316 Rankin and Merwin — 



ing temperature (1575° ±5°) is based on values obtained 

 from various compositions as given in Table 21. 



Table 21. 



Melting temperature: Quintuple point 7 for spinel, Al 2 3 .Si0 2 , and aAL0 3 

 as determined from various compositions. 



fMgO 100 150 150 140 15-0 



Composition, weight, per cent \ ALO, 700 600 440 47-0 48-0 



[SiO.. 200 250 41-0 49-0 37-0 



Temperature 1575 1585 1570 1568 1568 



The Composition - and Properties of the Ternary 

 Compound (Cordierite). 

 The temperatures in the field of the ternary compound 

 are highest along the boundary toward sillimanite, but 



Fig. 3. 



there is no maximum within the field, therefore the com- 

 position of the crystals of the compound which begin 

 separating near the point 3 must be between sillimanite 

 and the maximum in the boundary curve 2-3, that is 

 within the triangle III, IV, V of fig. 3. But the courses 

 of the crystallization curves of Table 14a show that the 

 composition must lie within about the limits IV, V, VI, 

 VII. The simplest formulas that can be written for com- 

 positions within this quadrilateral are MgO.Al 2 3 .3SiO.. 

 and 2Mg0.2Al 2 3 .5Si0 2 . 



If the crystals separating in the other parts of the field 

 of the ternary compound have compositions thus repre- 

 sented then there should be a maximum on boundary 

 curve 1-5. If the composition of the crystals of the ter- 



