8 Rankin and Wright — Ternary System OaO-Al 2 0-Si0 2 . 



twinning lamellae parallel with the cleavage were observed; 

 showing small extinction angles not exceeding v : 7' = 3° ; 

 usually the extinction is parallel. Crystal system, probably 

 monoclinic. Refractive indices : a Na = 1*642 rfc 0*002, /3 Na = 

 1-645 =b 0*003, 7 Na = 1-654 d= 0*002; birefringence medium 

 about 0*015 ; optic axial angle 2E approximately 52° ; plane of 

 optic axes normal to cleavage direction ; optical character nega- 

 tive. This form of the orthosilicate is readily distinguished 

 from the a and ft forms by its lower refractivity, the position 

 of its plane of optic axes, and its optical character. 



$"2CaO.SiO^ is an unstable form which was obtained only 

 by quenching an old hydrated sample from a temperature of 

 about 1425° ; it could not be obtained from fresh preparations, 

 indeed we have obtained it only from one particular sample, 

 and then in fine equant grains without apparent cleavage. 

 The average refractive index is about 1*715 and the birefring- 

 ence very weak. Interference figures are difficult to obtain 

 because of the fine granularity and the weak birefringence, but 

 the optic axial angle appears to be small or zero and the opti- 

 cal character positive. The form appears to be monotropic, 

 but the evidence thus far obtained can hardly be considered to 

 have established beyond question the existence of this form. 



4. Tricalcic silicate SCaO.SiO, (CaO 73*6, Si0 2 26*4) is 

 unstable at its melting point, but unlike the compound 3CaO. 

 2Si0 2 it does not occur as a primary phase at any point on the 

 liquidus curve of the binary system lime-silica. Although a 

 definite binary compound, it is found as a primary phase only 

 within the ternary system. It dissociates at 1900° =t 20° 

 into CaO and 2CaO.Si0 2 ; the fact that this temperature is 

 below that of the eutectic between CaO and 2CaO.Si0 2 (2065°, 

 composition CaO 67*5, Si0 2 32*5) explains why 3CaO.Si0 2 

 does not occur in contact with the melt in the binary system. 



This compound occurs in small equant grains not over 0'03 mm 

 in diameter, colorless and apparently without cleavage. Aver- 

 age refractive index approximately 1'715; birefringence very 

 weak, not over 0*005. The grains appear uniaxial or biaxial 

 with small optic axial angle ; optical character negative. In 

 certain of the preparations of ternary mixtures, fine twinning 

 lamellae with low extinction angles were observed on some of 

 the grains, indicating that the crystal system is possibly mono- 

 clinic. 



It is an interesting fact that in the formation of this com- 

 pound from the mixed oxides at 1400° or 1500°, orthosilicate 

 is first formed and unites on longer heating with free lime to 

 form the tricalcic silicate. 



