6o 



ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS. 



CONCLUSIONS. 



It now remains for us to gather the results together and to draw- 

 such conclusions as they appear to justify. 



(i) If the melting points are now plotted in a system of which they 

 form the ordinates, while the percentage compositions of the different 

 feldspars form the abscissas (fig. 14), we discover, within the limits of 

 accuracy of possible measurement at these temperatures, a nearly 

 linear relation ; the melting point varies very closely with the compo- 

 sition. We have no maximum, no minimum, no branching of the 

 curve, but from each fusion there separates a solid phase of the same 



1600 



xa 1400 



4J 1300 



1200 



I- 



1100 



, _^___ , ^. ^ 



Ab,An 2 

 68.0 

 32.0 



Percentage composition 

 Fig. 14. Curve of melting temperatures of the soda-lime feldspars. 



composition as the vitreous matrix. In Abi An 5 it will be remembered 

 that this was proved by the separation and analysis of the two phases ; 

 in Ab!An 2 partial crystallization was accomplished in the first cooling 

 and the remainder in a subsequent reheating and cooling, the two 

 groups of crystals proving optically identical; a small quantity of 

 Ab 4 Ani, which admits of absolute identification optically, was crystal- 

 lized out of a melt of that composition and readily recognized. More- 

 over, evidence to show that the same phase always separated was 

 likewise presented. 



Stated in this way, the relation appears to be a simple additive one 

 in which liquid and solid phases of like composition are stable in all 

 proportions of the components and behave like a series of separate 

 feldspars. But as soon as we consider it with reference to the laws of 

 solution and the phase rule, it can not be explained in this simple way. 



