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

Abstract

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 <20 nm), of which violarite precisely preserves not only the overall morphology but also textural details (e.g., lamellae) of pentlandite; while violarite precipitation being rate limiting in strong acidic conditions (pH 1) the reactions produces low degree pseudomorphism (length scale ~10 μm), of which the overall morphology is only roughly preserved without preservation of textural details. The principle of pseudomorphic replacement reactions have been applied to the syntheses of two complex thiospinels, violarite, (Ni,Fe)₃S₄, and linnaeite, Co₃S₄. Violarite is very difficult to prepare by the traditional dry synthesis route, which requires several months’ annealing and still only results in an impure product. By contrast, pure violarite was synthesized by hydrothermal pseudomorphic replacement within a few days, and the composition is tunable by simply changing temperature, the compositions of the solution and of the pentlandite precursor. Pseudomorphic replacement reactions have also been applied to the syntheses of zeolite monoliths composed of three-dimensional ordered arrays of nanocrystals with uniform size and crystallographic orientation. Such materials have potential applications but have never been prepared. This work demonstrates that pseudomorphic replacement reactions are suitable routes for this purpose by synthesizing monoliths of analcime (NaAlSi₂O₆.H₂O) as an illustration using natural leucite (KAlSi₂O₆) crystals as precursors. The leucite crystals have inherent three-dimensional hierarchical structure of uniformly sized lamellar twins arising from the cubic to tetragonal phase transition. Such uniform lamellar texture was precisely preserved during hydrothermal pseudomorphic replacement reactions in pH buffered NaCl solutions, resulting in three-dimensionally ordered arrays of cubic analcime nanocrystals. Moreover, these analcime nanocrystals have uniform size and crystallographic orientation, which is due to epitaxial nucleation and growth controlled by the leucite precursors. Pseudomorphic mineral replacement reactions make possible the syntheses of zeolites monoliths with very sophisticated shapes and could be used to synthesize other advanced functional materials.