26 Rankin and Wright — Ternary System CaO-Al 2 3 -/Si0 2 . 



THE TERNARY COMPOUNDS. 



Of the three ternary compounds, two were found to be 

 stable in contact with liquid, and as the field for each of these 

 two compounds exhibits a maximum, the composition of the 

 compound is represented by the position of the maximum. 

 The molecular compositions of these two compounds are 

 CaO.Al 2 3 .2Si0 2 and 2CaO.Al 2 3 .Si0 2 . The third ternary 

 compound was found to be unstable in contact with liquid, so 

 that in order to determine its composition it was found neces- 

 sary to crystallize glass of various compositions until a com- 

 position was obtained in which the compound was the only 

 phase present. The molecular composition of this compound 

 was thus found to be 3CaO.A] 2 3 .Si0 2 . 



Artificial anorthite, CaO.Al 2 O s £Si0 2 (CaO 20*1, A1 2 3 36*6, 

 Si0 2 43*3). Pure artificial anorthite melts at 1550° ± 2° ; its 

 ternary field is bounded by the ternary fields for tridymite, 

 aCaO.Si0 2 , 2CaO.Al 2 3 .Si6 2 , Al 2 3 .Si0 2 , A1 2 3 ; it forms quin- 

 tuple point mixtures with tridymite and aCaO.Si0 2 , with 

 tridymite and Al 2 3 .SiG 2 , with Al 2 3 .Si0 2 and A1 2 3 , with 

 A1 2 3 and 20aO.Al 2 O 3 .SiO 2 , and with 2CaO.Al 2 3 .Si0 2 and 

 aCaO.Si0 2 . 



It appears in the preparations either in lath-shaped in- 

 dividuals elongated after the edge (001) (010) or as tabular 

 crystals after the side pinacoid M(010). Polysynthetic twin- 

 ning after the albite law is common and characteristic. Crystal 

 system triclinic. Refractive indices : 7 Na = 1-589 ± 0*001 ; 

 /3 Na = 1-585 =b 0*001, a Na == 1-576 =b 0*001. (These values are 

 identical, within the limits of error, with those of the natural 

 anorthite.) Birefringence not strong, y-a about 0'013. Ex- 

 tinction angle on basal pinacoid approximately 35° ; optic 

 axial angle large, optical character negative. 



The 'compound 2CaO.Al 2 0,. Si 0/(CaO 40-8, A1 2 3 37*2, 

 Si0 2 22) melts at 1590° ± 2° ; its ternary field is bounded by 

 the ternary fields for CaO.Al 2 3 .2Si0 2 , Al 2 3 ,3Ca0.5Al 2 0' 3 , 

 CaO.Al 2 3 , a2CaO.Si0 2 , /32CaO.Si0 2 , 3Ca0.2Si0 2 , aCaO.SiO, ; 

 it forms quintuple point mixtures with CaO.Al„0 3 2Si0 2 and 

 A1 2 3 , with A1 2 3 and 3Ca0.5Al 2 3 , with 3Ca0.5Al 2 3 and 

 CaO.Al 2 3 , with CaO. A1 2 3 and /32CaO.Si0 2 , with «2CaO.Si0 2 

 and /32CaO.Si0 2 , with /32CaO.Si0 2 and 3Ca0.2Si0 2 , with 

 3Ca0.2Si0 2 and aCaO.Si0 2 , with aCaO.SiO, and CaO.Al 2 3 . 

 2Si0 2 . 



It crystallizes in clear, glassy, colorless grains which show 

 distinct but not perfect basal cleavage. Fracture, conchoidal. 

 Hardness about 6. Specific gravity about 3*038. Crystal 

 system, tetragonal. Refractive indices : w Na = 1-669 =h 0*001, 

 e Na = 1*658 db O'OOl ; birefringence not strong, interference 



