30 Foote and Bradley — Solid Solution in Minerals. 



Table IV. 



Summary of Katios from Analyses by Bradley and Morozewicz. 



No. Si0 2 A1 2 3 etc. Na 2 etc. 



I. 2-23 1-00 0-98 



II. 2-21 " 0-99 



III. 2-21 " 0-99 



IV. 2-21 " 1-00 

 V. 2-12 " 1-00 



VI. 2-11 " 1-02 



VII. 2-15 " 1-03 



In this table the ratio of Na 2 : A1 2 3 is as nearly 1 : 1 as 

 could be desired. There can be no question that soda and 

 alumina are present in this proportion. The ratio for silica 

 varies from 2 - ll to 2*23, and this variation is greater than can 

 be accounted for either from errors of analysis, or from the 

 presence of impurities. For instance, analysis No. I contains 

 more than two per cent excess of silica if the ratios were to be 

 the same as in ISTo. VI. There is no case kuown, we believe, 

 where silica can be considered as replacing isoinorphously 

 either alumina or soda, and if it did in this case, the ratio 

 between these two would not be simple. The same general 

 conclusion as regards composition may be drawn from the 

 ratios derived from older analyses given in Table III, though 

 many of the analyses are probably not as good as the more 

 recent ones. 



Morozewicz* has shown that the nephelites may be con- 

 sidered as consisting of two series of compounds, a normal 

 series and a basic one. The normal series should be repre- 

 sented by the formula K Q Na n Al n+;i Si n+ 3O4 n+10 , in which %=8, 9, 

 10, and 11, and the basic series by the formula K 4 Na ]8 Al. 23 O 90 . 

 By this series of variable formulae, the variation in composition 

 can be expressed. This method of representing the com- 

 position is open to the serious objection that a chemical com- 

 pound, so far as we know, does not vary in type. Isomorphous 

 replacement, for instance, varies the composition, but the type 

 of compound remains the same. 



If nephelite be considered a solid solution, the case becomes 

 very different. A solution may be defined as a homogeneous 

 mixture of substances which cannot be separated by mechani- 

 cal means and whose composition varies continuously within 

 certain limits. This definition distinguishes a solution from a 

 suspension on the one hand and from a chemical compound on 

 the other. It characterizes a solution of a salt in water, and a 

 solid solution of ferric chloride in ammonium chloride and we 



*Loc. cit. 



