THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 515 



there are no mixing-heat effects and that the heat of solution of any 

 plagioclase in liquid plagioclase is simply the latent heat involved in 

 the change from solid to liquid/ 



In Figure 2 is plotted the equilibrium diagram for anorthite and 

 diopside. Now we know the latent heat of anorthite from the cal- 

 culated results of Figure i and we may calculate, using this value, a 



CaM^Si^lP^ 



Fig. 2. — Equilibrium diagram of diopside and anorthite. Determined curves in 

 full lines. Broken curves calculated on the assumption of no mixing heats. 



curve of freezing-point depression for anorthite according to the 

 equation for ideal concentrated solutions, viz. : 



T= 



To 



I — jr- Inx 



where T is the melting temperature (saturation temperature) of 

 anorthite in a solution of mol fraction x in anorthite; Q is the 

 latent heat of melting per mol of anorthite ( = 29,000 cal.) and 

 To its melting-point (=1550°). As a result of this calculation we 

 obtain the right-hand dotted curve. It will be noted that the 

 determined curve corresponds with the calculated dotted curve in 

 the upper portion (as far as about 80 per cent anorthite) and then 

 falls below. In other words, we could have calculated the latent 



' See N. L. Bowen, Melting phenomena of the plagioclase feldspars. Amer. 

 Joitr. Sci., Vol. XXXV (1913), p. 590. 



