346 P. Eskola — Silicates of Strontium and Barium. 

 In pure crystals obtained by cooling a melt of the compo- 



sition Ba0.2SiOo were found 



1.617 and a = 1.598. 



The melting point was determined by the heating curve 

 method at 1426 °C. Bowen suggests that the slightly 

 lower values of the indices of refraction of the crystals 

 in the optical glass may represent a real difference and 

 that when formed in the glass they take a small amount 

 of alkaline disilicates into solid solution. 







Fig. 



7, 





1460 



BaO-4 « a *3Si0 2+ liquid 



■ 



1440 



" and \ I^-t 

 Jiquid V/* 



\^~r^-^/ Mixer y st ais of 



1\>^\ SBaOaSiOi-BaOgSiOa 

 >\^ ^\ +li<^uid 











1450 









Tridymite / " 

 and liquidv/ " 



1400 









f\ - 





Ba0SlO £ + 



Mixcrystals of 



BaO-SSiOg+li^uid 





1380. 



2BaO-35)0 A 



SBaO-SSiO^-BaO^SiOg 





* N. / 







+ 



1360 







BaO 25io £ + 



t ridymite 



1 . 



67Wt%Ba0 65 

 SBVt^oSiOj 



6£.90 

 2830-3510^ 



60 



55.98 55 

 BaO-ZSiOg 



50 



46wt%BaO 

 54Wt°/oSi0 t 



Fig. 7. — Part of the binary system BaO-Si0 2 showing the melting of the 

 mix crystals of barium disilicate and dibarium trisilieate. 



I prepared crystals of Ba0.2Si0 2 from a BaCl 2 -melt, 

 and also obtained well-formed crystals embedded in 

 Ba-silicate glass from a mixture containing 50 per cent 

 BaO, held at 1385°. In both cases the crystals were 

 bounded only by the three orthorhombic pinacoids, were 

 very much elongated in the direction of « and had almost 

 square cross sections (fig. 8b and 8c). There are notable 

 cleavages in all the pinacoidal directions, but by far the 

 most perfect is the one parallel to a/?, so that when 

 crystalline masses of this substance, as for example, that 

 obtained by cooling of the pure melt, are ground, there 

 are formed almost exclusively scaly cleavage splinters 

 normal to y. 



