Dolomitization reaction, brines

The dolomitization reaction had been observed to take place relatively rapidly and completely in modem evaporating seawater tidal flats when calcite sand or mud is present, the brine had evaporated to a fairly high strength, and the brine pool was quite warm (Levy, 1977 Kinsman, 1966 others). Normally, however, for seawater or similar low-concentration brines, the dolomitization reaction appears to take place only at very high temperatures (e.g., deep sea vents), or with exceedingly long residence times and some temperature elevation (e.g., in some coastal aquifers or oil field brines). 

In the resulting reaction (Fig. 25.3), calcite progressively dissolves as the Fountain brine mixes into the Lyons formation. The Ca++ and HCO3 added to solution drive precipitation of anhydrite and dolomite by a common ion effect. The overall reaction,... 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

In the process used by Norsk Hydro, magnesium hydroxide extracted from seawater with the aid of calcined dolomite is mixed with charcoal and magnesium chloride brine and is heated to 1000-1200°C in the presence of chlorine produced during subsequent electrolysis of magnesium chloride. The main reactions are [266]... 

Many authors have also proposed different reactions for the contact of very hot seawater in the rocks of deep sea vents, including Hardie (1990) who considered the primary reaction to be with basalt to form spilitic greenstone. With vents and various other hot calcium chloride brines Shvartsev and Bukaty (1996), Stura (1977,1974), Azizov (1975), Kissin and Pakhomov (1969), and Pastushenko (1967) have proposed reactions with a wide variety of other rocks. Several authors have also discussed rock leaching, adsorption, precipitation or ion exchange to add or remove components in the dolomitization brine. 

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