N. L. Bowen — The Binary System. 567 



in determining the refractive indices ; for example, both 

 indices of crystals varying in composition from Ne b0 An 20 to 

 Ne 75 An 25 appear to match the F537 liquid, but Ne 80 An 20 is 

 weakly birefracting and determinable in the prismatic section 

 of a crystal as definitely negative and therefore &)>e. In a 

 similar manner it was shown that JNe 75 An 25 is positive and 

 therefore e>o>. The positive character of crystals Ne 65 An 35 

 indicated by the interference figure was confirmed in these 

 crystals of known orientation. 



A curious fact which gave rise to some difficulty in some of 

 the quenchings may be noted here. The nephelite mix-crystals 

 with 23 per cent CaAl 2 8i 2 8 (approx.) are in equilibrium at 

 1335° with a melt containing 36 per cent CaAl 2 Si 2 8 (approx). 

 (Area CBFG, Diagram II.) When quenched the crystals are 

 sensibly isotropic and have an index 1*537 and the glass, it so 

 happens, has the same index.* Neither by a difference of 

 index nor by the presence of birefringence do the crystals 

 become distinguishable from the glass. Thus, in several 

 charges quenched from 1335° the presence of nephelite in the 

 glass could not be detected, although the necessity of its pres- 

 ence could be proven from charges quenched at a slightly 

 higher or lower temperature, in which charges the extreme 

 similarity of nephelite with the glass was approached, but not 

 quite attained. For this reason the attempt to accurately 

 locate the intersection of the liquidus B Gr on the ordinate of 

 35 per cent CaAl 2 Si 2 8 failed. The curve may, however, be 

 regarded as sufficiently established by the other known points. 



The Work of Earlier Investigators. 



Several different workers have succeeded in preparing the 

 nephelite form of NaAlSi0 4 , but the carnegieite form has 

 seldom been encountered. This is rather a curious fact, for 

 carnegieite crystallizes readily from the pure melt, whereas 

 nephelite is obtained only with very slow coolingf or with the 

 aid of fluxes. Carnegieite was first prepared by Thugutt 7 by 

 fusing an artificial ' nepheline hydrate.' He obtained a mineral 

 with polysynthetic twinning and high extinction angles, prob- 

 ably triclinia Following the suggestion of Lemberg, this 

 mineral was called soda-anorthite. 



In 1905, carnegieite was prepared at the Geophysical labor- 

 atory and the new name proposed. (Published 191 0.) 8 But 



* The index of this glass corresponds approximately with that calculated 

 for it from the indices of anorthite glass and carnegieite glass in the propor- 

 tion 36 : 64. 



f With slow cooling, the temperature of the preparation is for a long 

 period in the region slightly below the inversion point, permitting the for- 

 mation of the low temperature form, nephelite. 



