AET. 17 



MINERALS OF OBSIDIAN CLIFF — FOSHAG 



15 



other siigfrests that the sohibility curves do not lie very far apart, 

 for the rate of transformation of an unstable to a stable phase is 

 dependent upon their relative solubilities. A <rreat difference in solu- 

 bility brinfrs about a rapid transformation to the stable phase. The 

 difference in solubility as shown by the curves in the dia^^ram is, 

 therefore, probably somewhat exag^xerated. 



The form which actually separates out is, in the absence of 

 nuclei, determined by which ever nucleus first forms. It is ob- 

 viously necessar}^ for the solution to be supersaturated with respect 

 to the stable phase before the less stable form can possibl)^ separate 

 out. Further, the unstable forms are likely to appear only if their 

 solubility is not oreatly different from that of the stable forms. This 

 is a further indication that the solubility of the silica minerals is 



X ------- 



K70 /62.S 



T 



Fio. T). — Diagram showing kelative solubility of the forms of silica 



not very jjreatly different. In fact, judging from the ease with 

 which the unstable phases of silica form, their solubility is very 

 nearly the same. If this actually be the case, it may even be pos- 

 sible for the unstable phase to form in equilibrium with the stable, 

 provided the former produces large crystals while tlie latter separates 

 as very fine particles. This apparent ambiguity is due to the fact 

 that the smaller particles have a higher vapor tension and hence a 

 greater solubility than the larger, and may even equal in solubility 

 the larger crystals of the unstable phase. This, of course, can only 

 hai)])en if the solubility of the two phases differs but slightly. 



It is well known that the various allotropic modifications may be 

 obtained from a solution by merely varying the conditions of crys- 

 tallization. The theory of allotropy proposed by Smits-^ is based 



23 A. Smits, Zeit. Pliysik. Chem., vol. 82, 1!)13, p. 657. 



