290 HAROLD L. ALLING 



solution diagram, having the minimum at a point about 64 per cent 

 albite. The writer believes that Makinen's diagram roughly repre- 

 sents the conditions prevailing during the crystallization of plutonic 

 feldspars. Douglas^ has suggested that the eutectic point should 

 be placed at 40 per cent orthoclase, 60 per cent albite. Such a dia- 

 gram, all other matters being the same, may be, therefore, inserted 

 in the three-dimensional model between that of Vogt and of Maki- 

 nen. It is hoped that in spite of the many uncertainties concerning 

 the thermal diagram of the potash-soda series, the suggestions 

 offered in Figure 2 will be helpful in securing a proper conception 

 of the mode of crystallization of all feldspars of this range of compo- 

 sition. 



THERMAL STUDIES 



Audley,^ from commercial experience, says: 



The destruction by heat of the crystalline condition of a silicate does not 

 always result in the immediate production of a homogeneous and transparent 

 substance — that is a glass. (Potash) feldspar, for example, between 1100° and 

 1200° C. gives a white material resembling porcelain or devitrified glass in 

 appearance. The feldspar has really become decomposed. 



Morey and Bowen^ have investigated this phenomenon in the 

 Geophysical Laboratory and have shown that potash feldspar, either 

 orthoclase or microchne, has no true melting point, that is, it melts 

 incongruently at about 1170° C, for pure (artificial) potash feldspar, 

 breaking up into liquid and leucite. This temperature is lowered a 

 little with increasing amounts of other feldspar components. This 

 necessitates the use of a binary diagram to show the thermal prop- 

 erties of orthoclase (or microcline) which is a binary compound of 

 leucite and silica (Fig. 3, Part II). 



KAISi.06+ SiOai^KAISijOg 



Leucite+ Silica ^ K-f eldspar 



^ J. A. Douglas, "On Changes of Physical Constants in Minerals [by heating]," 

 Qiiar. Jour. Geol. Soc, LXIII (1907), 159. 



2 J. A. Audley, "Sihca and the Sihcates," Van Nostrand, 1921, p. 46. 



3 G. W. Morey and N. L. Bowen, "The Melting of Potash Feldspar," Amer. 

 Jour. Sci. is), IV {1922), 1-22. 



