12 THE ROCK-FORMING MINERALS 



The optical properties of minerals are of great value in the 

 study of rocks, and by the aid of the polarizing microscope very 

 minute crystals may be identified. 



Most substances which are solid under any circumstances are 

 capable of assuming a crystalline form, so that solidification and 

 crystallization are usually identical. For the formation of large 

 and regular crystals, it is necessary that the process be gradual and 

 that space be given for the individual crystals to grow. Usually 

 crystallization begins at many points simultaneously, and the crys- 

 tals crowd upon one another, resulting in a mass of more or less 

 irregular crystalline grains. The same substance which, when very 

 rapidly solidified, forms an amorphous glass, will give rise to dis- 

 tinct crystals, if slowly solidified. 



Crystallization requires that the molecules be free to move upon 

 each other, and thus to arrange themselves in a definite fashion. 

 It may take place either by the deposition of a solid from solu- 

 tion, by cooling from a state of fusion, or by solidification from 

 the condition of vapour. In all cases the size and regularity 

 of the crystals depend upon the time and space allowed for 

 their growth. In a manner not yet understood, amorphous 

 solids may be converted into crystalline aggregates. This has 

 been observed in the case of certain glassy volcanic rocks, which, 

 though amorphous when first solidified, have gradually become 

 crystalline, without losing their solidity. This process is called 

 devitrification. 



The actual steps of crystallization may be observed by slowly 

 evaporating a solution of some crystalline salt under the micro- 

 scope. The first visible step in the process is the appearance of 

 innumerable dark points in the fluid, which rapidly grow, until 

 their spherical shape is made apparent. The globules then begin 

 to move about rapidly and arrange themselves in straight lines, 

 like strings of beads, and next suddenly coalesce into straight rods. 

 The rods arrange themselves into layers, and thus build up the 

 crystals so rapidly, that it is hardly possible to follow the steps of 

 change. In certain glassy rocks, which solidified too quickly to 

 allow crystallization to take place, the incipient stages of crystals, 



