Natural calcium orthophosphates occur in most geological envi

… Natural calcium orthophosphates occur in most geological environments, usually as accessory minerals (< 5%). Concentrations sufficient for economic use (> 15%) are also available. The largest world deposits of natural apatites are located in Russia (the Khibiny and Kovdor massifs, Kola peninsula61,62), Brazil and Zambia, while the largest world deposits of natural phosphorites are located in Morocco, Russia, Kazakhstan, USA (Florida, Tennessee), China and Australia, as well as in the oceans.36-40 Most of natural calcium orthophosphates occur as small polycrystalline structures (spherulitic clusters). Larger crystals are rare.63 They usually have the crystal structure of apatites (hexagonal system, space group P63/m).

Giant crystals, including ��a solid but irregular mass of green crystalline apatite, 15 feet long and 9 feet wide��64 and a single euhedral crystal from the Aetna mine measuring 2.1 x 1.2 min with an estimated weight of 6 tons,65 were found. None of them are pure compounds; they always contain admixtures of other elements. For example, ions of calcium might be partially replaced by Sr, Ba, Mg, Mn, K, Na, Fe; ions of orthophosphate may be partly replaced by AsO43-, CO32- and VO4;2-30,33-66 ions of hydroxide, chloride, bromide, carbonate and oxide may, to a certain extent, substitute for fluoride in the crystal lattice of natural apatites.48 Furthermore, various organic radicals have been found in natural apatites.67,68 In principle, the crystal structure of apatites can incorporate half of the periodic table in its atomic arrangement.

In medicine, this property might be used as an antidote for heavy metal intoxication.69 Ease of atomic substitution for apatite leaves this mineral open to a wide array of compositions. This might be related to the fact that the apatite structure type displays porous properties.70 The substitutions in apatites are usually in trace concentrations, but large concentrations and even complete solid solutions exist for certain substituents (e.g., F- and OH-). To make things even more complicated, some ions in the crystal structure may be missing, leaving crystallographic defects, which leads to formation of non-stoichiometric compounds. Figure 2 shows examples of polycrystalline and single-crystalline samples of natural FA. Figure 2. Polycrystalline (A) and single-crystalline (B) FA of a geological origin.

The single crystal has a gray-green color due to incorporated ions of transition metals. Manufacturing of elementary phosphorus (white and red),71,72 phosphoric acids,37,73-76 various phosphorus-containing chemicals and, especially, agricultural fertilizers (namely, superphosphate,77-79 ammonium orthophosphates80) are the major industrial Brefeldin_A applications of natural calcium orthophosphates. The annual consumption of a phosphate rock has approached ~150 million tons, and about 95 percent of this production is utilized in the fertilizer industry.

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