354 P. Eskola — Silicates of Strontium and Barium. 



The minimum melting temperature in this series was 

 found to be 1474 ± 3° at the composition 44 per cent 

 CaO.SiO ? -56 per cent SrO.Si0 2 . 



The mix crystals were found to be crystallographically 

 very like a-CaO.Si0 2 . Considerable care was taken to 

 find out whether they show that twinning on the base 

 which, in the case of pseudowollastonite, indicates mono- 

 clinic symmetry. 



In a-CaO.Si0 2 I found frequently poly synthetic twin- 

 ning along the basal plane, as described by Wright (loc. 

 cit.). In mix crystals with 12.5 per cent SrO.Si0 2 the 

 twinning was no less clear; in one case I measured an 

 extinction angle of 3°. In the mixture with 25 per cent 

 SrO.Si0 2 twinning was clearly observed, while, in 

 mixtures with 37.5 per cent, 56.25 per cent and 62.5 per 

 cent SrO the extinction angles were apparently so small 

 that the twinning lamellae, though occasionally positively 

 identified, always appeared extremely faint. In a 

 preparation with 75 per cent SrO.Si0 2 the occurrence of 

 faint twinning along the base was still clearly observed. 

 In the pure SrO.Si0 2 I did not succeed in finding definite 

 twinning appearing as oblique extinction, in spite of 

 much search. 



In summary, therefore, it may be stated that all the 

 crystals containing calcium and strontium metasilicates 

 in solid solutions are pseudohexagonal and really mono- 

 clinic. In the pure strontium metasilicate, although it 

 may belong to the same class of symmetry as the mix 

 crystals, no deviation from hexagonal symmetry was 

 observed (cf. p. 337). 



The indices of refraction for the solid solution series 

 were determined with the following results. 







Table V. 









Wt. % CaO.SiO, 



100 



75 



50 



43.75 



25 







Wt. % SrO.Si0 2 







25 



50 



56.25 



75 



100 



a(F) 



1.618 



1.617 



1.614 



1.612 



1.609 



1.606 



a(Tl) 



1.614 



1.612 



1.609 



1.608 



1.605 



1.602 



a(D) 



1.610 



1.608 



1.606 



1.6045 



1.602 



1.599 



«(0) 



1.607 



1.604 



1.602 



1.6015 



1.599 



1.596 



7(F) 



1.663 











1.646 



Y(T1) 



1.667 











1.641 



7 <D) 



1.654 



1.651 



1.646 



.... 



1.642 



1.637 



7(C) 



1.649 











1.634 



