh elementakh v fosforitakh. Moscow, 1963. (Tr. In-ta mineralogii, geokhimii i kristallokhimii redkikh elementov, fasc. 17.)
Mansfield, G. R. “Origin of the Western Phosphates of the United States.” American Journal of Science, 1918, vol. 46, no. 274.

V. N. KHOLODOV

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