CE 



S8 



Foote and Bradley — Solid Solution in Minerals. 31 



can see no reason why nephelite should not be treated in the 

 same class. This method of considering the composition of 

 nephelite has the advantage of being much more simple than 

 using a series of complicated formulae, and it appears to us to 

 agree with the facts. It need hardly be said that a chemical 

 formula could be assigned to any solution but a different one 

 would have to be used for each change in concentration of the 

 solution, just as Moroscewicz uses a different formula for each 

 nephelite. 



From what has been said, we think it fair to consider that 

 nephelite as it occurs in nature is not a pure compound but a 

 solid solution analogous to the solid solution of ferric chloride 

 in ammonium chloride. It then becomes of interest to con- 

 sider the probable formula of the pure compound which forms 

 the basis of nephelite. This appears to be the orthosilicate 

 NaAlSi0 4 . This formula is supported in two ways: (1) ISTephe- 

 lite has the same crystalline form as eucryptite LiAlSi0 4 and 

 kaliophilite KAlfSi0 4 which are in the same group, making it 

 very probable that the type of formula is the same in all three 

 cases. (2) Artificial nephelites have been prepared by Doel- 

 ter* which have the same general characteristics as natural 

 nephelite and vary in composition from the formula NaAlSi0 4 

 to compounds containing potash and an excess of silica corre- 

 sponding to the mineral. 



Perhaps the point should be emphasized that nothing what- 

 ever is known about the actual condition of the dissolved 

 silica, whether it is present as dissolved albite or silica or 

 leucite or in any other form, just as very little is known about 

 the condition of dissolved substances in liquids as to whether 

 they are combined with the solvent. It is certain, however, 

 that the dissolved silica does not have the properties of either 

 ordinary quartz or albite, since it is soluble in hydrochloric 

 acid. In the same way, the properties of a dissolved salt are 

 entirely different from the properties of the solid. 



The excess of silica which can be taken up by nephelite to 

 form a saturated solution can apparently be determined from 

 the data given by Morozewicz and ourselves. Where albite is 

 found intimately mixed with nephelite it is evident that the 

 nephelite must be saturated with silica and the excess of the 

 latter has formed albite. 



In this case, therefore, the nephelite should have a constant 

 ratio of silica to alumina, and these nephelites should contain 

 the maximum amount of silica that can be taken up. The 

 influence of temperature in determining the composition of 

 the saturated solution can apparently be neglected. In our 



*Zeitschr. f. Kryst., ix, 321, 1884. 



