Synthesis and characterization OF Ca, Pb, Zn, Cu and Cd chlorapatites and Pb-Cd chlorapatites solid solutions

Abstract

The only minerals among divalent metal chlorapatites are: chloroapatite Ca₅(PO₄)₃Cl and pyromorphite Pb₅(PO₄)₃Cl. To date, no occurrence of zinc, copper and cadmium chloroapatites have been noticed in hypergenic environments. Literature provides only scattered data on hydrothermally synthesized Zn and Cd chlorapatites. There is also no data on solid solutions of Pb and Cd in chlorapatites. The aim of this research was the aqueous synthesis of calcium, lead, zinc, copper and cadmium chlorapatites. Additionally, solid solutions of Pb-and Cd-apatites (Pb_5-xCd_x) (PO₄)₃Cl (x from 0 to 5) were prepared. The syntheses were prepared in aqueous solutions under normal conditions by mixing metal nitrates Me(NO₃)₂ (Me=Ca, Pb, Zn, Cu, Cd), potassium phosphate KH₂PO₄ and sodium chloride NaCl in proportions corresponding to the stoichiometry of the expected product. The precipitates were characterized by X-ray diffraction, infrared spectroscopy and scanning electron microscopy with an EDS microanalyser. Partial substitution of Pb²⁺ ions by Cd²⁺ in the pyromorphite structure was observed only in the Pb₅(PO₄)₃Cl - (Pb₄Cd₁)(PO₄)₃Cl series. However, instead of the expected Zn, Cu and Cd chlorapatites, precipitation of hopeite Zn₃(PO₄)₂∙4H₂O, libethenite Cu₂(PO₄)OH and cadmium phosphate Cd₅H₂(PO₄)₄ x 4H₂O were observed.