Minerals of the Composition MgSiO % . 



413 



Monotropy. — In fig. 8, 



OT = axis of temperatures. 



OP = axis of vapor pressures. 



abed = vapor pressure curve of liquid. 



eb = vapor pressure curve of unstable solid. 



fc — vapor pressure curve of stable solid. 



T a == melting point of stable form. 



T 2 = melting point of unstable form. 



T 



Fig. 8. Vapor pressure curves in a case of monotropy ; eb is curve for 

 unstable form ; fc is curve for stable form. 



This figure represents the relations between the vapor pres- 

 sure curves of two monotropic solids and their corresponding 

 liquid. The curve differs from the preceding case only in that 

 fb and ed do not intersect before reaching the melting point 

 curve abc. At b the vapor pressure of one solid is equal to 

 that of the liquid, and the two* are in equilibrium ; T 2 is there- 

 fore the melting point of this form. Similarly, T 1 is the melt- 

 ing point of the second (stable) solid. It will be noted that 

 for a given temperature the vapor pressure of the first solid is 

 always greater than that of the second ; the two curves do not 

 intersect below either melting point and therefore are not in 

 equilibrium at any temperature ; the solid of lower vapor 

 pressure is more stable than the first throughout. The melt- 

 ing point of the unstable solid is always lower than the other, 

 but in practice the unstable form often changes into the more 

 stable before its own melting point (T a ) is reached. 



