140 THE ROYAL SOCIETY OF CANADA 



found explicitly or implicitly referred to, but a fuller insistence on 

 their meaning may lead to a clearer conception of the fundamentals of 

 the problem. 



THE CONDITIONS OF STABILITY OF CALCITE AND DOLOMITE 



A concise formulation of the probable process of dolomitization is 

 that recently suggested by Linck * in his article on the formation of the 

 carbonates in the first volume of Doelter's Handbuch des Mineralchemie, 

 a work of reference gladly welcomed by all who are interested in ques- 

 tions of mineral statics. The statement is as follows: "Dolomite is 

 formed in the presence of CaCOg in solution or in the presence of labile 

 modifications of CaCOg as solid phase: it is the product of equilibrium 

 between MgCOg in solution and in solid form, while CaCOg goes 

 simultaneously into solution." Linck seems to consider that though 

 CaCOg is not stable in the presence of dolomite, MgCOg may be present 

 as stable solid phase when the dolomite is in process of formation. 

 He suggests that in cases where MgCOg is present in small quantity in 

 a limestone, the dolomitization may be due to the dissolving of the CaCOg 

 and recrystallisation of the remainder into dolomite. Skeatsf shares 

 this opinion with reference to the dolomitization of calcareous algae. 

 On the other hand the view may be advanced, as was done in the in- 

 stance of local dolomitization above referred to, that the Mg salts present 

 in the algae go into solution, till the concentration of Mg ions in the 

 solution is sufficiently great to cause the precipitation of the double 

 salt. For purposes of simplicity, we shall at first leave out of con- 

 sideration the possibility — indicated above — of the formation of mixed 

 crystals between CaCOg, MgCOg, and CaCOg. MgCOg, and shall assume 

 that these three salts are completely nonmiscible in the solid state. 



It is evident that under the same conditions of temperature dolomite 

 — or at any rate limestones with high percentage of Mg — may form in 

 one place and ordinary limestones in another. The concentration of Mg 

 ions in the solution is evidently the determining factor in certain instances 

 of dolomitization. This is undoubtedly the case in the limestones 

 investigated in Manitoba. It will serve our purpose then, in investiga- 

 ting the dolomitization process, to deal mainly with constant temperature 

 and pressure, and to consider the effect of different concentrations of 

 Mg ions in the liquid phase. 



CaCOg in the form of calcite is stable in presence of seawater of 

 normal composition under low or moderate pressure, and even in pre- 

 sence of a saturated solution of calcium carbonate in water. Dolomite 



♦Doelter's Handbuch der Mineralchemie, Bd. I, 133. 



tHuIlctin of the Museum of Comparative Zoology (Harvard) 42, 53. 



