370 P. Eskola — Silicates of Strontium and Barium. 



■ Mol.vol. ( a +p+ y )/3 



Carbonates : Aragonite 34.01 1.632 



Strontianite 39.87 1.615 



Witherite 45.82 1.627 



Sulphates : Anhydrite 45.99 1.586 



Celestite 46.18 1.626 



Barite 51.96 1.641 



Metasilicates : £-CaO.Si0 2 40.09 36 1.625 



«-CaO.Si0 2 40.11 36 1.625 



SrO.Si0 2 44.91 1.612 



BaO.Si0 2 48.57 1.675 



The carbonate series exemplifies the rule found to hold 

 good in many other instances that the molecular volumes, 

 in an isomorphous series, increase regularly with the 

 atomic weights of the substituted elements. Compounds 

 which are not isomorphous often show a discrepancy in 

 their molecular volumes, as for example anhydrite in its 

 relation to celestite and barite. 



The metasilicate series, in the regular increase of the 

 molecular volumes with the atomic weight, behaves more 

 like an isomorphous series, although we know that barium 

 metasilicate is not isomorphous with the others. Our 

 knowledge of the solid solubility relations of the carbo- 

 nate and sulphate series is too incomplete to allow any 

 closer comparison. 



In the ref ringence one can hardly see any regular rela- 

 tion. The barium compounds have mostly -the highest 

 indices of refraction, but the carbonates make an excep- 

 tion, aragonite having higher indices than witherite. 

 Among the strontium compounds some have higher and 

 others lower indices than the corresponding calcium 

 compounds, without any apparent regularity. 



Schaef er 37 carried out a thermal study of the binary 

 systems CaCl 2 -SrCl 2 and CaCL-BaCL. He found, in the 

 former case, a complete solid solubility with a minimum 

 in the melting curve, while the latter do not mix at all, but 



36 These values have been obtained by computing the specific gravities at 

 25°, compared with water at 25°, as found by Allen and White (this Journal, 

 21, 103, 1906), for a -CaO.Si0 2 2.912 and /3-CaO.Si0 2 2.214, in terms of 

 density, i. e. comparing with water at 4° and making the correction for the 

 buoyancy of air. Thus, for a-CaO.SiOo, d = 2.901 and, for /3-CaO.SiO, 

 d = 2.903. 



37 Walter Schaef er, Neues Jahrb. Min. Geol., 1, 15, 1914. 



