44 Buddington — Natural and Synthetic Melilites. 



percent 3CaO.Al 2 3 .3Si0 2 these is a mere trace of inho- 

 mogeneity in material held for 16 hours at 980° ; and a 

 mixture of equal parts of the two components, treated at 

 the same temperature for the same length of time, appears 

 perfectly homogeneous. The upper range of homo- 

 geneity for this latter mixture, however, is limited, and 

 dissociation products appear in preparations heated for 

 one hour at 1150°, or over 100° below the eutectic. In a 

 mixture of 75 percent 2CaO.Al 2 3 .Si0 2 and 25 percent 

 3CaO.Al 2 G 3 .3Si0 2 the beginning of dissociation is not 

 well marked, and the material may be homogeneous up 

 to the eutectic. In summary, then, mixtures of 2CaO. 

 Al 2 3 .Si0 2 and 3CaO.Al 2 3 .3Si0 2 exhibit a limited range 



Fig. 2. 



o 



I 



/■66O 



/■6ZO 



o 



o 







1-670 



1-650 



/■630 



1-620 



O 20 4 6O SO IOO 



2CaO-/U 2 Oy^iO a WT- PERCENT 3CaO-Al z Oy33i O a 



2. — Curves for refractive indices plotted against composition in weight 

 percent for mixtures which show complete homogeneity when crystallized at 

 about 1000°. Indices of refraction for dominant phase of inhomogeneous 

 mixture are indicated where determined. 



for complete homogeneity extending from the 2CaO. 

 Al 2 3 .Si0 2 component to a little beyond a 50-50 mixture, 

 and within a temperature range that is limited upwards 

 by dissociation or the eutectic point, and of unknown 

 lower limits. Partial solid solution is found in mixtures 

 containing so much as 80 per cent 3CaO.Al 2 3 .3Si0 2 , as 

 is indicated by the indices of refraction determined on 

 the major constituent of preparations crystallized at 

 980°, and may extend further. But a break in the series 

 of completely homogeneous mixtures towards the 3CaO. 

 Al 2 3 .3Si0 2 component beyond a 50-50 mixture is indi- 

 cated by a break in the curves for indices of refraction. 

 In fig. 2 the indices of refraction are plotted against com- 



