Phosphorite deposits composition

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

Occurrence and Circulation op Phosphorus —-Mineral Phosphates—Assimilation by Plants—Sources of Phosphates—The Composition of Phosphorites —-The Distribution of Phosphatic Rooks—Ooeanio Deposits and Guanos—-The World s Production of Phosphate Rook. 

Insular phosphates of two different types exist, depending on whether guano interacts with igneous rocks of intermediate or basic types, or whether such action is confined to calcareous accumulation — such as coral. The first type, which will now be discussed, has more diversified mineralogical compositions. Rock phosphates of magnesium, aluminium and iron comprise less extensive deposits than phosphorites and, consequently, are of less economic importance. 

The commonest concentrated igneous apatite deposits consist mainly of fluorapatite, Ca,o(P04)6F2, but isomorphous chlorapatite, CamCPO igClj, and hydroxyapatite, Caio(P04)6(OH)2, are also found in close association or in solid solution (Chapter 5.1). Apatite occurs mostly as a sedimentary deposit with an approximate composition Caio(P04)6F2, which is named Phosphorite or "phosphate rock. Collophane is a term sometimes used to describe varieties of cryptocrystalline phosphorite which are fine grained and optically isotropic. 

Apatites form an important series of minerals. They occur as a minor constituent of many igneous rocks, although a few large igneous deposits are known, like the Kola peninsular. Apatite is also present in most metamorphic rocks, especial crystalline limestone. Less well crystallised deposits of rather variable composition, usually referred to as rock phosphates or phosphorite, occur in large deposits, some of which were formed by the reaction between phosphatic solutions from guano and calcareous rock, or precipitated from sea water. 

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