[WALLACE] 



STUDY OF DOLOMITIZATION 



143 



We are dealing here only with the final product of equilibrium between 

 a normal or concentrated seawater and the solid phase, no matter 

 whether the original deposition be due to a purely chemical or to a 

 biochemical action. Owing to the relative insolubility of the calcite 

 and dolomite, equilibrium conditions can only be established after 

 long continued contact between solid and liquid phases. 



Case I. Here the concentration of the seawater, before solution 

 of Mg salts takes place, is represented by the point A (Fig. 2). Calcite 

 as solid phase is stable in presence of this solution. MgCOg or other 

 Mg salts are, however, unstable, and will gradually go into solution, 

 the composition of the solution moving along AP to P. The stable 



Figure 2 



phases are now calcite and dolomite, and a certain amount of dolo- 

 mite will be precipitated. If evaporation and concentration does 

 not take place, the amount of dolomite precipitated will be small, 

 and the composition of the solution will remain constantly that repre- 

 sented by the point P. With a larger amount of Mg salt to go into 

 solution, or with slight concentration, calcite becomes unstable and 

 goes into solution, more dolomite is formed, and the composition of 

 the solution moves from P towards Q. It is to be noted then that 

 if MgCOg is present in small amount in calcareous algse in a seawater 

 represented by A, dolomitization would take place, not by the dissolv- 

 ing out of the excess of CaCOg, but by the dissolving of MgCOg and 

 precipitation of the double salt out of solution. The only way in which 

 dolomite can be conceived to have formed is as a precipitate out of a 

 solution saturated for the salt. 



