550 S. Taber — Growth of Crystals. 



with the side of the beaker, and the rate of growth was prob- 

 ably not uniform, as rings of growth varying in thickness and 

 also in fineness of crystallization could be distinguished. The 

 successive rings gradually increased in diameter so that the 

 formation was largest at the end in contact with the beaker. 

 The rings are probably due to variations in the rate of growth 

 resulting from changes in temperature and humidity. In this 

 case the material for growth diffused under the growing mass 

 from within instead of from without. 



The shape of a crystal is the resultant of several different 

 factors, and under varying conditions these factors vaiw greatly 

 in their relative importance. In the ideal case of a growing- 

 crystal, everywhere in contact with a solution of uniform sat- 

 uration, it is probable, as suggested by Lehmann, that the form 

 assumed is the resultant of the forces of surface tension and 

 molecular orientation.* The tendency is always to build that 

 form for which the total surface energy is a minimum, and in 

 crystals the surface tension is not the same in different direc- 

 tions. Otherwise the stable form would be a sphere. 



In nature this ideal condition can seldom be attained, and in 

 most cases the form of a growing crystal is determined by the 

 external forces resisting growth and the accessibility of the 

 material from which the crystal is built rather than the tendency 

 to form crystal faces. The tendency to form crystal faces is 

 much stronger in some substances than in others, but it is never 

 so strong as to cause growth on a face which is not in contact 

 with a supersaturated solution, and, even if a growing surface 

 is in contact with a supersaturated solution, the relative rate of 

 growth is chiefly controlled by the rapidity with which the 

 material for growth is made available. 



If a solution remains in contact with two substances, A and 

 B, it will in time become saturated with respect to both, and 

 should the conditions change so that the solution is supersatu- 

 rated with respect to A, then the crystals of A will begin to 

 grow. With increase in size a crystal of A will finally 

 approach so close to a crystal of B that the latter will offer 

 resistance to further growth in that direction. If, under exist- 

 ing conditions, A has a relatively weak tendency to form crys- 

 tal faces and a relatively rapid rate of growth, the crystal of A 

 will grow around the crystal of B, which will then become an 

 inclusion ; but on the other hand, if A has a relatively strong 

 tendency to form crystal faces and a relatively slow rate of 

 growth, so that there will be sufficient time for diffusion to 

 supply new material to the surface of A along its contact with 

 the crystal of B, then B must make room for the crystal of A. 



* Quoted in J. Y. Elsden's "Principles of Chemical Geology," p. 60. Lon- 

 don, 1910. 



