JV. L. Bowen — The Binary System. 563 



slightly above the eutectic point and will show on quenching 

 the excess phase. By examining all, the excess phase for the 

 composition used will be determined, and by repeating with 

 other mixtures the eutectic may be placed within narrow 

 limits. In this manner it was shown that in the mixture with 



45 per cent CaAl 2 Si 2 8 nephelite was the excess phase, and with 



46 per cent, anorthite. The eutectic composition is, therefore, 

 between the two, approximately at 45 per cent CaAl 2 Si 2 8 . 



The Limits of Solid Solution {Table II (h) (k) & {I).— A 

 solution may be defined simply as a homogeneous mixture, and 

 with this in mind it is readily seen that the limit of solid solu- 

 tion might be determined by finding the first composition 

 (concentration) which shows any trace of inhomogeneity. This 

 method is, as a rule, the best in mineral work. The most 

 important solid solutions encountered in this system are the 

 nephelite mix-crystals. At a temperature slightly below the 

 eutectic point the first mixture which showed anorthite in 

 excess was that with 35 per cent CaA] 2 Si 2 8 ; in this the 

 anorthite was a mere trace. The extreme solid solution is, 

 therefore, approximately Ne 65 An 35 . 



The maximum concentration of CaAl 2 Si 2 8 in carnegieite is 

 obtained at the temperature of the ' knickpunkt ' 1352°, where 

 the composition of the carnegieite mix-Crystals is Cg 95 An B 

 (approx.). 



The Liquidiis of Anorthite {Table 11(e) ). — The upper por- 

 tion of the anorthite liquidus was established by heating-curve 

 breaks. With lesser concentration of anorthite the magnitude 

 of the eutectic break obscured the upper point, and in these 

 mixtures the liquidus was located by quenching from various 

 temperatures and finding that temperature at which the last 

 trace of anorthite melted. 



In a similar manner part of the carnegieite liquidus was 

 located (Table II (d)). 



Optical Study. 



In the microscopic examination of the product of each 

 experiment the method employed for identifying constituents 

 was that of determining refractive indices in the crushed 

 grains by immersion in liquids of known refractive index. 

 This method is especially useful for detecting small quantities 

 of any component. If the preparation be placed in a liquid 

 which matches the mean index of the excess constituent, even 

 very minute quantities of another constituent show up markedly, 

 except in the rare case of different substances with nearly 

 identical refractive indices. Moreover, it is possible to deter- 

 mine in the crushed grains most of the properties by which 

 minerals are identified in thin section, such as order of bire- 

 fringence, optical character, etc. 



