The Ternary System MgO-Al^O t -SiO v 303 



determinations of their primary crystalline phases ; that 

 is, the determinations of the crystalline substance which 

 is the first to crystallize from a completely melted cool- 

 ing charge,* or the last to melt in a crystalline charge 

 when heated. The determination of the primary crys- 

 talline phase in a given composition is carried out by 

 heating, a small charge of the material to various con- 

 stant temperatures for a period of time sufficient to 

 establish equilibrium, and suddenly cooling (quenching) 

 so as to prevent the crystallization of any liquid which 

 may have formed. By proceeding systematically in this 

 way one eventually obtains a quench which contains 

 a single crystalline substance embedded in glass. 



In this manner the primary phases which could be 

 obtained below 1600 ° 5 were determined in the first com- 

 position prepared. These data give one an idea not 

 only as to the different crystalline compounds which will 

 be found in the system MgO, A1 2 3 , Si0 2 but also an idea 

 as to the melting temperatures throughout this system. 

 With the preliminary data, thus obtained, as a working 

 basis, other compositions were made up and the primary 

 phases determined. Proceeding systematically in this 

 way to prepare charges, whose compositions are such as 

 to give further information as to the crystalline sub- 

 stances found in this system, the range of compositions 

 in which each such substance occurs as a primary phase 

 was determined. In other words, using phase rule 

 terms, the equilibrium field of each such crystalline sub- 

 stance was established. 



For the study of the composition of the ternary com- 

 pound, glasses were required which contained no crys- 

 tals. Many of these glasses required preparation at 

 temperatures considerably above 1600°. Small charges 

 were melted in the gas blast furnace and subsequently 

 rapidly chilled by immersing in water. 



Limits of the Fields. 



Data which determine the limits of each field are given 

 in tables 1-7 inclusive. These data are presented graph- 

 ically in the triangular diagram, figure 1. 



5 1600° C is about the upper limit of the platinnm-resistanee furnace from 

 which the charges are quenched by dropping into a, dish of mercury. 



