180 _ZV. L. Bowen — Crystallization of Haplobasaltic, 



position of the plagioclase is now T (Ab ]5 An 8 .), NT being a 

 three-phase-boundary. As the temperature is lowered, both 

 plagioclase and diopside crystallize until at 1237° all the liquid 

 is used up. The composition of the final liquid is O and the 

 feldspar has attained the composition L (Ab ]8 An g „.) 



It should be noted that the crystallization curves DPM and 

 ERN apply to the liquids D and E respectively and to no 

 other liquids. Thus the crystallization curve of the liquid P 

 is not the curve PM but the new curve PL, i. e., if we start 

 with a liquid P free from crystals the composition of the 

 liquid follows the course PL. Only when the liquid P eon- 

 tains in it the crystals formed during the change from D to P 

 does the further course of the liquid coincide with PM. More- 

 over, the liquid P when originally free from crystals becomes 

 on cooling completely crystalline, not at 1200° (H), as before, 

 but at a somewhat lower temperature. 



Crystallization with Zoning. 



Throughout the foregoing discussion of crystallization per- 

 fect equilibrium is assumed. The conditions are supposed to 

 be such that crystals of plagioclase can change their composi- 

 tion through and through in response to the demands of equi- 

 librium. It may be considered, however, that crystallization 

 takes place in a quite different manner. When plagioclase of 

 a certain composition has separated it may remain as such and 

 become surrounded by layers of different composition deposited 

 by the continually changing liquid. The liquid is in equi- 

 librium at any instant only .with the material crystallizing at 

 that instant and not with crystals already formed.* A plagio- 

 clase crystal, once separated, does not participate further in the 

 equilibria. As far as any effect on the course followed by the 

 liquid is concerned the crystal may be considered absent. The 

 course of crystallization of the liquid P, fig. 13, in the absence 

 of crystals has been compared in the foregoing with that fol- 

 lowed in the presence of crystals. If we examine also the 

 liquid M, say, we find that if it crystallizes in the presence of 

 crystals formed during the change in composition of the liquid 

 from D to M, it then becomes completely crystalline at 1200° 

 and the final liquid has the composition EL On the other 

 hand, if the crystals referred to are separated from the liquid 

 M complete crystallization does not take place until the tem- 

 perature 1170° is attained and the final liquid has the composi- 

 tion X, i. e., is very much richer in albite. If in this latter 

 case a second removal of crystals took place when the liquid 



* This corresponds with " Erstarrung erster Art'' of Schreinemakers, Zs. 

 phys. Chemie, 1, p. 189, 1905. . 



