66 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



sented by this tangent. Crystallization will then begin at a (fig. 18, 

 II), with the mixture richest in the higher melting component, crys- 

 tals of composition a will be in equilibrium with the liquid phase b in 

 all proportions, and solidification (or melting) will not take place at a 

 single temperature, but through a range of temperature. If we now 



100 A 



100 B 



Fig. 19. 



plot the length of the abscissa cor- 

 responding to ah in a separate dia- 

 gram with the observed tempera- 

 ture range of solidification, adding 

 all the other possible cases which 

 will arise from the continued dis- 

 placement of the C-curves, we ar- 

 rive at the accompanying diagram 

 (fig. 19) of Roozeboom's Type I. 

 Types II and III appear in the same 

 way when the form of the C-curves 

 changes as indicated in figs. 20 

 and 21. 



The physical side of the system of 

 reasoning is readily inferred from 

 the figures. If we start with a mix- 

 ture of the composition indicated 

 by m (fig. 22) and temperature 

 above the melting point, crystalliza- 

 tion will begin at a, the separating 

 crystals will have the composition 

 b, while that of the remaining melt approaches d. Upon cooling to e, 

 solidification ends with crystals of this composition. Melting is ex- 

 actly the reverse operation. Whether these first crystals of compo- 

 sition b remain stable as such or undergo solid transformation or wholly 

 or partly redissolve appears to remain undetermined in any general 

 way by Roozeboom's theory, and may be radically influenced by 



00 B 



Fig. 18. 



