THE MINERALOGRAPHY OF THE FELDSPARS 



361 



triangular base) is controlled by the conditions prevailing during 

 crystallization. These Knes are farther away from the sides of the 

 diagram when excessive undercooling obtains, in fact they may come 

 together near the center as in certain anorthoclases, producing a 



KAISi308 s*' 



Triclinic'Series 



NaAlSiaOs «, y> ^^ «P *,^ ^^ ^^<> ^ ^Mf^ CaM^SisOs 



KAISiaOa 4? 



^sfv^v^ 



"Monoclinic" Series 



NaAISiaOs ^o 



CaAlaSiaOs 



Fig. 5. — Plot of /3 values of indices of refraction of the potash-soda-lime feldspars. 

 These diagrams are to be regarded as tentative and subject to revision. 



series of solid solutions with a minimum, or may be close to the sides 

 when equiHbrium and exsolution are unhampered. Rapidly cooled 

 albites, andesines, and oligoclases may thus contain a higher per- 

 centage of the potash component than the more slowly cooled 

 equivalents. As the viscosity of such melts decreases with increased 



