Biomedical Engineering Reference
In-Depth Information
staffelita) belonging to carbonate-rich fluorapatites (chemical
formula: Ca
]) [5, 57] and DCPD [58]. Furthermore,
calcium orthophosphates were found in meteoric stones [59]. The
world deposits of natural calcium orthophosphates are estimated to
exceed 150 billion tons; from which approximately 85% belong to
phosphorites and the remaining ~15% belong to apatites [39].
[(F,O)(PO
,CO
)
5
4
3
3
Figure 1.1
Simplified schematic of the phosphorus cycle from apatitic
igneous rock to phosphorite sedimentary rock through
chemical or physical weathering. Life forms accumulate
soluble phosphorus species and can produce apatite through
biomineralization. Reprinted from Ref. [41] with permission.
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 peninsula [60, 61]), 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 [35-39]. most of natural
calcium orthophosphates occur as small polycrystalline structures
(spherulitic clusters). Larger crystals are rare [62]. They usually
have the crystal structure of apatites (hexagonal system, space
group
/m). Giant crystals including “a solid but irregular mass of
green crystalline apatite, 15 ft. long and 9 ft. wide [63] and a single
euhedral crystal from the Aetna mine measuring 2.1 × 1.2 m with
an estimated weight of 6 tons [64] were found. None of them is a
pure compound; 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
P
6
3
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