Vibrational spectroscopic study of mimetite–pyromorphite solid solutions T. Bajda a,⇑ , W. Mozgawa b , M. Manecki a , J. Flis a a Department of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland b Department of Material Science and Ceramics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland article info Article history: Received 20 May 2011 Accepted 21 June 2011 Available online 12 July 2011 Keywords: Apatites IR spectroscopy Raman spectroscopy Arsenate Phosphate abstract A number of compounds of the mimetite Pb 5 (AsO 4 ) 3 Cl–pyromorphite Pb 5 (PO 4 ) 3 Cl solid solution series were synthesized at room temperature and analyzed with Raman and infrared spectroscopy. The funda- mental antisymmetric stretching (m 3 ) and bending (m 4 ) vibrations of the mimetite–pyromorphite solid solutions occur in the regions 720–1040 cm 1 and 400–580 cm 1 . The band originating from the m 3 and m 4 modes shifts to higher wavenumbers between mimetite and pyromorphite because the atomic mass of As is higher than P and As–O bonds are longer than P–O bonds. The position, shape and relative intensity of the bands vary primarily as a function of As/(As + P) in the analyzed solid. The strong corre- lations between the positions of the vibrational modes and the As/(As + P) ratio can be used to determine the composition of the investigated natural and synthetic samples of the minerals from the mimetite– pyromorphite series. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Lead apatites form a family of isomorphous compounds, and well-known members of the group are mimetite Pb 5 (AsO 4 ) 3 Cl and pyromorphite Pb 5 (PO 4 ) 3 Cl. Isostructural with vanadinite Pb 5 (VO 4 ) 3 Cl, these three compounds form a ternary system within the apatite group of P6 3 /m symmetry (hexagonal bipyramid). The mimetite and pyromorphite structures can incorporate numerous admixtures, mainly Ca, Ba, As, V, P and others. The most common substitution is isovalent replacement of part of Pb with Ca, and As with P and V. Extensive substitution of the (AsO 4 ) 3 group by the tetrahedrally coordinated and isovalent (PO 4 ) 3 ion has been well established by the existence of a complete solid solution between mimetite and pyromorphite [1–7]. Pyromorphite and mimetite have recently gained considerable attention as metal sequestration agents in water treatment and con- taminated soil remediation because they are the most stable forms of Pb(II) and As(V) in the environment [7–13]. One of the most effec- tive methods used to remove arsenate ions from solution is the precipitation of mimetite. The method is based on two subsequent reactions: 1 – the controlled addition of lead and chloride ions to the polluted solutions which results in precipitation of mimetite; 2 – the removal of excess Pb(II) by the addition of phosphates which results in precipitation of pyromorphite [14,15]. Consequently, this leads to the formation of a physical mixture of the two phases and their solid solutions. Identification of the composition of so formed mixture is extremely difficult because of the similarity of the mimetite and pyromorphite structures. The formation of mimetite and pyromorphite in contaminated soils significantly reduces the Pb(II) and As(V) bioavailability [9,14–17]. It is known, however, that the introduction of phosphates into As(V) contaminated soils leads to secondary arsenic remobilization [18–22] and mimetite– pyromorphite solid solutions are formed instead of the expected pure end members of the series. This observation has been recently confirmed by laboratory experiments [23]. The identification of mimetite–pyromorphite solid solutions in soils and solids is a necessary, though sometimes problematic, step of the validation of the remediation scheme. There are many meth- ods used for the recognition of lead phosphates and arsenates in the environment, e.g. electron microprobe techniques, transmis- sion electron microscopy, X-ray absorption spectroscopy, X-ray diffraction [10,24–26]. However, these are often useless because of too low detection limits or problems with conclusive identifica- tion of the results obtained. For these reasons, we decided to test the Raman and IR spectroscopic measurements as the most useful method for identification in this class of minerals. Infrared and Raman spectroscopy are particularly useful for the examination of in situ coordination of arsenate and phosphate ions, specifically their for protonation and solvation reactions, which other cited techniques lack [27–31]. The use of the vibrational spectroscopy is possible because of the extensive substitution of the (AsO 4 ) 3 group by (PO 4 ) 3 in the structure of lead apatites resulting in a complete solid solution between mimetite and pyromorphite. 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.06.034 ⇑ Corresponding author. E-mail address: bajda@geol.agh.edu.pl (T. Bajda). Polyhedron 30 (2011) 2479–2485 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly