Carbonates substitution in lead apatites – IR spectroscopic study

Similar to Ca-apatites, carbonate substitutions are also possible in Pb-apatites although they are still not fully characterized [1,2]. Therefore, for the first time, a comprehensive comparison of the IR spectra of synthetic Pb-apatite analogs has been carried out with a detailed examination of the rather often ignored carbonate substitutions. CO₃^2- ions can be substituted in the apatite structure in two different positions: by substitution of an anion located in the X-position in the channel, such as OH- or halogen (A-type substitution), or by substitution of an anion in the tetrahedral position, such as PO₄^3- or AsO₄^3- (B-type carbonate substitution). This can be illustrated by the following chemical formula: Pb_10-y(Na,K)_y[(PO₄)_6-y(CO₃)_y][(X)_2-2x(CO₃)_x], where x≈y. IR studies have shown that in apatites prepared at high temperature, most of the carbonates are located in the channel. Under low-temperature environmental conditions, the formation of AB-carbonated Pb apatites is much more plausible [3]. Some Pb apatites, such as fluoride apatites e.g. Pb₁₀(AsO₄)₆F₂, do not tend to incorporate carbonate at all, at least not during precipitation from solutions with CO₃^2- concentrations similar to environmental (pCO₂=10^-3.5 atm). This is likely due to the greater ordering in apatite channels, where the fluoride anion occupies the mirror plane.

In the present work, various Pb-apatites containing As and V were prepared by precipitation from an aqueous solution in the presence of Na⁺ cations at room temperature and open to the air. The type of A- or B- carbonate substitution was determined in IR spectra collected at room temperature based on asymmetric stretching of the carbonate (ν₃) and out-of-plane bending (ν₂) modes. The high-frequency component of the ν₃ region of the A-type carbonate for Pb apatites showed the greatest variability with chemical composition. For example, in Pb₁₀(AsO₄)₆OH_0.86(CO₃)_0.07 hydroxylmimetite, the bands at 1462 and 1421 cm^-1 are attributed to A-type carbonate substitution, while the bands occurring at 1385 and 1348 cm^-1 are associated with B-type. Compared to lead phosphates and vanadates, carbonate bending oscillations ν₂ at 870 cm^-1 are not apparent in arsenate Pb apatites due to overlap with the As-O stretching mode. In situ IR measurements were also carried out during temperature rise to 500 °C to determine the thermal stability of individual carbonate substitutions in the Pb-apatite structure. In general, most Pb-apatites begin to release carbonates around 300 °C. As the temperature increases, bands in the ν₂ and ν₃ carbonate regions begin to disappear until the apatite structure completely disintegrates.

 

References

[1] Yoder, C. H., Bollmeyer, M. M., Stepien, K. R., & Dudrick, R. N. (2019). The effect of incorporated carbonate and sodium on the IR spectra of A-and AB-type carbonated apatites. American Mineralogist: Journal of Earth and Planetary Materials, 104(6), 869-877.

[2] Kwaśniak-Kominek, M., Manecki, M., Matusik, J., & Lempart, M. (2017). Carbonate substitution in lead hydroxyapatite Pb₅(PO₄)₃OH. Journal of Molecular Structure, 1147, 594-602.

[3] Lempart, M., Manecki, M., Kwaśniak-Kominek, M., Matusik, J., & Bajda, T. (2019). Accommodation of the carbonate ion in lead hydroxyl arsenate (hydroxylmimetite) Pb₅(AsO₄)₃OH. Polyhedron, 161, 330-337.