Rankin and Wright — Ternary System CaO-Alfi-SiO^. 9 



THE BINARY SYSTEM '. ALUMINA-SILICA. 



The general behavior of the system alumina-silica is obvious 

 from the diagram, fig. 2. In this binary series only one com- 

 pound, Al O 3 .SiO 2 , sillimanite, appears; it melts at 1816° ± 10° 

 and forms eutectic mixtures both with alumina and with silica. 

 The eutectic between sillimanite Al o 3 .Si0 2 and A1 2 3 , has the 

 composition A1 2 3 64, Si0 2 36 and melts at 1810° db 10° ; that 



Fig. 2. 



2000 



" 







S -i 



1900 







s 



/ _ 



s Al 2 3 +Melt 





Cnstobaht + Melt^, ' ' AI 2 S!0 5 + Melt 



c , 



1800 

 1700 





D 



- f- 





A4 __B^'"' 



G 





- 



1600 



-H 



1500 



Cristobal ite + AI 2 Si 5 



I 



J 





AI 2 3 + AI 2 S.0 5 



1400 



Tndymite + AI 2 Si0 5 







- 



1300 









1 1 1 



S,0 2 



Al 2 3 



Fig. 2. Concentration-Temperature Diagram for the System Alumina- 

 Silica. 



between sillimanite and cristobalite (the only form of Si0 2 

 occurring on the liquidus curve in this system) consists of 

 A1 2 3 13, Si0 2 87, and melts at 1610°. Two other forms of 

 Al 2 3 .Si0 2 , namely, andalusite and cyanite or disthene, occur 

 in nature, but they have not been observed with certainty in 

 the laboratory nor has anything bearing on their relation to 

 sillimanite been established beyond the fact that on heating 

 they pass into sillimanite. The laboratory and field evidence, 

 so far as it goes, indicates that they are monotropic with respect 

 to sillimauite. 



Artificial sillimanite crystallizes from the melt as fine- 

 grained, fibrous to lath-shaped individuals, which, like the 

 natural mineral, are often in close parallel groups. Terminal 



