Mechanisms and kinetics of pseudomorphic mineral replacement reactions and their applications in materials syntheses.

Although pseudomorphic mineral replacement reactions are common in all geological environments and many industrial processes, few studies have been devoted to understanding the mechanisms and kinetics of these reactions. The subjects of this thesis are to understand the mechanisms and kinetics of pseudomorphic replacement reactions by detailed experimental studies on model systems, and to apply the principle of these reactions in the syntheses of novel materials. The mechanisms of pseudomorphic replacement reactions were revealed by a thorough kinetic and textural study of the replacement of pentlandite, (Fe,Ni)₉S₈, by violarite (Ni,Fe)₃S₄, under mild hydrothermal conditions (80°C to 210°C). Reaction kinetics shows a complex dependence on various physical and chemical parameters including temperature, sample texture of mineral assemblage, pH, and concentrations of various reaction species (e.g., oxidants, metal ions). Textural observations show a sharp phase boundary and a porous product. Both kinetic and textural results indicate a coupled dissolution–reprecipitation mechanism. The coupling between pentlandite dissolution and violarite precipitation is controlled by local solution chemistry as well as the epitaxial nucleation of violarite onto the pentlandite substrate. The latter was confirmed by electron backscatter diffraction (EBSD) analysis that pentlandite and violarite share a common crystallographic orientation. The rate limiting step depends on solution chemistry and controls the degree and length scale of pseudomorphism: pentlandite dissolution being rate limiting at mild acidic to neutral conditions (1 < pH < 6) results in high degree pseudomorphism (length scale