P. Eskola — Silicates of Strontium and Barium. 375 



minimum. The indices of refraction show a continuous 

 though not linear variation with the composition. 



The strontium and calcium metasilicates form a contin- 

 uous series of solid solutions with a minimum in the melt- 

 ing curve. The indices of refraction and the densities of 

 the mix crystals show continuous variation with the 

 composition. 



The barium metasilicate is not isomorphous with the 

 calcium and strontium metasilicates, being optically 

 biaxial, with low birefringence and probably of ortho- 

 rhombic symmetry. It forms no solid solutions with 

 a-CaO.Si0 2 . Instead, there occurs a double compound, 

 2CaO.Ba0.3Si0 2 which melts incongruently, breaking up 

 into a-CaO.SiO and liquid. This compound is optically 

 uniaxial and negative, unlike all the other metasilicates. 

 In its refringence it is similar to BaO.Si0 2 . 



Xeither strontium nor barium metasilicate forms, with 

 magnesium metasilicate, a double compound analogous to 

 diopside, CaO.Mg0.2Si0 2 . This is considered as a 

 special case of the more common rule which seems to 

 obtain generally in the compounds of the alkaline earth 

 elements, namely, that calcium, in those compounds in 

 which it can be replaced by magnesia and ferrous oxide, 

 cannot be replaced by strontia or baryta, while the iso- 

 morphous series including strontium or barium com- 

 pounds may have isomorphous and miscible analogs 

 among sodium, potassium, or lead compounds. 



Both strontium and barium form feldspars, analogous 

 to anorthite. The strontium feldspar has indices of 

 refraction exactly like those of anorthite, and the two 

 seem to be completely miscible. The barium feldspar is 

 monoclinic, like the natural celsian. It forms commonly 

 Carlsbad twins of the contact type and the minute twin 

 crystals therefore appear as if orthorhombic. 



During this work I had pecuniary assistance, beside 

 that from the Geophysical Laboratory, from two funds 

 for the advancement of scientific research in my native 

 country, Finland, namely, Alfred Kordelin's General 

 Trust for the Advancement of Progress and Knowledge, 

 and Herman Rosenberg's Traveling Bursaries Trust of 

 the University of Helsingfors. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, 

 April. 1922. 



