P. Eskola — Silicates of Strontium and Barium. 351 



calculation from the indices of refraction gives 2V = 

 53° 30'. The density was found to be 3.93. 



In summary, it appears that the two silicates BaO. 

 2Si0 2 and 2Ba0.3Si0 2 are very perfectly isomorphous, 

 have orthorhombic symmetry and pinacoidal cleavages. 

 Their relations bear some resemblance to the plagioclase 

 feldspar series. The melting diagrams of the solid solu- 

 tion series are quite similar, cleavage variations are 

 similar. The variation of the refractive indices and 

 optic axial angles is irregular in the plagioclases, though 

 in a smaller degree than we have found here. 



Very rarely do two compounds of the same element so 

 different in chemical constitution form so perfect an 

 isomorphous series. 



Eutectic 2Ba0.3Si0 2 -BaO.SiO ? .—The mixture of 60 

 per cent BaO and 40 per cent silica was found to have 

 very exactly the eutectic composition between the barium 

 metasilicate and the dibarium trisilicate, as appears from 

 the fact that a melt of this composition gives the two 

 crystalline phases simultaneously. 



Barium metasilicate, BaO.Si0 2 . — In the BaO-Si0 2 

 mixtures in wihch BaO.Si0 2 is a primary phase, this 

 compound was found in the form of rounded globules or 

 short rods with rounded ends. The latter show a nega- 

 tive elongation and parallel extinction. In glasses 

 composed of mixtures of barium and calcium silicates 

 (in the system CaO.Si0 2 -BaO.Si0 2 ) it was frequently 

 observed as small crystallites looking like short needles 

 sharpened on both ends. These always show a parallel 

 extinction and negative elongation. There is a well 

 developed cleavage parallel to a/3. Grains placed parallel 

 to this plane show the axial figures, with 2E = 50° 

 approximately, or 2V = 29°. Dispersion of the optic 

 axes is strong: p > v. Birefringence is very weak. 

 Grains parallel to /3y y showing the trace of the good 

 cleavage along af3, display abnormal blue interference 

 colors. The indices of refraction were determined as 

 follows : 





a 



P 



7 



F ... 



.. 1.682 



1.684 



1.688 



Tl .. 



.. 1.677 



1.678 



1.682 



D .. 



.. 1.673 



1.674 



1.678 



C ... 



.. 1.669 



1.670 



1.673 



