Synthesis and Characterization of Arsenate/Phosphate Fluorapatite Solid Solutions

Abstract

A series of arsenate-substituted fluorapatite (FA) was directly prepared through the aqueous precipitation method. Characterization studies from elemental analysis, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, and scanning electron microscopy (SEM) have confirmed the formation of the arsenate/phosphate fluorapatite solid solutions (AsPFAs), i.e., Ca 5 (P x As 1-x O 4 ) 3 F, which had a single phase of apatite structure. The analysis of the measurements indicated that the unit cell a and c parameters did not vary smoothly with the degree of substitution, as expected by Vegard's law. The morphology was dependent of the arsenate/phosphate ratio. The solids with high As/(P + As) molar ratio of 0.29 to 1.0 were typically needlelike crystals, which developed along the c-axis; all apatites with the As/(P + As) molar ratio of <0.09 were large tabular. The phosphate peaks and the arsenate peaks in the FT-IR spectra were observed for the entire series of substituted apatites. The area of the phosphate peak was gradually suppressed and that of the arsenate peak increased as the proportion of the arsenate increased.

As-RICH APATITE FROM MT. CALVARIO: CHARACTERIZATION BY ΜICRO-RAMAN SPECTROSCOPY

“…In igneous and metamorphic rocks and in ore deposits, apatite group minerals occur mostly as accessory minerals, with substantial potential for ionic substitutions. With respect to arsenic, the presence of the element in F-dominant apatites has been reported by Zhu et al (2009) and Perseil et al (2000). In particular, Perseil et al (2000) described As-bearing fluorapatites in the manganiferous deposit of St. Marcel-Praborna (Italy) and related their formation to the late circulation of fluids enriched in As and Sr.…”

“…The difference in length, and therefore the strength, of the chemical bonds in [PO 4 ] 3− or [AsO 4 ] 3− tetrahedra may account for the noticeable variation in the frequency of the ν 1 vibrational modes of the two types of tetrahedra. On the other hand, Zhu et al (2009) studied a series of arsenate-substituted fluorapatite (FA) crystals obtained by the aqueous precipitation method. Combining X-ray diffraction data (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM), they confirmed formation of the arsenate/ phosphate fluorapatite solid solutions with an apatitelike structure.…”

“…Combining X-ray diffraction data (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM), they confirmed formation of the arsenate/ phosphate fluorapatite solid solutions with an apatitelike structure. In addition, Zhu et al (2009) investigated the variations of the cell parameters in the apatite Ca 5 (P x As 1−x O 4 ) 3 F solid solutions versus the amount of the incorporated arsenate. As the tetrahedral covalent bonds in [AsO 4 ] 3− groups are longer (1.18 Å) than those in [PO 4 ] 3− (1.10 Å), these replacements modify the cell parameters, increasing the a 0 and c 0 values.…”

“…5a). In the IR spectra from arsenate substituted fluorapatite, Zhu et al (2009) reported bands around 842-876 cm −1 and 422-432 cm −1 ascribed by the authors to the vibrations of [AsO 4 ] 3− tetrahedra. Moreover they observed that increasing amounts of As in the crystals involved increasing intensity in the IR band produced by [AsO 4 ] 3− and, at the same time, decreasing intensity in the band at 964 cm −1 , ascribed to [PO 4 ] 3− tetrahedra.…”

“…The cathodoluminescence technique enables progressive replacements of P by As (up to 10 wt.% As 2 O 5 ) to be investigated. Arsenic (V) may totally replace P (V) at the tetrahedral sites in svabite, a fluorarsenate mineral belonging to the apatite group with the chemical formula Ca 5 (AsO 4 ) 3 F. As the arsenate group is tetrahedral [AsO 4 ] 3− , iso-structural, and iso-electronic with [PO 4 ] 3− groups, ionic P-As substitutions are not surprising, as shown by a study of the entire compositional range from fluorapatite to svabite (Dunn et al 1980, Hughes & Drexler, 1991, Zhu et al 2009). Using FT-IR spectra, recorded from samples crystallized over the entire series of substituted apatites, Zhu et al (2009) proved that increasing amounts of arsenate involves increasing intensity of the bands produced by the arsenate groups, while the bands ascribed to the phosphate groups gradually decrease in intensity.…”