350 P. Eskola — Silicates of Strontium and Barium. 



Table III. 



Wt. % 2Ba0.3Si0 2 29 58 76.1 94.2 100 



Wt. % Ba0.2Si0 2 100 71 42 23.9 5.8 



o(F) 1.602 1.602 1.619 1.622 1.625 1.627 



a(Tl) 1.599 1.599 1.615 1.619 1.622 1.623 



a(D) 1.597 1.597 1.612 1.616 1.619 1.620 



o(C) 1.595 1.595 1.610 1.613 1.616 1.617 



/3(F) 1.617 1.618 1.621 1.627 1.631 1.632 



j8(Tl) 1.614 1.614 1.618 1.623 1.627 1.628 



/3(D) 1.612 1.612 1.615 1.620 1.624 1.625 



/3(C) 1.610 1.610 1.612 1.617 1.621 1.622 



7 (F) 1.632 1.644 1.645 1.647 1.652 1.652 



7 (T1) 1.625 1.639 1.641 1.642 1.647 1.648 



7 (D) 1.621 1.636 1.638 1.639 1.644 1.645 



7 (C) 1.618 1.633 1.635 1.636 1.640 1.641 



Density^ 3.73 3.80 3.93 



a The determinations of density in this series were made on only 0.5 g. of 

 substance and are therefore less accurate than in the other cases. 



It might seem probable, from the diagram, that a and 

 p should exchange roles at abont 65 per cent 2Ba0.3Si0 2 . 

 Drawn thus, the curves for p would be nearly straight 

 lines. I have not, however, drawn the curves across each 

 other because it was found that the twinning along a pris- 

 matic face occurs in the same relation to the three main 

 optical directions, in the two mixtures containing 58 and 

 76 per cent 2Ba0.3Si0 2 . This indicates that the indices 

 a and p have not exchanged roles. 



Dibarium trisilicate, 2Ba0.3Si0 2 - — This pure com- 

 pound was observed only in the form of a granular mass 

 and I did not see any crystalline forms. It does not seem 

 to have any disposition for prismatic or scaly develop- 

 ment, the grains being rounded and equant. The only 

 cleavage that appears perfect is that along Py while the 

 two other pinacoidal cleavages are poorly developed. 



The twinning and composition plane in the zone of a 

 and inclined 30° from p forms polysynthetic lamellae. In 

 many cases just a few such lamellae occur and they are so 

 narrow that they appear as what is often called parting 

 planes. 



The indices of refraction (cf. table 3) are as follows: 



o(D) =1.620; /?(D) =1.625; y(D) =1.645. 



The optical character is positive and the axial angle is 

 of about the same size as in diopside, or 2V = 58°. A 



