A total of seven compounds of the hydroxylpyromorphite Pb10(PO4)6(OH)2 - hydroxylmimetite Pb10(AsO4)6(OH)2 (HPY-HMI) solid solution series were synthesized at 80°C from aqueous solutions and characterized using Raman spectroscopy. The positions of the bands in all spectra of the series depend on the content of arsenates and phosphates shifting to lower wavenumbers with substitution of (AsO4)(3-) for (PO4)(3-). This shift results from the decreasing bond strength of X-O (where X=P, As) and higher atomic mass of As than P. The position and intensity of major (PO4)(3-) and (AsO4)(3-) bands in Raman spectra exhibit linear correlation with As content, while the ratio of the intensities of these peaks shows exponential correlation. This results due to different polarizability of (PO4)(3-) and (AsO4)(3-) molecules. A small carbonate band develops with increasing As content indicating that hydroxyl lead apatites adopt the (CO3)(2-) ions, particularly at the arsenate end of the series.

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A total of seven compounds of the hydroxylmimetite Pb 10 (AsO 4) 6 (OH) 2hydroxylpyromorphite Pb 10 (PO 4) 6 (OH) 2 (HPY-HMI) solid solution series were synthesized at 80 °C from aqueous solutions and characterized using XRD, SEM-EDS and FTIR. The substitution effect of [PO 4 ] 3− anions by [AsO 4 ] 3− on systematic changes in lattice parameters and spectral properties has been explained based on correlation between chemical composition and the peak positions. The substitution results in systematic linear increase in unit cell parameters and unit cell volume. Isomorphic substitutions are apparent in IR as a change in the position and intensity of bands derived from phosphates, arsenates and hydroxyl ions. Main bands attributed to stretching

Abstract:Five crystalline members of the hydroxyapatite (HAP; Ca 5 (PO 4 ) 3 OH)-johnbaumite (JBM; Ca 5 (AsO 4 ) 3 OH) series were crystallized at alkaline pH from aqueous solutions and used in dissolution experiments at 5, 25, 45, and 65 • C. Equilibrium was established within three months. Dissolution was slightly incongruent, particularly at the high-P end of the series. For the first time, the Gibbs free energy of formation ∆G

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