Properties of the Feldspars. 



135 



^\ 



I 





Ibo A 



n 



i 

 i 

 i 



loo B 

 L 



loo A 



looB 



These types are for the moment purely hypothetical and are 

 a product of the method of analysis, though they are being 

 rapidly identified for various isomorphous pairs by pupils of 

 Roozeboom and by others. 



The method of reasoning which 

 yields these three possible types 

 will be briefly described with the 

 help of the van Alkemade graphi- 

 cal analysis : 



If we indicate the potential (f) 

 of a particular mixture by the 

 length of the ordinate (fig. 16), 

 and the number of molecules of 

 A and B by subdividing the hori- 

 zontal axis (A 4- i?=100) in the 

 proper proportion, assuming 

 atmospheric pressure and constant 

 temperature for each diagram, then 

 every point within the coordinates 

 represents a particular phase of 

 known composition and potential. 

 Suppose now (Roozeboom) a tem- 

 perature is assumed above the melt- 

 ing point of the higher-melting 

 component, clearly, whatever the 

 composition, only the liquid phase 

 can have a stable existence. If 

 potential difference represents the 

 measure of the tendency to change 

 and the tendency of all change is 

 toward the minimum potential, 

 for this temperature all change 

 will be toward the liquid, and the 

 potential of a solid, if one existed 

 there, would be greater than that 

 of the liquid for all compositions 

 — hence the curve 8 (solid) above 

 the curve L (liquid) throughout. looA 



Suppose the potential to be f 16 



lowered to a point where crys- 

 tallization can begin, the tendency to melt no longer obtains 

 for all compositions, the two curves will be displaced relatively 

 and, being of different form, will intersect. Draw a common 

 tangent to the two curves and apply van Alkemade's reason- 

 ing above noted. The trend of the potential of both phases 

 between the points of tangency, i. e. of all mixtures between 

 these limits of composition, is toward the minimum repre- 

 sented by this tangent. Crystallization will then begin at a 



