P. Eskola — Silicates of Strontium and Barium. 367 



celsian. No evidence was found of the existence of a 

 nephelite analog, a hexagonal form of BaAl 2 Si 2 O s 

 described by Ginsberg. 



The indices of refraction indicate a negative character. 

 They were determined as follows and are compared with 

 Strandmark's result on the natural celsian: 



a(D) /3(D) 7 (D) 



Artificial barium feldspar' 3 ...... 1.587±0.002 1.593±0.002 1.600±0.002 



Natural celsian 1.5835 1.5886 1.5941 



The differences in the indices of refraction are some- 

 what greater than the probable errors, but not more than 

 may be accounted for by the fact that the natural celsian 

 carries a few tenths of a per cent of alkalies and lime. 



Artificial strontium feldspar, Sr0.Al 2 O 3 .2SiO 2 . — The 

 strontium feldspar obtained from the vanadate flux 

 formed a crystalline mass showing radiating fibrous 

 development but no well-formed crystals. No twinning 

 could be discerned. Therefore nothing can be said about 

 its crystallographic characters. The indices of refrac- 

 tion, however, were readily determined and are stated 

 below, compared with those of anorthite. 



a(D) /3(D) 7 (D) 



SrO.Al,0 3 .2SiO, 1.574±0.002 1.582±0.002 1.586±0.002 



CaO.AlA.2SiO, ..1.576 1.584 1.588 



It was thus found that the strontium feldspar is, in its 

 optical properties, exactly like the calcium feldspar 

 within the limits of the possible errors. This is not very 

 surprising in itself, as we have found in some cases the 

 indices of refraction of the strontium compounds some- 

 what lower and in other cases somewhat higher than 

 those of the calcium compounds. 



A mixture of 50 weight per cent SrO.Al 2 3 .2Si0 2 and 

 50 weight per cent CaO.Al 2 3 .2Si0 2 was prepared and 

 heated at about 1500°. It formed clear homogeneous- 

 looking grains whose indices of refraction agreed with 

 those of anorthite. It did not melt at this temperature 

 and it is therefore probable that the strontium feldspar 

 forms a complete series of solid solutions with anorthite. 



33 a (F) =1.593; a (Tl) = 1.590; a (D) =1.587; a(G) =1.585 



