Dolomite

Dolomite occurs widely as the major constituent of dolostones and dolomite marbles. As mentioned above, the origin of dolomite-rich rocks in marine sequences remains an unresolved problem of petrogenesis.

Dolomite—actually protodolomite—is known to have formed fairly recently in restricted environments such as on supratidal flats that occur in The Bahamas and Florida Keys. Also, no dolomite has been synthesized in an environment comparable to natural conditions. Thus, the explanation for the formation of dolomite in these marine units remains in question. It is now thought that dolostones may be of various origins. Indeed, several different models have been suggested for dolomite formation, each based on diverse considerations, combined with empirical and/or experimental data.

Except for models invoking formation of dolomite by direct precipitation, a process thought by most geologists to apply to only a small percentage of all dolostones, each model is based on the assumption that the dolomite of dolostones has been formed by conversion of CaCO3 sediment or sedimentary rocks to dolostone. Thus, the models have been formulated to account for this conversion, which is known as dolomitization.

The most widely discussed models for dolomitization, either partial or complete, involve four chief variables: time, location with respect to the sediment-seawater interface, composition and derivation of the solutions involved, and fluxing mechanisms. The time ranges from dolomitization that occurs penecontemporaneously with deposition to that which takes place subsequent to relatively deep burial of the precursor sediments. The location ranges from at or very near the sediment-seawater interface to well beneath some overlying sediments that were deposited at a later time. The solutions supply the magnesium needed and must have the appropriate pH and concentrations of other necessary ions; these solutions are generally considered to be seawater (either “normal” seawater or brines concentrated by evaporation), connate water, meteoric water, or some combination of these waters. (Connate refers to water that becomes enclosed within sediments upon their deposition; meteoric water is derived from the atmosphere as rain or snow, which often occurs in pore spaces within rocks.) Another important variable is the presence of dissolved sulfate (SO4− 2) ions, as this retards the dolomitization process. The fluxing mechanisms are generally attributed to density differences of the solutions involved and the permeability characteristics available for percolation through the precursor sediment. In addition, the presence of a geothermal heat source in a basin may enhance both fluid flux and the rate of dolomitization. There also are additional direct and indirect controls—e.g., climate, biochemical processes, and HDO:H2O and/or D2O:H2O ratios in the water. (The symbol D represents deuterium, the hydrogen isotope with a nucleus containing one neutron in addition to the single proton of the ordinary hydrogen nucleus.) Bacteria may also play a role in the formation of dolomite. In any case, it has been shown that some dolostones have gained their current characteristics as a consequence of certain combinations of these conditions and processes.

Criteria involving factors such as the identity of associated rocks and the coarseness of the grains of dolostones have been suggested for use in attributing one versus the other hypothesized models to certain occurrences of dolostone. None, however, has been accepted as an absolute criterion by many carbonate petrologists.

The desire for an understanding of dolomitization of sedimentary strata has been based on economic as well as scientific interests. In many places, dolomitization has led to increases in permeability and porosity and thus increased the potential of such rock strata as good oil, gas, and groundwater reservoirs and, in some cases, even as hosts of certain kinds of ore deposits.

The other fairly common dolomite occurrences include the following: Dolostones have been metamorphosed to both dolomite and calcite marbles; dedolomitization processes account for the latter. Some dolomite marbles are nearly pure dolomite. Dolomite carbonatites are of the same general origin as calcite carbonatites. The dolomite present in dolomite veins has also been ascribed diverse origins; some appears to have been deposited by percolating connate or meteoric groundwater, and some seems more likely to have been deposited by hydrothermal solutions charged with magmatic volatiles.