Buddington — Natural and Synthetic Melilites: 41 



The succeeding tables present only selected data which 

 are considered significant for the problem. The indices 

 of refraction are all for sodium light. Where indices of 

 refraction are given for mixtures which are slightly dis- 

 sociated, the observations apply to the dominant crystal 

 phase. 



Mixtures of 3Ca6.Al,0 8 .3SiO, ; 2CaO.Mg0.2SiOo, and 



2CaO.Al 2 3 .Si0 2 . 



A preliminary series of experiments made on mixtures 

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

 3Si0 2 gave unsatisfactory inhomogeneous crystalline 

 aggregates at the temperatures of the experiments. 

 Work on these mixtures, accordingly, was temporarily 

 abandoned. The constant presence of a sodium com- 

 pound in the natural melilites and the existence of the 

 natural mineral sarcolite, containing about 10 percent of 

 the compound 3Na 2 O.Al 2 3 .3Si0 2 , suggested that mate- 

 rial of the composition of sarcolite be tried in place of the 

 calcium compound 3CaO.Al 2 3 .3Si0 2 . In the progress 

 of this later work it was discovered that certain mixtures 

 which were heterogeneous at temperatures just below 

 the solidus were homogeneous at lower temperatures. 

 With this information as a guide, work on mixtures of 

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

 3SiOo, was resumed and significant results were obtained. 



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

 which contained 70 percent or more of the former com- 

 ponent, are homogeneous below the dissociation point 

 except for a few grains with a much higher index of 

 refraction. Whether these grains actually mean an 

 incomplete solid solution or are the result of slight 

 impurities in the melt, or whether they are compounds 

 formed at lower temperatures and existing in unstable 

 equilibrium at higher temperatures, was not ascertained. 



It has been previously shown 9 that the compounds 

 2CaO.Al 2 3 .Si0 2 and 2CaO.Mg0.2Si0 2 are isomorphous 

 and form a complete series of solid solutions with a 

 minimum melting point. In table II the optical charac- 

 ters and thermal data for mixtures of these two compo- 

 nents are given, and in fig. 1 the indices of refraction are 

 plotted against the composition in weight percent. The 



• J. B. Ferguson and A. F. Buddington, loc. cit. 



