6 Rankin and Wright — Ternary System CaO-Alft-SiO^. 



1190° while the silica raises it to 1210°. The eutectic between 

 aCaO.Si0 2 and tridymite (the stable form of silica at the 

 eutectic temperature) has the weight percentage composition, 

 CaO 37, Si0 2 63 ; its melting temperature is 1436° db 2°. The 

 other general relations are obvious from the diagram. 



aCaO.SiO^ crystallizes in equant grains without decisive 

 crystal outline ; polysynthetic twinning is not uncommon and, 

 together with the optical evidence, indicates that the crystal 

 system is probably monoclinic. Refractive indices : 7 Na = 

 1-654 =b 0-002 ; « Na = 1'610 do 0*002 ; birefringence, strong, 

 j-a about 0*041 ; optical axial angle, small, nearly uniaxial ; 

 optical character positive. 



/3CaO.Si0. 2 occurs in lath-shaped individuals similar in every 

 respect to the natural mineral wollastonite. Cleavage parallel 

 to the elongation of the laths and fibers is well marked. 

 Crystal svstem monoclinic ; refractive indices : y m = 1*631 db 

 0-002, &J a = 1-629 ± 0-002, a Na = 1-616 db 0-002 ; birefringence 

 medium, j-a about 0015 ; extinction parallel to elongation ; 

 optic axial angle 2E, about 70° ; optical character negative ; 

 plane of optic axes normal to cleavage lines. 



(2) The compound SCaO.VSiO, (CaO 58-2, Si0 2 41-8) dis- 

 sociates into 2CaO.Si0 2 and liquid at 1475° db 5°, and con- 

 sequently shows no true melting point. In the binary series it 

 occurs as primary phase on the liquidus only with mixtures of 

 composition ranging from 54'5 to 55'5 per cent lime. It is 

 best formed by holding glass of the composition 3Ca0.2Si0 2 at 

 a temperature slightly below 1475° until crystallization occurs. 



This compound crystallizes in rounded irregular grains, 

 without definite crystal outline and without distinct cleavage. 

 Crystal system, probably orthorhombic. Refractive indices : 

 7 ^ a = 1-650 ± 0-002, a Na = 1*641 ± 0*002 ; birefringence fairly 

 weak, y-a being about 0-01 : optic axial angle large ; optical 

 character positive. 



It was at first thought that this substance might be pure 

 akermanite, whose formula, according to Vogt, is 4Ca0.3Si0 2 , 

 but akermanite is tetragonal and uniaxial with refractive 

 indices: 7Na about 1-640, a Na about 1-635. The presence 

 of magnesia in Vogt's akermanite may account for the 

 observed differences in optical behavior of the two substances ; 

 and if so, the formula for pure akermanite should be 

 3Ca0.2Si0 2 rather than 4Ca0.3Si0 2 . 



The compound 3Ca0.2Si0 2 forms withaCaO.Si0 2 a eutectic 

 mixture of the weight percentage composition CaO 54*5, SiO, 

 45'5, which melts at 1455° ± 2°. It does not form a eutectic 

 with 2CaO.Si0 2 ; the quadruple (invariant) point at which 

 these two compounds are stable in coexistence with liquid and 

 vapor is at the composition CaO 55*5, Si0 2 44'5, the correspond- 

 ing temperature being 1475° db 5°. 



