Phosphorites elemental 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... 

Other resolubilized trace metals precipitate as replacement ions in existing solids such as fecal pellets and bone. Examples of these fiassilized materials include barite, phosphorite, and glauconite. These precipitates contain small amounts of a variety of trace metals as well as other elements. As a result, their chemical composition is variable and their structure is usually amorphous, making it difficult to assign them an empirical formifia. 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

The average composition of marine phosphorites on continental margins is summarized in Table 2, and the enrichment factors E m (phosphorite/shale) for various elements are plotted in Figure 23. (phosphorite/shale) values of 1 (within a factor of 2) for Li, B, Mg, Al, Si, K, Ti, Fe, Ga, and Bi indicate their aluminosilicate origin. The moderate enrichment of Sc, V, Cr, Y, Zr, Nb, REEs, and Th, and high enrichment of Ag, Cd, and U probably result from their replacement of Ca in CFA, facilitated by their similar ionic radii. However, the enrichment of biophile elements S, Se, Cu, Zn, Mo, Sn, Sb, Au, and Hg is also related to the occurrence of sulfides and organic matter in the samples (Li, 2000). 

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