Phosphate rock igneous apatites

Lehr and McClellan [17] studied a suite of sedimentary phosphate rocks and an igneous phosphate rock concentrate and concluded that carbonate substitution in apatite was the dominant factor influencing reactivity measurements. This study also concluded that reactivity... 

The reactivity of phosphate rock is of more importance in TSP production than in phosphoric acid production. Unreactive rocks may require unusually fine grinding or long reaction times or both. Even so, acceptable completion of reaction may be difficult to achieve with some igneous apatites. 

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. 

Apatite also occurs (less abundantly) as igneous phosphate rock which is highly crystalline and much purer than sedimentary phosphorite. Commercially important igneous rock formations of crystalline fluorapatite are found in the Kola peninsula (Khibiny) of Russia, South Africa (Palabora), Brazil (Jacupiranga) with smaller deposits in Uganda, Finland, South Norway and South Sweden. All these deposits, however, at present account for less than 15% of the world total of mined apatite. 

Deep groundwaters from igneous rocks are usually saturated with respect to CaC03, which limits the maximum Ca2 -concentration and thereby also indirectly determines the F"- and H2P04"-concen-trations due to the limited solubility of CaF2 and Ca-phosphates. Both F" and H2PO4" are usually found in the groundwater due to the presence of accessory minerals like fluorite and apatite in the rock. 

Apatite A phosphate mineral (ideally, Ca5(P04)3(F,Cl,0H)) that may occur in sedimentary, metamorphic, and igneous rocks. Arsenate may substitute for phosphate. 

Several hundred phosphate minerals (see Mineralogy) are known, virtually aU of which are orthophosphates An exception is Canaphite, CaNa2P20y dHyO. The important mineral Apatite, Cas(P04)3X is found in igneous pegmatite rocks and hydrothermal veins. Fluoro-, Chloro-, and Hydroxyapatites are found (X = F, Cl, OH, respectively) as well as many substituted varieties. 

The phosphate content of soils tends to remain roughly constant during soil development. Phosphate is only slowly leached from soils, at about the same rate as silica loss, so the total phosphorus content of soils varies little with soil maturity. The form of phosphate, however, changes from predominantly apatite (Cas(OH,F)(P04)3) in igneous rocks to AI(III) and Fe(III) phosphates in moderately to strongly weathered soils. 

Non-phosphatic 0.1-1.0 Widely distributed apatite in almost all igneous rocks... 

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

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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