42 Buddington — Natural and Synthetic Melilites. 



Table II. 



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



2SiO,. Compositions expressed in weight percent, 



temperatures in degrees Centigrade. 



2CaO. 



2CaO. 









Crystal 



Glass 





AL0 3 . 



MgO. 







All 



-)- trace 



-f- trace 





Si0 2 . 



2Si0 2 



rico 



ne 



crystal 



Glass 



crystal 



All gla 



100 







1.669 



1.658 









1590° 



80 



20 



1.664 



1.657 



1498° 



1502° 



1553° 



1557° 



60 



40 



1.657 



1.654 



1441° 



1445° 



1506° 



1512° 



50 



50 



1.653 



1.652 







1475° 



1478° 



40 



60 



1.648 



1.649 



1400° 



1404° 



1430° 



1432° 



30 



70 



1.645 



1.648 



1390° 



1392° 



1394° 



1397° 



25 



75 



1.643 



1.647 



1385° 



1388° 





1390° 



10 



90 



1.637 



1.643 



1416° 



1420° 



1424° 



1427° 







100 



1.632 



1.639 









1458° 



optical properties are a continuous function of the com- 

 position, and crystals of a certain intermediate mixture 

 are isotropic for sodium light, constituting a transition 

 phase from the positive akermanite to the negative 



gehlenite. 



Fig. 1. 



<o /-660 

 ^ I-650 



K 



*S 1-630 









CO^ 



















J^r^l 



~~-o — "^^^""^ 



> 







SJ^er 











/■670 



I- 600 



1-650 



I-04O 



1-630, 



10 20 30 40 50 60 70 SO 90 IOO 



2CaO- Mg 0-2.Sc 2 WT- PERCE N T 2CaO-/l/ z Oj- ^Sc O z 



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

 percent for mixtures crystallized just below the solidus. 



From the data given by Eankin and Wright 10 in their 

 study of the ternary system CaO-Al 2 3 -Si0 2 , it is found 

 that the point of beginning of melting for all mixtures of 

 2CaO.Al 2 3 .Si0 2 and 3CaO.Al 2 3 .3Si0 2 is the eutectic 

 melting at 1265° and that no solid solution exists above 

 this point. Mixtures of these two compounds are the 

 only ones investigated in the present work which failed 

 to show evidence of some solid solution above the begin- 

 ning of melting, either accompanied or unaccompanied by 



10 G. A. Eankin and F. E. Wright, this Journal, 39, 1-79, 1915. 



