Buddington— Natural and Synthetic MeMites. 47 



with more rapid nudeation and crystallization at higher 

 temperatures (about 1100 o ±50°), much more air is 

 included and the crystalline material is very cloudy. As 

 soon as dissociation begins, the air is expelled and the 

 preparations are clear but minutely heterogeneous with 

 the dissociated products. 



The optical characters and thermal data for mixtures 

 of the three components are given in Table V. In fig. 3 



Table Y. 



Optical and thermal data for mixtures of 2CaO.ALC .SiO.:2CaO.AIgO. 



2SiCV.3CaO.ALO .3SiO ; . Compositions expressed in weight 



percent and temperatures in degrees Centigrade. 



3CaO. 2CaO. 2CaO. One Trace of 



ALO : . AUO. Al.Oj crystal disso- — trace of 



jgi 2SiO a SiO," ■■_■ -: phase eiation crystals All glass. 



: 50 1.652 1.645 1170° 1190° 1194= 1501° 



(16 hrs.) (16 hrs.: 

 20 60 20 1.637 1.639 1100° 1110 c 1378 e 13S3 2 



(16 hrs.) (16 hrs.^ 

 40 20 10 1.617 1.612 * 1170 e 1480° I486 



980° 1: hrs (16 hrs. 



60 2( :: *!L641 *1.634 980 e 1408° 1115- 



(16 hrs.) 

 10 10 *1.63S *1.633 Inhomogene his 

 (16 hrs." at 9-.~ : 

 20 1.610 1.637 11S0 : 1110= 1111= 



_ hrs :: hrs 



15 15 70 ' 1.657 1.652 1150= 1200 c 1511= 1517 3 



(16 hrs.) (16 hrs 



* Determined on major crystal phase. 



the compositions in weight percent of the mixtures 

 investigated are plotted in triangular coordinates with 

 isotherms for the temperatures of complete melting. A 

 point of possible significance bearing on the origin of the 

 natural melilites is that the mixtures which are closest in 

 composition to the natural minerals lie in the zone with 

 lowest temperatures of complete melting. In fig. 4, 

 lines connecting similar indices of refraction of the ordi- 

 nary ray and the extraordinary ray have been plotted 

 against the composition in weight percent for those mix- 

 tures which show complete solid solution within a definite 

 temperature range above 9S0 : (the approximate lower 

 working limit under the conditions of the experiment). 

 Lines connecting those mixtures which show the same 

 birefringence have also been plotted. Both 2CaO.AL0 3 . 



