SUMMARY OF CONCLUSIONS. 



71 



SUMMARY OF CONCLUSIONS. 



Reviewing this discussion briefly : (1) The triclinic feldspars are solid 

 solutions and form together an isomorphous series. It is a sufficient 

 condition for the latter that the curve of melting points is continuous 

 (Bruni, loc. tit.). Like Kuster's curves for organic compounds, the 

 curve of melting points does not follow Van't Hoff's law of dilute solid 

 solutions and does approximate closely to a straight line joining the 

 melting points of the components. The case appears to fall under 

 Type I of Roozeboom's theoretical classification of isomorphous mix- 



2.900 



2800 



2700 



> 2.600 



2.500 



2.400 



2.300 



2.200 



An 

 An 100 

 Ab 



Ab,An 5 

 84.1 

 15.9 



Ab,An E Ab,An, 



68.0 51.5 



32.0 48.5 



Percentage composition 



AbjAn, Ab 3 An, 

 34.7 26.1 



65.3 73.9 



Ab 





 100 



Fig. 23. Curves of specific gravity of the feldspars and feldspar glasses. 



tures, in which case the line can not become exactly straight unless 

 the melting points of the components are nearly or quite identical, nor 

 the solidification absolutely homogeneous without reducing the num- 

 ber of phases to three and destroying the equilibrium. The theory 

 also accounts for an absence of sharpness in the intermediate melting 

 points of the feldspars, but the fact that this lack of sharpness culmi- 

 nated in albite instead of terminating there shows that the viscosity 

 was the chief factor in our difficulties from this cause. Albite was 

 clearly shown to melt through a variable range of 1 50 or more, while 

 the intermediate feldspar bytownite (AbiAn 5 ) melted almost as 

 sharply[as anorthite, as one would expect it to do in view of the flat- 



