Precipitation of metastable phases and kinetics of solution-mediated phase transformation

Abstract

This study summarizes the principal factors on nucleation, growth and polymorphic transformation termed “solution-mediated phase transformation” of some thermodynamically metastable phases in nature. The formation of the metastable phases occurring occasionally in hydrothermal deposits and sedimentary rocks is explained by the empirical Ostwald step rule, where the surface free energy and degrees of supersaturation are important factors. Transformation from a metastable phase to a stable polymorph in a solution can be kinetically analyzed by the relative rates of dissolution and growth of the two phases.
     The solubility of opal-CT as an example of metastable phase is usually higher than that of quartz as the stable phase, but this relation reverses in the case of nm-size grains according to the mesoscopic effect. In the latter case both solubilities of opal-CT and quartz become distnctly higher, and so the polymorphic relation among the silica minerals in nature is inferred to be in accordance with the Ostwald step rule. In contrast to solid-state transformation, metastably crystallized wurtzite appears to transform readily into stable sphalerite under hydrothermal conditions. This suggests that the solution-mediated transformation can be rapidly attained as compared with the solid-state transformation, showing that the dissolution-precipitation mechanism is predominant in the low-temperature polymorphic transformations.