440 Foote and Bradley — Chemical Composition of Nephelite. 



ent in solid solution, leaving the question entirely open as to 

 what form the excess takes. It might, for instance, be present 

 as hexagonal albite as Schaller suggests, just as it might be 

 present in some other form. Schaller, on the other hand, 

 suggests that the particular form which the excess of silica 

 takes is that of an hexagonal albite and Bo wen suggests albite 

 and leaves the system of crystallization undecided. 



Considering the fact that the greatest uncertainty still pre- 

 vails in regard to the actual molecular condition of dissolved 

 substances in other types of solution which have been much 

 more investigated, it seems impossible to decide definitely in 

 regard to what molecular aggregates are actually present in 

 nephelite. It appeared to us, when writing the first article 

 (p. 31), as it does now, that it is better to avoid any definite 

 assumption which cannot be proved, regarding the molecular 

 condition of the dissolved silica. Our main point was to show 

 there was an unusual case of solid solution and that it was 

 unnecessary to assume any definite molecular condition for the 

 excess of silica. While it may later be shown that albite 

 actually exists in solid solution, this at present must be regarded 

 as pure conjecture. The albite molecule, if present, would 

 have very peculiar properties, entirely unlike albite as it exists 

 either in pure condition or isomorphous with other feldspars, 

 since nephelite with its excess of silica is completely soluble 

 in N/4 hydrochloric acid. This evidence, however, we recog- 

 nize fully is not proof that the albite molecule is absent. It 

 cannot be proved either way at present. Hence our point, 

 that it is better to leave the question as to the molecular con- 

 dition of excess of silica entirely open. 



There is one point regarding the amount of silica which 

 nephelite can take up, which needs mention. Eowen states 

 (p. 53) : " It is therefore only in the presence of albite itself 

 that nephelites may be expected to be saturated with silica." 

 This is a conclusion which we also drew in our article, but later 

 Bowen states : " The conditions necessary for the saturation of 

 nephelite with albite are so unlikely to occur that it may 

 be safely said that natural nephelites are probably never 

 saturated." It was shown in our former article that three 

 nephelites, which were associated with albite, and one asso- 

 ciated with microcline-microperthite, exhibited a constant 

 maximum ratio for silica, and we suggested that this repre- 

 sented the saturation value. The objection could, perhaps, be 

 raised that the nephelite and albite were not formed simulta- 

 neously, so that the former was not necessarily saturated. 

 Probably no one will doubt the assertion that if a magma 

 deposits both nephelite and albite together, the former must 

 be saturated with silica at the temperature of solidification. 

 There is plenty of evidence that this process has taken place. 



