12 Morey & Bowen — Melting Potash Feldspar. 



librium. conditions in the following manner. At 1530° 

 ■(E) leucite begins to crystallize and it increases in amount 

 as the temperature falls, until at 1170° orthoclase begins 

 to separate and leucite to redissolve, or, stated better, 

 perhaps, the liquid reacts with the leucite, transforming 

 it into orthoclase. This continues at constant tempera- 

 ture until the liquid and leucite are entirely used up and 

 the mass consists entirely of orthoclase. 



A mixture lying on the leucite side of KAlSi 3 8 begins 

 to crystallize at a temperature higher than that of the 

 point E. In this case when the temperature 1170° is 

 reached the reaction between liquid and leucite proceeds 

 as before, but all the liquid is used up while still some 

 leucite remains and the completely crystalline mass con- 

 sists of orthoclase and leucite. 



A mixture on the silica side of orthoclase, if not richer 

 in silica than that represented by the point B (probably 

 corresponding to about 10 per cent free silica), also 

 begins to crystallize with separation of leucite but at a 

 temperature lower than that of the point E. At 1170° 

 the reaction is completed as a result of the disappearance 

 of leucite and the mass now consists of orthoclase and 

 liquid of composition B. The liquid then proceeds to 

 crystallize along the curve BC, with separation of ortho- 

 clase, until at C free silica separates as well and the whole 

 mass is now solidified as a mixture of orthoclase and 

 free Si0 2 . The exact temperatures and compositions are 

 in these parts quite unknown. Only in mixtures with a 

 greater excess of free silica than that represented by the 

 point B does the early separation of leucite fail. 



Such is the behavior of the various liquids when perfect 

 equilibrium obtains. Now in any of these mixtures 

 leucite might fail to react completely with the liquid at 

 the reaction point (1170°) as a result of the formation 

 of an armor of orthoclase about it. The consequence of 

 this would be that some liquid of the composition B would 

 be left over in all of the mixtures above discussed, even 

 that on the leucite side of orthoclase, and this liquid would 

 then form orthoclase and free silica. Or if leucite 

 crystals were locally segregated that part of the mass 

 from which they were removed would crystallize as ortho- 

 clase and free silica, even though the composition of the 

 original liquid were on the leucite side of orthoclase. 

 Conversely, too, even though the composition of the 

 original liquid were a moderate distance on the silica side 



