THE MINERAWGRAPHY OF THE FELDSPARS 



291 



With the above modification as to the nature of the melting of 

 potash feldspar, the diagrams so far offered represent the present 

 status of our knowledge. 



1800- 



1700- 



1600- 



1500- 



1400- 



1300 



1200 



1100 



1000 

 900 

 800 



Leucite 



and 

 Liquid 



Orthoclas 



or 



Microcline 



and 



Leucite 



On 



Mic 



Liquid 



SiOs 



and 



Liquid 



Orthoclase or Microcline 

 and SiOa 



— 1 1 1 \ 1 1 



10 20 30 40 50 60 70 80 90 



KAISisOe KAISiaOs 



SiOa 



Wt. Percent 



Fig. 3. — Thermal diagram of the system leucite-silica, showing incongruent melting 

 of the intercomponent compound KAlSiaOg (orthoclase-microcline), after Morey and 

 Bowen. , Amer. Jour. Sci. (5), IV (1922), 1-22. 



DIMORPHISM 



Orthoclase vs. microcline. — The writer, in Part I, agreed with 

 Vogt,^ Barbier,^ Clarke,^ and Harker"* that orthoclase and microcline 

 are dimorphous forms of the same substance. To secure evidence 



' J. H. L. Vogt, as cited by C. H. Warren, Proc. Amer. Acad. Arts and Sci., LI 

 (No. 3, 1915), 144. 



^ Ph. Barbier, "Researches sur la composition chimique de feldspaths potassique," 

 Bull. Soc. Franq. mineral., XXXI (1908), 152-67. 



3 F. W. Clarke, U.S. Geol. Surv. Bull. 588, 12. 4 op. ciL, p. 258. 



