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Apatite Formation in the CaO–PbO–P 2 O 5 –H 2 O System at 23°± 1°C
Author(s) -
Bothe James V.,
Brown Paul W.
Publication year - 2000
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2000.tb01240.x
Subject(s) - apatite , crystallinity , aqueous solution , mineralogy , precipitation , crystal structure , phase (matter) , lattice constant , materials science , lead oxide , analytical chemistry (journal) , nuclear chemistry , chemistry , crystallography , diffraction , metallurgy , environmental chemistry , organic chemistry , physics , meteorology , optics
Lead is a toxic heavy metal that is commonly used to manufacture products such as storage batteries and electroceramics. In an effort to minimize the health risks to humans, the low‐cost stabilization of lead via its incorporation into the apatite crystal structure has been explored. Therefore, apatite formation in the quaternary system CaO–PbO–P 2 O 5 –H 2 O at room temperature (23°± 1°C) and atmospheric pressure from PbO, CaHPO 4 , Ca 4 (PO 4 ) 2 O, and H 3 PO 4 was studied. Apatites were prepared via precipitation from aqueous‐based mixtures with a constant (Ca + Pb):P molar ratio of 1.67. Aqueous mixtures that contained up to 1 mol of PbO (massicot) per mole of apatite (Ca 10− x Pb x (PO 4 ) 6 (OH) 2 ) resulted in the precipitation of weakly crystalline Ca 10 (PO 4 ) 6 (OH) 2 as the predominant phase. Highly crystalline Pb 10 (PO 4 ) 6 (OH) 2 was formed from mixtures that contained 8–9 mol of PbO per mole of apatite. Intermediate proportions of lead oxide, which entailed 4.5–7 mol of PbO per mole of apatite, produced a stable quaternary apatite phase of moderate crystallinity, with an approximate composition of Ca 5 Pb 5 (PO 4 ) 6 (OH) 2 . The lattice parameters of this quaternary apatite were calculated using a least‐squares refinement program and other existing X‐ray diffraction software. Experimental observation, along with computer modeling of the solution chemistry, enabled the mechanism of apatite formation to be established. Dissolution of lead oxide and calcium‐bearing minerals, such as monetite (CaHPO 4 ), with the aid of H 3 PO 4 , resulted in the precipitation of lead‐rich apatites. This phenomenon was followed by their slow conversion to the stable quaternary apatite (Ca 5 Pb 5 (PO 4 ) 6 (OH) 2 ).