414 K T. Allen, F. E. Wright and J. K. Clement— 



Although many solids, like the minerals under discussion, 

 have a vapor tension much too low for measurement at these 

 temperatures, the relations are as represented in the diagram.* 



The stability relations of the four polymorphic forms of 

 magnesium silicate may be shown in the same way by a simple 

 diagram (fig. 9). These curves do not, of course, represent 

 measured vapor pressures, but simply the order of the stability 



tZJk/ -mL 



Fig. 9. fa is vapor pressure curve for orthorhombic amphibole ; hb is 

 vapor pressure curve for monoclihic amphibole ; mc is vapor pressure curve 

 for orthorhombic pyroxene ; nd is vapor pressure curve for monoclinic 

 pyroxene. 



of the forms. As is proved by the experiments described 

 below, the monoclinic pyroxene, at atmospheric pressure, is 

 the most stable, and the others bear a monotropic relation to 

 it, their relative stability being in the order indicated by the 

 vapor pressure curves. 



Two lines of evidence lead to this conclusion : (1) a. Ensta- 

 tite and the amphiboles, while still in the solid state, pass over 

 at high temperatures into monoclinic pyroxene which cannot 

 be changed back without passing through the amorphous state ; 

 b. At much lower temperatures (about 800°) the same three 

 forms can be dissolved and recrystallized simultaneously into 

 the monoclinic pyroxene by means of fluxes ; (2) enstatite and 

 the two amphiboles change into monoclinic pyroxene with 

 evolution of heat. 



(1) Although it was shown conclusively that the amphiboles 

 change into monoclinic pyroxene above about 1150°, and 



*Koozeboom, Heterogen Gleichgewichte, Heft. 1, 158-159, Braunschweig, 

 1901. 



