Phosphate minerals, natural

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 occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

Phosphorus compounds occur widely in nature, with some of the most common forms being phosphate rocks and minerals, bones, and teeth. Phosphate minerals include calcium phosphate, Ca3(P04)2 apatite, Ca5(P04)30H fluoroapatite, Ca5(P04)3F and chloroapatite, Ca5(P04)3Cl. Elemental phosphorus was first obtained by H. Brand, and its name is derived from two Greek words meaning "light" and "I bear" because of the phosphorescence of white phosphorus due to slow oxidation. 

Thorium is a radioactive metal that occurs naturally in several minerals and rocks usually associated with uranium. However, it is approximately three times more abundant in nature than uranium. On average, soil contains 6 to 10 ppm of thorium. Thorium is most commonly found in the rare-earth thorium-phosphate mineral, monazite, which contains 8% 10% thorium. Current production of thorium is, therefore, linked to the production of monazite, which varies between 5500 and 6500 tonnes per year, with approximately 300 to 600 tonnes of thorium recovered (NEA/IAEA, 2006a). 

Another naturally occurring phosphate mineral is sombrerite (whitlock-ite), which is tricalcium phosphate, Ca3(P04)2. 

Ceramicrete cures to create final waste forms that are analogs of naturally occurring phosphate minerals. These minerals have been shown to be relatively insoluble over geologic time scales. The final waste form is stronger than typical room temperature, hydraulic cements and performs in the manner of high-temperature fused ceramics. The technology has been evaluated in bench-and operational-scale tests on contaminated wastewater, sedimenL ash, and mixed wastes. 

Forms a strong, dense, leach-resistant final product that is an analog of naturally occurring phosphate minerals. 

Fig. 2. Elements found in naturally occurring phosphate minerals (data derived from Nriagu, 1984). Hie number shown for an element indicates the number of naturally occurring phosphate minerals that contain the element. For instance, there are 16 naturally occurring phosphate minerals that contain Pb.

As in the case of hazardous contaminants discussed in Chapter 16, CBPC treatment converts radioactive constituents of waste streams into their nonleachable phosphate mineral forms. It follows the philosophy [7] that, if nature can store radioactive minerals as phosphates (apatite, monozites, etc.) without leaching them into the environment, researchers should be capable of doing the same by converting radioactive and hazardous... 

Phosphate minerals are the main source of phosphoric acid. Unlike nitrogen, phosphorus is found in nature exclusively in a combined state, principally as the P043- ion in phosphate rock, which is mainly calcium phosphate, Ca3(P04)2, and fluorapatite, Ca5(P04)3F. Fluorapatite can be converted to phosphoric acid by grinding up the phosphate rock and forming a slurry with sulfuric acid ... 

Sites of prehistoric human activity may be evident where the human activity has disrupted natural chemical processes. This is particularly true in areas where the weathering processes do not include a great deal of leaching by aqueous solutions. For example, the ultimate source of all phosphates in soils is the breakdown of the phosphate minerals that are weathered from surrounding geological formations (9). Human activity may lead to decreases in the soil s phosphate levels (e.g., by agricultural activities) or increases in these levels when low-solubility phosphate compounds are deposited in the soil (e.g., by burials). The acidity of a soil may also be affected by human activities. 

Tribasic calcium phosphate occurs naturally as the minerals hydroxylapatite, voelicherite, and whitlockite. Commercially, it is prepared by treating phosphate-containing rock with sulfuric acid. Tribasic calcium phosphate powder is then precipitated by the addition of calcium hydroxide. Tribasic calcium phosphate is alternatively prepared by treating calcium hydroxide from limestone with purified phosphoric acid. It may also be obtained from calcined animal bones.Some tribasic calcium phosphate products may be prepared in coarser, directly compressible forms by granulating the powder using roller compaction or spray drying. 

A method is being developed to transform actinide ions in the near surface environment to less soluble, less reactive, thermodynamically stable phosphate minerals phases through application of organophosphorus complexants. These complexants decompose slowly, releasing phosphate to promote the formation of stable phosphate mineral phases, particularly with the more soluble trivalent, pentavalent, and hexavalent actinide ions. The complexant of choice, myo-inositol(hexakisphosphoric acid) or phytic acid, is a natural product widely used as a nutritional supplement. We have determined that phytic acid decomposes slowly in the absence of microbiological effects, that crystalline phosphate minerals are formed as a consequence of its decomposition, and that the formation of actinide (lanthanide) phosphates reduces the solubility of trivalent and hexavalent metal ions under environmental conditions. 

Phosphorus is always combined in nature. Phosphorus is present in all living organisms—as organophosphates and in calcium phosphates such as hydroxyapatite, Ca5(P04.)3(0H), and fluorapatite, Ca5(P04.)3F, in bones and teeth. It also occurs in these and related compounds in phosphate minerals, which are mined mostly in Florida and North Africa. 

Phosphates of most metal ions and other cations are known. Some of these are of enormous commercial and practical importance, for example, ammonium phosphate fertilizers, alkali phosphate buffers, etc. Natural phosphate minerals are all orthophosphates, the major one being fluorapatite hydroxo-... 

However, all weak inorganic and organic acids and their salts ( phosphates, borates) and cations of metals being hydrolysed to release H" ions are titrated at the same time. In the majority of less-mineralized natural waters with pH > 4.5 which do not originate from peat moor regions, the content of free CO2 in water can be calculated from BNCg 3 (so-called acidity). 

The extensive and detailed analytical results given in Table 1 of Forster (1998a) illustrate the extreme compositional diversity exhibited by natural monazites. In fact, Forster has even reported the existence of a monazite-group mineral that is intermediate between monazite and huttonite. Forster s analysis indicated that complete miscibility exists between common monazite-(Ce) and the phosphate mineral brabanite [Ca,Th,U(P04)2]- The monazite/xenotime compositional systematics have also been examined by Fleinrich et al. (1997). Additional data related to the compositional diversity of monazites can be found in the work of Bea (1996), Bea et al. (1994), and Hinton and Patterson (1994). 

Byrappa K (1983) The possible reasons for absence of condertsed phosphates in Nature. Bhys Chem Minerals 10 94-95... 

Gratz R, Heinrich W (1997) Morrazite-xenotime thermobarometry experimental cahbration of the miscibihty gap in the system CeP04-YP04. Am Mineral 82 772-780 Harlov DE, Forster HJ, Nijland TG (2002) Fluid-induced nucleation of (Y+REE)-phosphate minerals within apatite nature and experiment. Am Mineral 87 245-261 Heinrich KFJ (1985) A simple, accrrrate absorbtion model.Microbeam Analysis-1985 20 79-81 Heinrich W, Andrehs G, Franz G (1997) Monazite-xenotime miscibility gap thermometry. I. An empirical calibration. J Metamor Geol 15 3-16... 

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