26 Foote and Bradley — Solid Solution in Minerals. 



the mineral comparable to the salt solution. If such an 

 impurity were present in appreciable amount, it is obvious that 

 the formula of the pure compound could not be calculated 

 correctly from the analysis. This type of solid solution must be 

 clearly distinguished from isomorphous replacement, which is 

 also commonly considered as solid solution. In the latter case, 

 the formula of the compound can be derived directly from 

 the analysis, as previously mentioned. 



Before considering the application of these statements to 

 nephelite, we wish to mention a simple case of solid solution 

 which is known in artificial crystals. It has been shown by 

 Roozeboom* that when ammonium chloride crystallizes from a 

 solution containing ferric chloride, the crystals deposited are 

 colored and may contain as much as seven per cent of ferric 

 chloride. Here there can be no cpiestion of isomorphous 

 replacement, and on the other hand the ferric chloride is not 

 mechanically enclosed by the ammonium chloride. The latter 

 point is proved partly by the fact that the crystals appear 

 perfectly homogeneous, and it is proved much more definitely 

 by the fact that the solubility of such crystals varies with their 

 composition. If a mechanical mixture were present, the solu- 

 bility would not vary with the composition of the mixed crystals, 

 but there would be a definite solubility at a given tempera- 

 ture independent of the composition. The colored crystals 

 are to be regarded as one homogeneous phase in which 

 the ferric chloride is held in solid solution by the ammonium 

 chloride. Similar occurrences have been noted in artificial 

 minerals with a good deal of probability. Day and Shepherdf 

 have observed an artificial calcium metasilicate crystallizing 

 with tridymite which differs slightly in optical properties from 

 the pure silicate. The variation appears to be due to the 

 presence of silica taken up in solid solution by the metasilicate. 

 The same metasilicate is also capable apparently of absorbing 

 a considerable amount of the orthosilicate and still appear 

 homogeneous. Again, Shepherd and Ranking have shown 

 that artificial corundum may take up a limited amount of 

 sillimanite (or silica) in solid solution and also a small quantity 

 of calcium oxide. We believe such cases also exist in certain 

 minerals such as nephelite. 



Several years ago, the late Prof. S. L. Penfiekl suggested to 

 one of us (Bradley) that the reason for the variation in the 

 composition of nephelite might be due to the presence of 

 mechanical impurities and that if material of undoubted purity 



* Zeitschr. f. phys. Chem., x, 145, 1892. 

 + This Journal (4), xxii. 265, 1906. 

 {This Journal (4), xxviii, 293, 1909. 



