52 JY. L. Boiven — The Composition of Nephelite. 



Foote and Bradley,* in a recent paper, express the opinion 

 that the excess silica of natural nephelites must be regarded as 

 present in solid solution. Their conclusion has the great 

 advantage of being stated in modern phraseology, but there 

 appears to be nothing in this view which is essentially differ- 

 ent from and, therefore, refutes the older view of the pres- 

 ence in variable amount of some compound of higher silica 

 content. 



Foote and Bradley do, however, point out certain facts 

 which suggest to the reader that the compound determining 

 the excess silica is the soda compound albite and not a potash 

 compound as the older views have insisted upon. One of 

 these facts is that the nephelites are ' saturated ' with silica 

 when found in contact with the polysilicate albite, but Foote 

 and Bradley themselves apparently consider that contact with 

 sanidine should bring about a like result. 



Still more recently Schaller,f seizing the truth in Foote and 

 Bradley's discussion, has made the explicit statement that 

 nephelites may be considered as composed of the three mole- 

 cules JSIaAlSi0 4 ,KAlSi0 4 and NaAlSi 3 8 . The random posi- 

 tion of the points on the diagram shows that the expression in 

 terms of three components is a step in the right direction. 

 Moreover the conclusion agrees with that indicated by experi- 

 mental results. 



Schaller supports his conclusion by testing it against actual 

 analyses and finds a satisfactory concordance. But it should 

 be pointed out that this test alone is not decisive. Had 

 Schaller tried KAlSi 3 8 , or KAlSi 2 6 , or Na 2 A] 2 Si 3 O 10 , or 

 NaAlSi 2 6 instead of NaAlSi 3 8 ,he would have found equally 

 good concordance. This fact becomes obvious by reference to 

 the diagram, since all the points representing compositions of 

 nephelite fall within the triangles having for their common 

 apices soda-nephelite and kaliophilite, and for the third apex 

 the compounds mentioned respectively. 



It so happens that all natural nephelites on whose analyses 

 we may rely are doubtful to this extent, and at first sight it 

 would appear that there is no reason for choosing any one as a 

 third component rather than any other. The synthetic nephe- 

 lite (A), however, leaves us to choose only between Na 2 Al 2 Si 3 O 10 , 

 ^aAlSi 2 6 and NaAlSi 3 8 , of which the most likely is, as we 

 have seen, albite. It is probable that there are natural nephe- 

 lites whose composition would lie above the soda-nephelite, 

 orthoclase line and for which albite would again be the only 

 choice.;): 



* This Journal, xxxi, 25, 1911. f Jour. Wash. Acad. Science, Sept. 19, 1911. 



:{: Nephelite from Denise, Haute Loire, France (anal. 7, Dana's System), 

 has a composition which would place it barely above the orthoclase, soda- 

 nephelite line. The low summation in this analysis makes it probable, 

 however, that no great significance can be attached to this fact. 



