C. Edmund Marshall 75 



result was obtained in varying degrees with all the clay minerals ex- 

 amined. The influence of potassium on the bonding energy of calcium 

 depends greatly on the relative amounts of these cations and on the clay 

 mineral under investigation. 

 The montmorillonite reported on by McLean and Marshall afforded 



Ratio 12 

 A k/ /, ca 



O/lOO 



10/90 



20/80 



30/70 



% Exchangeable K / % Exchangeable Ca 



Figure 6. The potassium-calcium activity ratios for vari- 

 ous proportions of the two exchangeable cations on a variety 

 of clay minerals. 



a clear-cut case of the depression of calcium activity with increasing 

 potassium and, hence, of an increase in the calcium ion mean bonding 

 energy. The other clays are less regular and may show an increase over 

 one part of the range and a decrease over another. 



A general review of these potassium-calcium relationships may be 

 obtained by plotting the ratio of the activities against the ratio of the 



