568 JV. L. Bowen — The Binary System. 



few thermal determinations were made, so that its melting 

 point and its relation to nephelite were not ascertained. It 

 was not known at that time that some soda might be volatilized 

 during the preparation of the compound, but a reexamination 

 of some of the samples then made gives evidence that there is 

 a slight shortage in soda demonstrated bj a slight excess of 

 alumina. The optical properties of this material vary slightly 

 from those of the purer material, since prepared, the figures 

 for which are given here. For the same reason the density 

 2'513 found here is probably more accurate than that formerly 

 found (2*571). 



Gerh. Stein, 9 by crystallizing " soda-nepheline" directly from 

 the melt, obtained a granular, strongly double-refracting mass. 

 It is possible that Stein obtained carnegieite, of which the 

 double refraction, though hardly strong, as ordinarily under- 

 stood, is higher than that of nephelite. 



Nephelite. — The formation of soda-nephelite was first accom- 

 plished by Fonque and Levy in 1878 by simple fusion of its 

 constituents. With the addition of a flux (sodium vanadate) 

 Hautefeuille (1880) succeeded in making measureable hexago- 

 nal crystals." 



In 1884 Doelter 12 prepared not only the simple orthosilicate, 

 NaAlSi0 4 but mixtures with excess silica and the potash and 

 lime content of natural nephelite. 



Many others have succeeded in making nephelite as one 

 constituent of a complex mixture, or by the action of aqueous 

 carbonate solutions on various compounds. This work has no 

 special interest from the point of view of this paper, and will 

 merely be referred to here. 



Doelter found that JSfa 2 Al 2 Si 2 8 (JSTaAlSi0 4 ) and CaAl 2 Si 2 8 

 were capable of forming mix-crystals, as did the present writer. 

 It appears that Doelter found a somewhat greater range of 

 miscibility. 



Wallace, 10 by the slow cooling of a melt of composition NaAl 

 Si0 4 , obtained a completely crystalline mass with low refrac- 

 tive index and low birefringence which he refers to as soda- 

 nephelite. At the same time it should be noted that Wallace 

 observed in thin section a microline-like structure and since 

 this is a structure charcteristic of carnegieite there can be no 

 doubt that some carnegieite was present. 



On account of the long time necessary to obtain complete 

 inversion in the direction carnegieite-nephelite, it may be stated 

 as more than probable that much of the material prepared by 

 direct fusion and described as soda-nephelite in the literature 

 has contained some carnegieite. A rough similarity in optical 

 properties is the reason for the failure to differentiate the two. 

 The method of identification by immersion in refractive liquids 

 eliminates this possibility. 



