Accommodation of the carbonate ion in lead hydroxyl arsenate (hydroxylmimetite) Pb5(AsO4)3OH

Abstract

Synthetic hydroxylmimetite Pb5(AsO4)3OH – a mineral isostructural with hydroxylapatite Ca5(PO4)3OH – was assessed for its ability to accommodate carbonate substitution. Hydroxylmimetite was synthesized by precipitation from an aqueous solution in the absence and presence of carbonate ions (solutions containing dissolved NH4HCO3). The solids obtained were examined using X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, thermal analysis (TG/DTA) and evolved gas analysis (TG/EGA). The structure of hydroxylmimetite turned out to be very susceptible to carbonate substitution. Despite attempts to avoid or minimize the presence of CO2 in solutions, hydroxylmimetite precipitated in the absence of carbonates contained 1.3 wt% of CO32− ions. The maximum content of CO32− ions incorporated under the conditions of the experiment was equal to 3.05 wt%. The IR results indicated that the substitution of carbonate ions in the hydroxylmimetite structure occurred in two structural positions (A-B type): A-type substitution for OH in structural channels (IR bands at 1465 and 1421 cm−1) and B-type substitution for AsO4 tetrahedra (IR bands at 1388 and 1340 cm−1). However, the characteristic increase of the unit cell in the direction of the a-axis indicated the dominating nature of the A-type substitution. Simultaneous thermal analysis of the compounds allowed quantitative and qualitative characterization of carbonated hydroxylmimetite. The thermal decomposition of hydroxylmimetite is described for the first time.