CONSTRUCTION OF MINERALS. 



15 



Fig. 1. 



hedra, base. 



are perfect. The crystal of alum, for in- 

 stance, is a perfect double pyramid, the 

 shape being given in Fig. 1, which is an 

 octahedron that is, a body with eight 

 equilateral angles, the lines all being ex- 

 actly equal, and the spaces also. The name 

 is from two Greek words, octo, eight, and 

 In Fig. 2 you have the crystals of alum as 



Fig. 2. 



they appear when their formation is interfered with by 

 some disturbing cause. In the glistening white marble 

 the crystals are so crowded together as they form that 

 none of them are perfect. The same is true of the crys- 

 tals of quartz, mica, and feldspar in granite. 



14. Crystals of Different Sizes. As crystals increase 

 by additions upon the outside, the crystals of any sub- 

 stance may vary greatly in size. Quartz crystals are 

 sometimes very large. There is one at Milan which is 

 3i feet long, 5^ in circumference, and weighs 870 pounds. 

 Of course, the larger crystals of any substance will have 

 precisely the same shape with the smaller. 



