Factors Influencing the Precipitation of Dolomitic Carbonates

Abstract

Dolomitic carbonates having d-spacing values of disordered dolomite or protodolomite, but lacking major super-reflections, have been precipitated from solutions in the laboratory. Conditions of temperature, pressure, and concentration employed were far less extreme than has previously been reported in the literature. Reagents used were Ca(NO₃)₂, MgSO₄, and Na₂CO₃; activated charcoal was mixed with reacting solutions to reduce reaction rate so that effect of rate on the final product could be observed.

Experimental results indicate that the pH of the precipitating medium, as controlled by the amount of carbonate ion present, is the most important factor affecting the precipitation of dolomite. Better products were obtained from 1.0M solutions mixed in a Ca⁺⁺: Mg⁺⁺: CO₃⁼ ration of 1:1:2 where the pH of the precipitating medium ranged from 9.7 at 25°C to 9.1 at 100°C; when the ratio was changed to 1:1:1, other conditions remaining the same, proto-dolomite was the carbonate formed. Increase in temperature of reaction from 25°C to 100°C and in concentration of reagents from 0.1M to 1.0M abet the ordering and crystallinity of the dolomitic precipitate. Reduction in the rate of reaction allowed larger crystals to form; crystal size ranged from about 0.5 to 30 microns.

That the presence of sulfate may be essential to the precipitation of some dolomites was indicated when other salts of magnesium were substituted for the sulfate and the resulting products included no dolomitic carbonate. The genesis of some dolomites, especially those associated with evaporites or evaporative conditions, might require the presence of a natural suspension or gel-like phase.