[WALLACE] 



STUDY OF DOLOMITIZATION 



141 



is stable in presence of seawater with a fairly high Mg percentage. The 

 stability seems to be most marked in presence of ammonium salts, par- 

 ticularly ammonium sulphide. But experiments carried out by Pfaff * 

 show that dolomite may be obtained from solutions containing CaCOg, 

 MgCOg, and NaCl. We may then consider that in seawaters of normal 

 temperatures and at moderate depths the stability of calcite or dolomite 

 depends mainly on the concentration of Mg salts. With regard to mag- 

 nesite, the difficulties encountered in obtaining the mineral experi- 

 mentally render its stability limits a matter of speculation. At ordinary 

 temperatures and pressures, salts with water of crystallisation are 

 obtained: under pressure of COj, more frequently at high temperatures 



Figure 1 



and in presence of NaCl, the solid phase has been found to be magnesite. 

 Some iiatural occurrences, such as the Hall dolomites of the Tyrol, point 

 to the probability that magnesite has been formed at comparatively 

 low temperature by the interaction of highly concentrated Mg salts 

 with dolomite: but the mineral unquestionably owes its origin in the 

 majority of cases to metamorphic processes. 



Incorporating in a diagram the above facts on the stability of 

 the minerals with which we are concerned, and taking the vertical and 

 horizontal lines as repsesenting concentrations of Ca and Mg ions 

 respectively in a normal seawater from which the Ca and Mg ions 

 have been removed, we obtain a figure of the general type represented 

 by Fig. 1. The vertical line represents a seawater with no Mg ions, 

 but a regularly increasing amount of Ca ions from O (where no Ca ions 



*Neues Jahrb. fur Miner., &c., 9, Beilbd., 485 (1894). 



