NATURE OF PRECIPITATION. 113 



potentialized minerals to lower potentialized forms. These changes are 

 illustrated by the passage of glass to a crystalline form, by the passage of 

 minerals from a strained to an unstrained condition, and by the passage 

 of minerals of low specific gravity to minerals of higher specific gravity. 



PRECIPITATION. 



From solutions, by changing conditions, solids may separate. This 

 process is called precipitation. Since precipitation from ground water 

 solutions is of the utmost importance in metamorphism, it is necessary to 

 consider fully the conditions under which precipitation takes place. It has 

 already been seen that in solutions the ingredient which is present in excess 

 is called the solvent and the ingredients which are subordinate are the sub- 

 stances dissolved. When from solutions the substance in excess, or the 

 solvent, separates, this is called a freezing of the solution. When in the 

 solution the substances dissolved first separate, this is called crystallizing 

 out of the materials dissolved. "The processes of freezing and of crystal- 

 lizing out are both to be considered from the same point of view ; and when 

 we are not dealing with dilute solutions where one ingredient is present in 

 large excess, but with a mixture, where both ingredients are present in 

 about the same proportions, then we would be in actual doubt whether the 

 separation should be regarded as a freezing (of the solvent) or a crystal- 

 lizing out (of the substance dissolved), or perchance of both processes."" 



The necessary condition for precipitation is supersaturation ; for if a 

 solution be not saturated it will take more material into solution ; but if a 

 solution be sufficiently supersaturated some of the material must be thrown 

 down or be precipitated. If solids are present similar to the compounds 

 in solution, considerable supersaturation does not occur. This is very 

 frequently the case with ground solutions. Under such circumstances 

 the salts in solution separate out upon the minerals already present, or the 

 minerals grow. At any given pressure and temperature, provided the 

 changes occur slowly, equilibrium is nearly retained by this continuous 

 adjustment. This relation between minerals already present and solutions 

 is one of the most important factors which control the growth of minerals 

 which are present. If, for instance, in a complex solution containing 



oNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 

 1895, p. 414. 



MON XLVII — 04 8 



