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1 2012

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Low temperature synthesis and thermodynamic stability of fluorpyromorphite Pb5(PO4)3F at 5-65°C

Dziura A., Kwaśniak-Kominek M., Manecki M., Bajda T.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

465

Fluorpyromorphite Pb5(PO4)3F belongs to the pyromorphite group of minerals which are isostructural with apatites. Their structure allows for variety of substitutions in both cationic and anionie positions. The purpose of this study is low temperature synthesis, mineralogical and thermodynamic characterization of fluorpyromorphite (FPY) and determination of its solubility product based on the results obtained from the dissolution experiment. FPY was synthesized by dropwise mixing of solutions containing Pb(NO3)2, K2HPO4 and NaF at 25°C. The only product of synthesis is FPY within the detection limits of X-ray diffraction. Calculated unit cell parameters are a = 9.1 ATI A, c = 7.3216 A. Homogeneous character of the precipitate was confirmed further with the use of scanning electron microscopy. Dissolution experiments were conducted in triplicates, open to the air, at 5, 25, 45°C and 65-C and at pH = 2.0. An aliquot of about 250 mg of FPY was placed in 250 mL of 0.05 M KNO3 with the pH adjusted using 0.1 M HNO3. Periodically taken samples were analyzed for [Pb] using atomic absorption spectrometry AAS as well as for [PO4] and [F] using colorimetry. Calculations of solubility products were performed with the aid of computer program PHREEQC with modified MINTEQ thermodynamic database. The activities of ionic species were calculated from measured concentrations of elements (assuming stoichiometric dissolution) with application of extended Debye-Huckel equation.