266 



BOWEN : BARIUM DISILICATE IN GLASS 



marked symmetry of outline and their common arrangement with 

 greater dimensions parallel to flow lines in the glass, constitutes a 

 specimen of much beauty and perhaps of some interest to the 

 mineralogist, though nothing could be more ruinous to the glass 

 for optical purposes than this incipient crystallization. Even 

 glass which comes from the melting furnace free from this defect . 

 may devitrify during subsequent heat treatment with formation 

 of crystals of the same nature, though in this case of much smaller 

 dimensions. One step in an investigation designed to discover 

 the best conditions for avoiding the formation of these crystals 

 involved a determination of their nature. 



Under the microscope the larger crystal 

 plates that form in the melting furnace are 

 found to be about 0.03 mm. thick in the trans- 

 parent central portion, which is a single crys- 

 talline unit of uniform orientation. Ar6und 

 the edges of the larger crystal, however, in- 

 numerable tiny crystals, each identical in 

 nature and in habit with the larger crystal, 

 have sprouted out in all directions. These 

 tiny crystals with their interstitial glass, 

 giving diffusion of light, constitute the white 

 opaque rims of the larger crystals. The 

 arrangement recalls the feldspar microlites 

 with fibrous edges sometimes seen in rocks.' 

 The crystal plates have the shape of an elongated hexagon as 

 shown in figure 2, and are about 3 mm. long and 2 mm. wide. 

 The terminal angles as measured under the microscope are 

 approximately 100° and the lateral angles 130°. As shown by 

 their symmetry, taken together with their optical properties, the 

 crystals are orthorhombic. There is a good cleavage parallel to 

 the elongation. The elongation is always negative. The plane 

 of the optic axes is parallel to the platy development and the 

 optical character negative with 2V = 70° approximately. The 

 refractive indices are: y = 1.613 and a = 1.595, as measured in 

 immersion liquids under the microscope. 



1 Figured by J. P. Iddings in Rock Minerals, p. 215, 1911. 



Fig. 2. BaSi205 show- 

 ing optical orien- 

 tation. 



