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Département des Sciences de la Terre, Université Louis Pasteur, Centre de Géochimie de la Surface, CNRS, 1 Rue Blessig, F-67084, Strasbourg Cedex, France
Experiments into apatite synthesis through bacterial mediation were carried out using the now well-established methods of either replacement of a carbonate precursor, or direct precipitation from solution. In one set of experiments, strontium was added in varying amounts, either to the solution, or through the carbonate precursor. In another set of experiments, barium was added. The final products were analysed by X-ray diffraction and chemical methods. It is evident that both Ba and Sr can enter the lattice of the synthetized apatite. Strontium is able to replace calcium in any proportion and there is a continuous solid-solution series between purely calcitic and purely strontic end-members. Conversely, barium can replace calcium in only small amounts (less than 10% by weight), and its introduction in the lattice soon decreases the crystallinity of the apatite. The barium phosphate which forms does not belong to the apatite group, but approximates to a phosphate having the formula BaHPO4. This difference in behaviour between Sr and Ba results from their different ionic radii and may help constrain the genesis of natural phosphorites.
