Synthetic lamellar mixed A-B type carbonate containing apatite with two different Ca/P ratios (1.67 and 1.82) has been produced from a lamellar calcium acetate phosphonate precursor to investigate the impact of carbonate substitution on apatite crystal structure. Examination of X-ray diffraction line profile broadening was used to determine microcrystallite size and microstrain at different temperatures and to relate these to biological apatite. Both crystallite size and microstrain contributed considerably to the diffraction peak broadening in samples prepared below 700 degreesC. In the 500-700 degreesC temperature interval the crystallite sizes demonstrated a minimum at 600 degreesC, while a maximum for the microstrain was observed. The decrease of the crystallite size and the increase of the microstrain at 600 degreesC, were associated with structural arrangements in the phosphate sublattice. Above 700 degreesC, larger crystals are formed possessing little microstrain. It is shown that apatite formed at 600 degreesC with a Ca/P ratio of 1.82 is comparable in morphology, carbonate type, content, crystallite size, and microstrain to those in biogenic apatites. Infrared spectral analysis was used to quantify the carbonate type and content.