Model of interface-coupled dissolution-precipitation mechanism of pseudomorphic replacement reaction in aqueous solutions based on the system of cerussite PbCO3 – pyromorphite Pb5(PO4)3Cl

Abstract

The results obtained from carefully designed experiments with the lead carbonate–lead apatite system presented in the study are used to propose a general model for the development of microstructures upon polycrystalline pseudomorphic transformation of minerals that are formed from the reaction with an aqueous solution. In several experiments, cerussite PbCO3 powder or crystals were reacted with a phosphate solution, in the presence or absence of Cl− ions, in an autoclave at 140 °C for up to 5 days at a near-neutral pH. In the presence of Cl− ions, cerussite powder is replaced by pyromorphite Pb5(PO4)3Cl, whereas in the absence of these ions, Pb3(PO4)2 precipitates first and recrystallizes into hydroxylpyromorphite Pb5(PO4)3OH within 3 days of transformation. The reaction of single cerussite crystals with phosphate solutions proceeds in stages via an interface-coupled dissolution–precipitation mechanism through which PbCO3 is replaced by lead phosphates. A structural control is imposed on the reaction progress when the dimensions of the dissolving crystal are much larger than the thickness of the outermost layer of the secondary phase formed by the reaction with unaltered bulk solution. Since various components of the solution exhibit different diffusivity and their migration through the porous layer is retarded to various degrees (a chromatography effect), the conditions at the reaction front differ from those at the contact with the bulk solution, resulting in the crystallization of transitional phases. With time, all the components of bulk solution diffuse through the porous layers and the transitional phases transform into thermodynamically favored reaction products. At the reaction front, spatially separated dissolution of the host mineral (associated with the formation of the porous layer) and heterogeneous precipitation of the secondary phase are observed. Several similar features of the lead carbonate–lead apatite system have been reported before, which suggests that the proposed model is universal and the interpretation can be extrapolated to other systems.