46 Buddington — Natural and Synthetic Melilites. 



Table IV. 



Optical and thermal data for mixtures of 2CaO.Mg0.2Si0 2 and 3CaO.Al 2 3 . 



3Si0 2 . Compositions expressed in weight percent and 



temperatures in degrees Centigrade. 



3CaO. 2CaO. *Essentially * Trace of Glass 



ALA. MgO. one crystal disso- -f- 



3Si0 2 2Si0 2 «w tie phase ciation crystals All glass 



100 1.632 1.639 1458° 



10 90 1408° 1420° 1525° 



20 80 1.634 1.639 1215* 1240° 1403° 1409° 



(1 hr.) (% hr.) 



30 70 1.629 1.632 1200° 1240° 1380° 1386° 



(1 hr.) (i/ 2 hr.) 



45 55 1.629t 1.630t 980° 



(16 hrs.) 

 65 35 Inhomogeneous 1317° 1324° 



(980° 16 hrs.) . 

 80 20 Inhomogeneous 1317° 1323° 



(980° 16 hrs.) 

 100 Inhomogeneous 1340° 1347° 



(980° 16 hrs.) 



* Trace of higher refracting birefringent dots present, and preparations 

 are therefore not completely homogeneous, 

 t Determined on major crystal phase. 



side, and incomplete towards the 3CaO.Al 2 3 .3Si0 2 com- 

 ponent, at temperatures above 980° (the lower limit at 

 which experiments were performed) and below the tem- 

 perature of dissociation. 



The intermediate mixtures of the three end mem- 

 bers containing 40 percent 3CaO.Al 2 3 .3Si0 2 proved 

 extremely unsatisfactory. Held for 16 hours at 980°, they 

 appeared as homogeneous fibrous aggregates, whereas 

 at 1100°, for the same length of time, they are so very 

 cloudy, with areas of brownish dots, as to make it impos- 

 sible to judge whether they are simply full of included 

 air or, as seems more probable, are mineralogically inho- 

 mogeneous. At 1170° the preparations are relatively 

 clear again and show distinct interstitial birefringent 

 material. It is possible but not probable that even in the 

 preparations at 980° the material is actually inhomogene- 

 ous but appears to be homogeneous because of the finely 

 fibrous character. The general rule is that the prepara- 

 tions rich in the 3CaO.Al 2 3 .3Si0 2 component, both in 

 this system and in those containing the 3Na 2 O.Al 2 3 . 

 3Si0 2 compound, with slow nucleation and crystallization 

 (about 1000°), are clear and quite free of air; whereas 



