Minerals of the Composition MgSiO % . 435 



crystallizes no matter what the solvent, but this is not true in 

 many other instances. It seems entirely reasonable to expect 

 that if a solvent by its viscosity can hinder a transformation in 

 the solid state, it may also in certain cases restrain the mole- 

 cules in the act of crystallization from assuming the configma- 

 tion characteristic of the stablest structure. And we might also 

 expect viscosity to be especially effective where the transfor- 

 mation of the unstable form into the stable is effected with 

 comparative difficulty, as it is in the case of the rnagnesium 

 silicates. This would explain why enstatite comes out of a 

 viscous silicate solution at a temperature much higher than that 

 at which the monoclinic form is obtained from thinner solutions. 

 At first thought one might be inclined to regard viscosity as 

 directly conditioning the form of the crystal, and to look upon 

 the temperature as merely influencing the viscosity, but further 

 reflection convinces that this can not be so, for we obtained 

 the am pinhole from aqueous solutions at 375°-475°. The 

 properties of aqueous solution in general, however, differ 

 widely from others. It is probable that those movements of 

 the molecule which depend directly upon temperature have a 

 very important influence on crystallization. Although tem- 

 perature and viscosity are certainly important factors in the 

 formation of unstable modifications, the knowledge of the sub- 

 ject is still insufficient, and what we have is too little systema- 

 tized to generalize in an entirely satisfactory way r . 



Formation of Amphiboles. — Regarding the formation of 

 forms III and IY, it seems quite beyond the bounds of proba- 

 bility that they should be formed in nature by the rapid cooling, 

 which, on a small scale, is effective. On the other hand, assuming 

 that these forms are really am pinholes, the formation of at 

 least one of them from aqueous solutions, at a temperature of 

 375°-475°, is consistent with recognized geological forces; at 

 any rate, our experiments indicate that the two amphiboles 

 form at lower temperatures than the pyroxenes. We are inclined 

 to regard the pressure in these experiments as an unessential 

 factor, except in so far as it is necessary to prevent the escape 

 of water at these temperatures, because in the first place it has 

 been seen that both substances (III and IY) could be obtained 

 without pressure, and secondly, the specific volume of these 

 amphiboles, as of all others, for that matter, is greater than that 

 of the corresponding pyroxenes. According to Le Chatelier's 

 principle, pressure should tend to produce the system of small- 

 est volume. 



Relation betvieen Pyroxenes and Amj)hiboles. — Though we 

 refrain from generalizing as yet in regard to the two great min- 

 eral groups, the amphiboles and the pyroxenes, we can say that 

 the demonstration of an irreversible (monotropic) relation 

 between each of the two magnesian amphiboles and the stable 

 pyroxenes accords with the experience of many other investi- 



