Buddington — Natural and Synthetic Melilites. 45 



position in weight percent, and the curves connect those 

 mixtures which show complete homogeneity for a tem- 

 perature range above 980° (the lower working limit 

 under the conditions of the experiment). The indices of 

 refraction for the major constituent of inhomogeneous 

 mixtures crystallized for 16 hours at 980° are also 

 plotted. 



Since dissociation takes place at or before the eutectic, 

 thermal data for the liquidus and solidus cannot be used 

 as evidence of solid solution. 



None of the crystallized mixtures of 2CaO.Mg0.2Si0 2 

 and 3CaO.Al 2 3 .3Si0 2 was found to be absolutely homo- 

 geneous within the temperature range explored. Mix- 

 tures containing 70, 80, and 90 percent of the 2CaO.MgO. 

 2Si0 2 compound, however, showed only a trace of some 

 included, minute, more highly refracting, low-biref ringent 

 grains, and are essentially homogeneous for a range below 

 the dissociation point, or the point of beginning of 

 melting, as the case may be. The lowest temperature at 

 which experiments were made was 980°, so that nothing 

 is known of" the relations below this point. A mixture 

 containing only 10 percent of the 3CaO.Al 2 3 .3Si0 2 com- 

 pound appears homogeneous up to the point of beginning 

 of melting (1417° ±6°) except for the qualification noted 

 above. Dissociation (in addition to the trace of grains 

 noted above) in mixtures richer in this compound, how- 

 ever, takes place much below this, and those containing 

 55 percent 3CaO.Al 2 3 .3Si0 2 show a trace of interstitial 

 inhomogeneity at 980° ; those containing a greater per- 

 centage of this compound show still greater inhomogene- 

 ity at the same temperature. In Table IV the optical 

 characters and the thermal data for mixtures of these 

 two compounds are given. Owing to the lack of positive 

 evidence of the complete homogeneity of any of the pre- 

 parations examined, no curves have been drawn to show 

 the relation of the optical characters to the composition. 

 It is believed, however, that the optical characters given 

 are those of material essentially the same in chemical com- 

 position as would be indicated by the mixture designated. 



Mixtures of the three components 2CaO.Al 2 Oo.Si0 2 , 

 3CaO.Al 2 p 3 .3Si0 2 and 2CaO.Mg0.2Si0 2 form an incom- 

 plete series of solid solutions, completely homogeneous 

 in the field along the 2CaO.Al 2 3 .Si0 2 -2CaO.MgG.2Si0 2 



