P. EsTcola — Silicates of Strontium and Barium. 341 



Strontium ortho silicate, 2SrO.Si0 2 . — The melting 

 point of 2SrO.SiQ 2 is far above the range of the platinum 

 resistance furnace. 



In a search for several polymorphic forms of this com- 

 pound I heated a preparation of crystalline 2SrO.Si0 2 at 

 different temperatures: 5 minutes at 1634°, 2 hours at 

 1415°, 1300°, and 990° respectively. The result was 

 always the same form, well characterized by its refrin- 

 o-ence and twinning. It was also obtained from a SrCl 2 

 melt at about 1000° (cf. above, p. 339). Nor did the 

 substance change when allowed to cool very slowly. The 

 phenomenon of "dusting" in the case of 2CaO.Si0 2 

 apparently has no analog here. 



For the determination of the optical properties the 

 preparation quenched from 1634° was used, with the 



ring results. 











a 



P 



y 



F ... 



. 1.740 



1.744 



1.766 



Tl .. 



. 1.7325 



1.737 



1.760 



D .. 



. 1.7275 



1.732 



1.756 



C ... 



. 1.722 



1.727 



1.752 



The crystals are optically positive with 2E = 58° 

 approximately, or 2 V = 32° 30'. 



In many of the crystals a twinning was observed, often 

 repeated (fig. 5). In sections normal to y, the acute 

 bisectrix, the trace of the axial plane forms, with the 

 composition planes of the twinning lamellae, angles of 17°. 

 Thus, if the crystal system is monoclinic, the plane of the 

 optic axes is normal to the plane of symmetry. 



The repeatedly twinned crystals, in their outlines, 

 often display the habit of orthorhombic crystals, and 

 when the lamellae are very narrow they have apparent 

 straight extinction. 



14 Free strontium oxide attacks platinum on heating. Every mixture does 

 so as soon as it contains an excess, however small, of SrO over 2SrO.Si0 2 , 

 while the more acid mixtures may be heated in platinum crucibles without 

 any danger. The phenomenon appears as a blackening of the surface of 

 the platinum and of the charge. This black substance is soluble in HC1, 

 forming chloroplatinic acid. 



A probable explanation is that the strontium oxide dissociates at high 

 temperatures, either into metallic strontium or into a strontium suboxide 

 which forms an alloy with platinum. This may be connected with the face 

 that the strontium oxide is somewhat volatile. 



Both the dissociation and the volatilization are, however, very slight at the 

 temperatures of the platinum resistance furnace. 0.300 g. SrO, wrapped 

 in platinum foil and held 4 hours at 1580°, lost only 0.0017 g. in weight. 



