P. Eskola — Silicates of Strontium and Barium. 337 



The refractive indices were determined in granular 

 crystals obtained by cooling a pure melt. They were 

 found to be : 



a y 



F 1.606 1.646 



Tl 1.602 1.641 



D 1.599 1.637 



C 1.596 1.634 



These values were repeatedly checked and are correct in 

 the third decimal place. Jaeger and Van Klooster give 

 less exact values : n x = 1.590 ± 0.003, and n, = 1.620 ± 

 0.003. 

 a-CaO.SiO 2 has 10 a(D) = 1.610; /3(D) = 1.611; 7 (D) =1.654. 



In the apparent uniaxial positive character and hex- 

 agonal tabular habit the strontium metasilicate is similar 

 to a-CaO.Si0 2 and, as stated later, they form a continuous 

 series of solid solutions. It would seem necessary, there- 

 fore, that they should be closely isomorphous. Now 

 Wright 11 has proved a-CaO.Si0 2 to be really monoclinic, 

 showing, in plates perpendicular to the optic normal, a 

 polysynthetic twinning along the basal plane. The optic 

 axial angle, though small, is not quite 0°. 



Much care was taken to find out whether the strontium 

 metasilicate shows a similar deviation from hexagonal 

 symmetry, but no such phenomena were noticed, although 

 apparent twinning was observed in the mix crystals of 

 SrO.SiOo and CaO.SiO, (cf. below). 



Euhedral crystals of SrO.Si0 2 were obtained from a 

 strontium chloride melt with an excess of silica (cf. 

 below). They are all thick tabular and apparently 

 dihexagonal pyramidal (hemimorphic), one end being 

 terminated only by the basal plane (fig. 2), sometimes 

 with additional narrow pyramid faces, while the other 

 end regularly shows well developed pyramids, usually 

 combinations of two, three, or four simple forms. The 

 pyramid faces are striated parallel to the edge with the 

 basal plane. The greater part of the crystals are 

 twinned on the base (fig. 3). 



As the crystals are perfectly hexagonal in aspect and 



10 G. A. Kankin and F. E. Wright: The ternary system CaO-ALO-SiO,, 

 this Journal, 39, 1, 1915. 



11 E. T. Allen, W. P. White, and F. E. Wright: On wollastonite and pseu- 

 dowollastonite, — polymorphic forms of calcium metasilicate, this Journal, 21, 

 89, 1906. 



