48 Buddington — Natural and Synthetic Melilites. 



Si0 2 and the 3CaO.Al 2 3 .3Si0 2 compound have negative 

 optical characters, whereas 2CaO.Mg0.2Si0 2 is positive, 

 so that the combination of the three end members gives a 

 certain definite series of mixtures which are isotropic to 

 a definite wave length of light. The lines connecting com- 



Fig. 3. 

 2CaO*Al 2 3 *Si0 2 



2CaO*MgO*2SO 



WT PERCENT 



3CaO*Al ? 0,*3SiO 



2 W 3 



3. — Triangular concentration diagram showing compositions of prepara- 

 tions investigated and the isotherms for the temperatures of complete 

 melting. 



positions whose birefringence is 0.000 and 0.005 are more 

 or less concentric to the 2CaO.Mg0.2Si0 2 component. 



Mixtubes of 2CaO.Mg0.2Si0 2 , 2CaO.Al 2 3 .Si0 2 , and 

 -[(90 3CaO.Al 2 3 .3Si0 2 ) (10 3Na 2 O.AL0 3 .3Si0 2 )] 



Mixtures of 2CaO.Mg0.2Si0 2 and the solid solution, 

 equivalent in composition to the natural mineral sarcolite, 

 which contain 80 percent or more of the 2CaO.Mg0.2Si0 2 



