Morey & Bo wen — Melting Potash Feldspar. 3 



with a refractive index only a little higher than that of 

 the glass in which they are embedded, which is about 1.49, 

 with regularly arranged inclusions of glass ; in fact, with 

 all the characters of leucite crystals. The series of forms 

 assumed by the leucite with increasing freedom of growth 

 (increasing temperature) is shown in fig. L This series 

 is strikingly similar to that observed by Pirsson in the 

 small leucite crystals in the groundmass of rocks from 

 the Bearpaw Mountains. 4 The more primitive skeletal 

 forms are not shown in Pirsson 7 s series. 



Fig. 1. 



o.o/mm. 



Fig. 1. — Grains of glass containing leucite. Showing increasing perfec- 

 tion of form, with increasing temperature, of leucite crystals as grown in a 

 mixture of composition KAlSi 3 O s . 



The rounded leucite crystals, obtained at higher temper- 

 atures, may have a diameter as great as 0.025 mm. and 

 are surrounded by a rim of strained glass which is 

 observed as a birefracting halo under crossed nicols. In 

 fact the birefringence shown by the glass immediately 

 adjacent to the crystals is decidedly greater than that of 

 the crystals themselves. The birefringence of the glass 

 fades out as the distance from a crystal increases, and 

 when the crystals are not numerous there are areas of 

 ordinary isotropic glass. When the crystals are closely 

 spaced, however, all of the interstitial glass is birefrac- 

 ting, which accounts for the fact that, when the crystals 

 grow as ramifying skeletal forms, the whole mass is bire- 

 fracting, with the cross-grating effect analogous to that 

 in microcline. 



In the birefracting halo about the crystals y is radially 

 disposed, which shows that the glass is subjected to radial 

 tension. _ This tension is no doubt caused by the abrupt 

 contraction of the leucite crystals in cooling through their 



4 L. V. Pirsson, this Journal, 2, 145-146, 1896. 

 in Bock Minerals, p. 249 (2d ed., 1911). 



Figured also by Iddings 



