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A Raman spectroscopic study of hydroxyl analogues of pyromorphite-mimetite solid solutions

Giera A., Manecki M., Kwaśniak-Kominek M., Marchlewski T., Borkiewicz O., Rakovan J.

Geology, Geophysics and Environment

2014

Vol. 40, no. 1

86--87

Pyromorphite Pb10(PO4)6Cl2 and mimetite Pb10(AsO4)6Cl2, minerals belonging to apatite group, receive increased attention recently. Induced precipitation of pyromorphite and mimetite in soil pore solutions or waste solutions belongs to the best remediation and reclamation methods (Ma et al. 1995, Maniecki et al. 2009). These phases are the most stable forms of Pb2+ and As5+ in the environment. Deficiency of Cl in the environment can cause formation of their hydroxyl forms: Pb10(PO4)6(OH)2 and Pb10(AsO4)6(OH)2 or their solid solutions. Apatite structure allows for extensive and varied ionic substitutions in all positions. The isomorphic substitutions affect unitcell parameters and chemical properties of these minerals (Botto et al. 1997). Solid solutions of hydroxyl analogues of pyromorphite and mimetite have not been sufficiently characterized to this day. A detailed description of phases from this series is, however, necessary for optimization of the remediation methods. A Raman spectroscopic study of mimetite-pyromorphite series demonstrated a strong correlation between the positions of the vibrational modes and the As/(As+P) ratio (Bajda et al. 2011). Such a correlation may be used to determine the composition of the examined samples of minerals from the series. The current research is based on the assumption that in the case of solid solutions of their hydroxyl analogues similar correlations occur. Therefore, the aim of this study is structural (X-ray diffraction) and spectroscopic (Raman) investigation of the effect of PO4-AsO4 isomorphic substitution on the structure and vibrational spectra. Seven phases were synthesized in computer-controlled chemistate at pH = 11 and 80°C by dropwise mixing of solutions containing Pb2+, PO4 3- and AsO4 3- in stoichiometric proportions. The composition of the final products was Pb10[(PO4)6-x(AsO4)x(OH)2, where x = 0, 1, 2, 3, 4, 5, 6. High-resolution powder X-ray diffraction data was obtained using the diffractometer at beamline 11-BM at the Advanced Photon Source (Argonne National Laboratory, Chicago). A detailed Raman spectroscopy was performed with the use of confocal Raman microscope and OMNIC software (AGH Kraków). The morphology and elemental composition of the samples were characterized by means of Fei Quanta variable pressure SEM/EDS (AGH UST Kraków). Moreover, the chemical composition of synthetic phases was determined by wet chemical analysis. Unit cell parameters increase with substitution of AsO4 for PO4 . Parameter a increases from 449.879 Å to 10.189 Å, while parameter c - from 7.427 Å to 7.516 Å. This is consistent with other solid solution series of lead apatites (Flis et al. 2009). The area under selected Raman effects is also strongly correlated with P and As content. Additionally, systematic shift of the position of Raman effects is observed. The band attributed to the (AsO4)3- ν1 symmetric stretching mode shifts from 808 cm-1 in Pb10(AsO4)6(OH)2 to 814 cm-1 in Pb10[(PO4)5(AsO4)](OH)2. The range of the peak positions for the (PO4)3- ν1 symmetric stretching mode is even wider: from 918 cm-1 in Pb10[(PO4)(AsO4)5](OH)2 to 926 cm-1 in Pb10(PO4)6(OH)2. The observed correlations may be used for semi-quantitative estimation of As and P content using non-destructive Raman spectroscopy.

Synchrotron microanalytical methods in the study of trace and minor elements in apatite

Rakovan J., Luo Y., Borkiewicz O.

Mineralogia

2008

Vol. 39, iss. 1/2

31--40

Synchrotron X-ray facilities have the capability for numerous microanalytical methods with spatial resolutions in the micron to submicron range and sensitivities as low as ppm to ppb. These capabilities are the result of a high X-ray brilliance (many orders of magnitude greater than standard tube and rotating anode sources); a continuous, or white, spectrum through the hard X-ray region; high degrees of X-ray columniation and polarization; and new developments in X-ray focusing methods. The high photon flux and pulsed nature of the source also allow for rapid data collection and high temporal resolution in certain experiments. Of particular interest to geoscientists are X-ray fluorescence microprobes which allow for numerous analytical techniques including X-ray fluorescence (XRF) analysis of trace element concentrations and distributions; X-ray absorption spectroscopy (XAS) for chemical speciation, structural and oxidation state information; X-ray diffraction (XRD) for phase identification; and fluorescence microtomography (CMT) for mapping the internal structure of porous or composite materials as well as elemental distributions (Newville et al. 1999; Sutton et al. 2002; Sutton et al. 2004). We have employed several synchrotron based microanalytical methods including XRF, microEXAFS (Extended X-ray Absorption Fine Structure), microXANES (X-ray Absorption Near Edge Structure) and CMT for the study of minor and trace elements in apatite (and other minerals). We have also been conducting time resolved X-ray diffraction to study nucleation of and phase transformations among precursor phases in the formation of apatite from solution at earth surface conditions. Summaries of these studies are given to exemplify the capabilities of synchrotron microanalytical techniques.