320 Rank I a and Merwin — 



Table 22 contains other details of the observations on 

 those forms. 



Thus the optical comparison of crystals from various 

 parts of the cordierite field with crystal aggregates 

 formed from glasses having compositions in which MgO, 

 AL>0 3 and Si0 2 range from 1:1:3 to 2:2:5, indicate that 

 the possibility of solid solution of nearly this extent 

 should be considered. Already it has been shown that 

 the crystallization curves, the temperatures, and the 

 optical properties of the crystals over about one-third 

 of the cordierite field are satisfactorily explained by 

 assuming the existence of a 2 :2 :5 compound. Solid solu- 

 tion over the range just indicated would account for the 

 optical properties of the rest of the crystals. A diagram- 

 matic representation of change of composition of crys- 

 tals and course of crystallization curves in this case is 

 shown in fig. 4. A case of more extensive changes is 



shown in 



fig. 5. 





Table 22. 





Quenehiiigs showing the characteristics of aggregates of the a- and 







/^-ternary compound. 



Composition 



Weight 



Per cent 



Temp. 



Phases 



MgO 



ALO3 



Si0 2 







17-5 



42-5 



400 



915 

 925 



1000 



fi 4. MgO.AlA -f 1 

 a + AlA-Sib, -f ? 



160 



400 



44-0 



925 

 950 

 975 



„, + a + f (tr.) 

 p + a + ? (tr.) 

 a + /* + MgO.AlA (tr.) 



14-5 



40-0 



45-5 



950 

 1000 

 1010 

 1025 



a + AlA.SiO, -f 1 

 Indefinable aggregate 



15-3 



38-8 



45-9 



1000 

 1050 

 1100 



p _|_ a + Al 2 0,-Si0 2 

 a + ALO s .SiO, + ? 



14-5 



36-5 



490 



950 



1050 

 1300 



a + AL0 3 -Si0 2 + ? 



14-4 



35-6 



50-0 



950 

 1050 

 1300 



a + AlA.Si'0* + MgO.AlA ? 



14-0 



350 



510 



1450 

 1460 



a 



Glass 4- AlA.SiO, 



160 



330 



51 



1360 

 1450 



a + MgO.AlA 

 o4-10%(?) glass 



16-5 



32-0 



51-5 



1430 



a. 4 5% ( ?) glass 



150 



320 



530 



1450 



a 4 5% ( ?) glass 



150 



300 



550 



900 

 1000 



fi 4- glass 



*< + <.+ » (tr.) 



1 The MgO.AlA and 



other binary compounds reported here were usually 



in small amounts and in very small crystals, 



so that their identification was 



not always certain. 









