Buddington — Natural and Synthetic Melilites. 49 



molecule, show a trace of grains with a much higher index 

 of refraction at temperatures below the dissociation 

 point. The same phenomenon has already been referred 

 to in the case of the mixtures of 2CaO.Mg0.2Si0 2 and 

 3CaO.Al 2 3 .3Si0 2 , and its meaning is not known. With 



Fig. 4. 

 2CaO*Al 2 3 *Si0 2 



2CaO*MgO*2SiO 



WT PERCENT 



3Ca0*Al ? 0o»3Si0 



2 V 3 



4. — Triangular concentration diagram showing compositions of prepara- 

 tions investigated. Lines connecting similar indices of refraction have been 

 plotted for those mixtures which show complete homogeneity within a 

 definite temperature range above 980° (the lowest temperature of experi- 

 ments) . Lines for equal birefringence also plotted. 



this possible exception, the mixtures of the compounds 

 2CaO.Mg0.2Si0 2 , 2CaO.Al 2 3 .Si0 2 , and the solid solu- 

 tion with the composition of sarcolite [(90 3CaO.Al 2 3 . 

 3Si0 2 ) (10 3Na 2 O.Al 2 3 .3Si0 2 )] form a complete series 

 of solid solutions within a temperature range above 

 980° — the approximate lower working limit under the 

 conditions of the experiments — and below the solidus or 

 the dissociation point, as the case may be. 



Am. Jour. Sci.— Fifth Series, Vol. Ill, No. 13.— January, 1922. 

 4 



