164 A. L, DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



The eutectic Relation 



The second possibility when the cooling solution reaches the tempera- 

 ture where one component is in excess has not yet been definitely proved 

 to exist among the minerals, but it has been studied by Miers and Miss 

 Isaac^' with a view to such application, and is included in all the major 

 treatises^^ on physical chemistry, with a variety of illustrations from 

 aqueous solutions at low temperatures. According to this view, although 

 one of the minerals is now in excess and free to crystallize out, the equi- 

 librium is of a metastable or neutral kind, so that the actual crystalliza- 

 tion of the mineral can only be precipitated by introducing solid crystal 

 fragments of the same mineral (or another isomorphous with it) from 

 without. These fragments, which may be minute in magnitude and few 

 in number, provide the crystal nuclei about which the molecules of the 

 excess component immediately group themselves. No other variety of 

 crystal or any form of mechanical disturbance will cause the separation. 

 The range of temperature over which this attitude of indifference to all 

 but its own kind obtains is relatively small and dijffers for different sub- 

 stances. After passing through this metastable region the excess mineral 

 enters a region of labile equilibrium where crystallization of the unstable 

 component may be expected to be rapid and spontaneous, and to continue 

 in step with the cooling until the eutectic temperature is reached. Here 

 the other component is due to crystallize side by side with it, and will do 

 so unless delayed for a longer or shorter time by similar limiting con- 

 ditions. 



In practice, at high temperature it is difficult to distinguish this case 

 from the preceding one (see Harker, loc. cit., page 209), where crystalli- 

 zation is merely delayed by viscosity or some form of molecular inertia. 

 Of course, the same situation may equally well arise among pure minerals 

 crystallizing alone. 



The third possibility is the normal one in which one component begins 

 to crystallize out the moment the cooling has reached a point where it is 

 present in excess, and equilibrium prevails throughout the whole process 

 of crystallization. The eutectic also appears promptly, and the entire 

 operation follows the simple diagram on page 152. Curiously enough, no 



"Miers and Isaac: Journal of the Chemical Society (London), Transactions, 89, 1906, 

 pp. 413. 



Miers: Science Progress, 2, 1907, pp. 121. (Also several other papers.) 



18 The best statement of this case is by Ostwald : Studien uber die Bildung und Um- 

 wandlung fester K5rper : 1, Ubersattigung u. TJberkaltung, Zeitschr. f. phys. Chem., 22, 

 1897, pp. 289. 



