Particulate Forms of Phosphorus

Phosphorus is the tenth most abundant element on Earth with an average crustal abundance of 0.1 % and may be found in a wide variety of mineral phases. There are approximately 300 naturally occurring minerals in which is a required structural 

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c axis. In its pure form, F, OH, or Cl occupy sites along this axis to form fluorapatite, hydroxyapatite, or chlorapatite, respectively. However, because of the open nature of the apatite crystal lattice, many substitutions are possible and pure forms of apatite as depicted by the general formula in Table 14-2 are essentially never foxmd. Of the possible substituting ions, carbonate ion is by far the most important followed by Na, SO , and Mg . The most common form of natural apatite is francolite, a highly substituted form of carbonate fluorapatite deposited in marine systems. The substitution of CO3 ions into the mineral lattice has a substantial effect on apatite solubility (Jahnke, 1984). More studies are required, however, before the effects of all substituting ions are understood and an accurate assessment of the solubility of complex, natural apatites can be made. 

The importance of this mineral is perhaps best demonstrated by the diversity of its sources. Assuming that nearly all of the P present is in the form of apatite, igneous rocks contain between 0.02 and 1.2% apatite. Apatite is also produced by organisms (including man) as structural body parts such as teeth, bones, and scales. After an organism dies, these components tend to accumulate in sediments and soils. In some locations, these constituents are reworked and concentrated by physical processes to form economically important deposits. By far the largest accumulations of P on the Earth s 

Phosphate is ubiquitous as a minor component within the crystal lattices of other minerals or adsorbed onto the surface of particles such as clays, calcium carbonate, or ferric oxyhydroxides (Berner, 1973). Therefore, in general, transport of these other particulate phases represents an important transport pathway of P as well. 

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