Some Hydrothermal Syntheses of Dolomite and Protodolomite

The formation of dolomite from a number of metastable carbonate materials has been investigated. Aragonite-basic magnesium carbonate mixtures, Ca-Mg carbonate gels, and magnesian calcites produced by calcareous algae and echinoids were treated in the presence of H₂O and H₂O + CO₂, in the temperature range from 25° C. to 450° C. The dolomite produced at the higher temperatures is the stable material of 1: 1 composition, ordered with respect to Ca and Mg. The dolomite-like materials, or protodolomites, produced in some runs at lower temperatures are somewhat calcium-rich and do not show superstructure reflections. They appear to be intermediate stages in the development of the stable configuration. We believe that the necessity of attaining an ordered arrangement of Ca and Mg ions at rapid rates of crystallization is responsible for the difficulty of forming dolomite at low temperatures. At temperatures above about 500° C, ionic mobility is great enough for dolomite to form, even by dry, solid-state cation diffusion, in reasonable experimental times. Diffusion rates at earth-surface temperatures are, however, so greatly reduced that an aqueous environment appears to be necessary. Even then, cation ordering must take place essentially by surface diffusion rather than by internal rearrangement. This greatly increases the difficulty of both dolomite nucleation and subsequent growth upon either ordered or simple rhombohedral carbonate substrates. As a result, a metastable phase or phase assemblage is the most probable initial precipitate.