Abstract
Experiments on the cerussite (PbCO3) - aqueous NaHS solution system were performed at 150°C. Scanning electron microscopic observations of run products revealed the formation of large hexagonal and small granular crystals of galena replacing cerussite. The replacement texture of cerussite by galena progressed toward the inside of cerussite without recognizable change in the form of the sample after the reaction, hence being was classified as a pseudomorphic and guided penetration replacement texture. It is reasonable to assume that cerussite is replaced easily by galena because the molar volume ratio of replacing mineral (galena) to replaced one (cerussite) is 0.78, and a large volume of void space was formed in the galena replacing cerussite. Our results also suggest that the replacement of cerussite by galena can be achieved if the NaHS concentration in the solution is sufficiently high (1.0 mol/L). The diffusion of solute species is thought to be the rate-determining process for the metasomatism for the chipped cerussite sample in 0.1 mol/L NaHS solution system. The diffusion constant D value for this case is approximately 3.0×10-9 m2/s. On the other hand, the dissolution is the rate-determining mechanism for the chipped cerussite sample in 1.0 mol/L NaHS solution system in the initial stage within about 1 day, while the diffusion is thought to be the rate-determining mechanism after 1 day with the D value of approximately 7.7×10-8 m2/s. The dissolution rate constant k for cerussite dissolution in the metasomatism for the powdered cerussite sample is approximately 7.3×10-8 and 1.2×10-6 mol/m2s for NaHS concentrations of 0.1 and 1.0 mol/L, respectively.
