Morey & Bo wen — Melting Potash Feldspar. 11 



system, but it presents many difficulties. On the leucite 

 side of orthoclase the temperatures are very high, on 

 the silica side the mixtures react very sluggishly and 

 require excessively long heating. In the meantime we 

 have thought it well to place on record our results on 

 orthoclase. 



The point E (fig. 3) is accurately fixed as is also the 

 temperature of the line AB. The composition of the 

 liquid B, which is in equilibrium with both orthoclase and 

 leucite, we have not yet fixed accurately. The proportion 

 of leucite in a mixture of composition KAlSi 3 8 , held at 

 about 1250°, is very difficult to estimate, but appears to 

 be about 20 per cent. We have placed the point B in 

 accordance with this estimate. The melting point of 

 leucite we know to be very high, above that of platinum 

 (1755 c ). s The melting point of Si0 2 (cristobalite) has 

 been fixed at 1710°. 9 Of the temperature and composi- 

 tion of the indicated eutectic between orthoclase and Si0 2 

 we know nothing. Except at the composition KAlSi 3 O s 

 we wish to stress only the general form of the diagram, 

 not its details. That it must assume this general form 

 cannot be doubted. 



Petrogenic Significance of the Incongruent Melting of 

 Orthoclase; by N. L. Bowen. 



The discovery of the fact that orthoclase melts with 

 decomposition into leucite and liquid is of great signifi- 

 cance to petrogenic theory. It is the first definite exper- 

 imental demonstration of a genetic relation between a 

 mass consisting of feldspar and feldspathoid and a mass 

 consisting of feldspar and free silica, in other words, 

 between what may be termed an alkaline and a sub-alka- 

 line mass. Moreover, it shows plainly the nature of this 

 relation as a fractional crystallization phenomenon. 



Before going into this matter more specifically it is 

 desirable to discuss the course of crystallization in mix- 

 tures of various compositions shown in fig. 3. 



Reference to that diagram shows that crystallization of 

 a liquid of composition KAlSi 3 8 takes place under equi- 



s Leucite crystals were prepared by one of us some years ago when investi- 

 gating kaliophilite. To melt the leucite it was found necessary to use an 

 iridium container (N. L. Bowen, this Journal, 43, 117. 1917). 



9 Ferguson and Merwin, this Journal, 46, 424. 1918. 



