THE MINERALOGRAPHY OF THE FELDSPARS 



5 

 KALSiaOs 1 



289 



Adularious 



NaAISisOs 



Adularious 



Drusy 



}Pegmatitic 



Plutonic 



Porphyritic 



Adularious 



Dittler 



Porphyritic KAISiaOs 



Fig. 2. — A. Three-dimensional compositional diagram showing fields occupied by 

 natural feldspars of different origins. It indicates that porphyritic feldspars have a 

 greater range in composition than those of any other origin and may even bridge the 

 eutectic gap as sanidine and anorthoclase. It shows why sanidines may be rich in 

 soda while adularias more closely approach the theoretical composition, KAlSi^Og. 

 Arranged from Makinen's diagrams. 



B. Projection of an arranged series of thermal diagrams of the potash-soda feld- 

 spars, showing possible variation in the position of the eutectic point for feldspars of 

 different origins. This diagram is offered with full appreciation that it is based upon 

 uncertain data. 



C Stereogram of arranged thermal diagrams of the potash-soda series showing 

 that no one diagram can express the crystallization of all feldspars. From these dia- 

 grams the reader is to see a solid model; any transverse cross section of which is a 

 diagram of feldspars of a particular origin. 



