236 N. L. Bowen — The Ternary System: 



having the composition K. The whole then consists of 

 pyroxene O and forsterite (OP produced passes through the 

 composition of forsterite). 



In the actual system under investigation the temperature 

 interval between the points S and K is only 2°-3° and the 

 behavior above described cannot be experimentally realized, 

 and, therefore, has no practical importance in the present sys- 

 tem. The necessity of such a behavior is, however, important as 

 a general fact in systems of like character inasmuch as it illus- 

 trates the possibility of the precipitation, complete solution 

 and reappearance of a crystalline phase as the normal result of 

 cooling. 



Crystallization without Interaction between the Liquid and 

 the Solid Phases Already Separated. — The crystallization of 

 any mixture of a system such as the present may be considered 

 to take place in a manner somewhat different from that out- 

 lined for the case of perfect equilibrium. It may be con- 

 sidered that when a crystal has separated it is never redissolved 

 and that when a mix-crystal separates its composition does not 

 thereafter suffer any change. The crystals which are separat- 

 ing at any instant are at equilibrium with the liquid but those 

 that have already separated may not be. 



When crystallization takes place in this manner a liquid of 

 composition M, P or D of fig. 17 would behave as follows : 

 Forsterite would crystallize out first" and the composition of the 

 liquid would change to K. At the temperature of the point 

 K pyroxene of composition L would begin to separate. In the 

 case of complete equilibrium the re-solution of forsterite would 

 now begin and the liquid would change along the boundary 

 curve. In the case we are now considering, liquid and forster- 

 ite do not interact and the composition of the liquid crosses 

 the pyroxene field and meets the boundary curve pyroxene- 

 tridymite at a point lower than E, say GL* In the meantime 

 the composition of the pyroxene separating has changed from 

 L to T and there exist in the mixture, pyroxene crystals of all 

 compositions varying from L to T. When the temperature is 

 that of the point .G- and the composition of liquid is G, tridymite 

 begins to crystallize and the composition of the liquid changes 

 along the boundary curve pyroxene-tridymite. Meantime the 

 composition of the pyroxene separating changes from T towards 

 pure diopside and final crystallization takes place only when 

 the temperature is that of theeutectic diopside-tridymite, when 

 the remaining infinitesimal amount of liquid has the composi- 



*The exact curve which indicates the change in the composition of the 

 liquid is, unlike FE, such that the tangent to it at any point passes through 

 the composition of the pyroxene in equilibrium with liquid of the composi- 

 tion and at the temperature represented by the point. 



