94 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



normal to the optical bisectrix y, and the plane of the optic axes is 

 almost parallel to the direction of elongation of the blade. 



(67 a-b ). This preparation consists of spherulites about 2 mm. in 

 diameter, and some interstitial glass. The spherulites are irregular 

 in outline, due to mutual interference (Plate XVI). They consist of 

 radiating sectors made up of rather distinct prisms, which start at 

 the center of the spherulite as stout prisms and become innumerable, 

 slightly diverging prisms which terminate at different lengths as 

 distinct prisms terminated by a pinacoid almost at right angles to the 

 axis of the prism, probably (201). In other cases they are terminated 

 by two pinacoids making an acute angle. In places the terminations 

 are somewhat rounded. The crystals in these spherulites are homo- 

 genous and correspond to the composition of the mixture. 



OUGOCXASE (Ab 4 ANi). 



(29). The preparation was heated to a temperature of about 

 1400 and allowed to cool for 15 hours to about 425 . The resulting 

 solid is a glass with abundant crystals of feldspar in crossed bun- 

 dles of blades or plates, and spherulite-like radiating aggrega- 

 tions (Plate XVII). There are numerous rounded and subangular 

 grains of colorless quartz in the glass which have no definite relation 

 to the feldspar crystals, and appear to be undissolved fragments of 

 quartz used in compounding the preparation. There are also small 

 lumps of white aggregates, probably undissolved powder, which in 

 many cases form centers of spherulitic crystallization, that is, they 

 become points at which feldspar crystallization began. 



The colorless glass has an index of refraction noticeably lower than 

 that of Canada balsam, 1.5393.* The length of the feldspar bundles 

 and diameter of the spherulitic aggregates is about 0.2 mm., the width 

 of the bundles about 0.0 1 mm. The narrow cross-sections of the 

 bundles of feldspar, which has a higher refraction than the glass, 

 exhibit low extinction angles, nearly parallel to the longitudinal 

 direction. The bundles are clearly composed of numerous parallel 

 or approximately parallel individuals, which spread out fan-like or 

 plumose at the extremities. In other positions these aggregates are 

 seen to be relatively broad, blade-like, or tabular, and made up of 

 subparallel plates of extreme thinness. This is shown by numerous 

 crystal edges almost parallel to one another, and also by the com- 

 posite character of the interference phenomena with crossed nicols, 

 the crystal blade being mottled instead of uniformly dark in the 



* x -5393 is the index of refraction of the balsam used by the U. S. Geological 

 Survey, as determined by Dr. J. E. Wolff, in 1896. 



