Foote and Bradley — Solid Solution in Minerals. 339 



Art. XXIX. — On Solid Solution in Minerals. V. The Iso- 

 morphism between Calcite and Dolomite ; by H. W. Foote 

 and W. M. Bradley. 



Solutions of liquids in liquids may, as is well known, con- 

 veniently be divided into two classes depending on whether 

 there is complete or incomplete miscibility in all proportions. 

 In the same way, it is convenient to distinguish two classes of 

 isomorphous mixtures or solid solutions. In one class, iso- 

 morphism is complete and solid solutions in any proportions 

 are possible, while in the other, each solid can take up but a 

 limited amount of the other. The alums form solid solutions 

 of the first class, and among minerals, anorthite and albite are 

 similarly capable of mixing in all proportions, forming the 

 plagioclase feldspars. A large number of salts form solid solu- 

 tions of the second class, each salt taking up only a limited 

 amount of the other, and among minerals, calcite and dolomite 

 belong in this class. In the present investigation, we have 

 endeavored to determine, at least approximately, to what 

 extent solid solution may take place between these two 

 minerals as they occur in nature. 



It will be well to consider first the possible effect of tem- 

 perature on the mixing limits. This influence has very com- 

 monly been disregarded in determining to what extent solid 

 solution may take place. For instance, Retgers* has deter- 

 mined the mixing limits of a large number of salts and has 

 considered the results as representing fixed values without 

 regard to temperature. Yan't Hoff,f however, pointed out 

 that the mixing limits of solid solutions, like the composition 

 of partially miscible liquids, should be a function of the tem- 

 perature and this has been demonstrated experimentally in a 

 number of cases. The determination of mixing limits might, 

 therefore, have little significance unless the temperature were 

 known. However, if the solubility of one solid in another is 

 slight, temperature will have but little influence on the abso- 

 lute amount dissolved and the mixing limit should be reason- 

 ably constant. This is the case, as will be seen below, with 

 calcite saturated with dolomite (or magnesium carbonate). 

 When a solid dissolves a considerable quantity of another, 

 temperature may affect the limit largely and in general a 

 greater absolute change would be expected than where the 

 solubility is slight. 



It is evident that for a given temperature each solid 

 will take up the maximum amount of the other when both are 



* A series of eleven articles, Zeitschr. phys. Chem., 1889-1895. 

 f Vorlesungen liber Chemie, I, 49. 



