348 P. Eskola — Silicates of Strontium and Barium. 



Solid solutions of barium disilicate, Ba0.2Si0 2 and 

 dibarium trisilicate, 2Ba0.3Si0 2 . — My first observations 

 pointing' to the existence of solid solutions in the system 

 Ba0-Si0 2 were made with the mixture containing 60 per 

 cent BaO. It was found to give only one kind of crystals 

 very similar but not identical with those of Ba0.2Si0 2 . 

 Furthermore I found that the crystals, when formed in 

 a solid state (below the solidus) had lower refractive 

 indices than they had when formed in the presence of a 

 liquid phase (above the solidus). These observations 

 made it probable that there exists a solid solution series 

 without a maximum or minimum. In the course of the 

 work this proved to be the case, the other end member 

 of the series being the compound 2Ba0.3Si0 2 . 



The gradual change in the physical properties will 

 appear from the following description. 



Mixture 29 per cent 2Ba0.3Si0 2 -71 per cent Ba0.2Si0 2 

 gives crystals which agree fairly well with the pure 

 disilicate. Among the three pinacoidal cleavages the one 

 parallel to a/3 is most perfect. The optical character, 

 however, is distinctly positive, with 2V = 70° approxi- 

 mately. 



Density = 3.80. 



The crystals of the composition 58 per cent 2BaO. 

 3Si0 2 -42 per cent Ba0.2Si0 2 show a different habit, 

 being usually somewhat elongated along p. The cleavage 

 parallel to Py is most perfect. Cleavage along a/? also is 

 good and that parallel to ay distinct. Many crystals are 

 composition twins along a prismatic face in the zone of a. 

 The angle between p and the twinning plane is about 30°, 

 or between p in both individuals about 60°. The optical 

 character is positive and the axial angle quite small. I 

 determined 2E = 44°, or 2V = 27°. 



Crystals of the composition 76.1 2Ba0.3Si0 2 -33.9 

 Ba0.2Si0 2 are equant grains, not elongated. Cleavage 

 along py is best, next to it that along a/3, while that 

 parallel to ay was hardly observed at all, The same pris- 

 matic twinning with the twinning plane parallel to a and 

 inclined 30° to p was observed frequently, and is some- 

 times poly synthetic. The axial character is positive and 

 the axial angle larger than in the former. I measured 

 2E = 56° or2V = 33°30'. 



The mix crystals containing 94.2 per cent 2Ba0.3Si0 2 

 are in all their characters closely similar to the former 

 ones. 



