CHAPTER III 

 PHYSICAL PROPERTIES 



Cleavage and fracture. When a mineral is broken by the blow 

 of a hammer on a sharp-edged instrument, as a cold chisel, held 

 on the specimen, it either breaks in a smooth plane face or irregu- 

 larly. The former is cleavage ; the latter is fracture. Cleavage 

 is caused by the separation along and between layers of molecules. 

 Taken in this sense, only crystalline minerals may possess cleavage. 



FIG. 371. A Crystal of Galena, showing the Perfect Cubical Cleavage. Aurora, 



Missouri. 



Cleavage is named from the crystal form to which the separation 

 is parallel, and is represented by the letter representing the form, 

 as cleavage m = prismatic or parallel to the unit prism, c = basal, 

 etc. Galena and halite have cubical cleavage, Fig. 371 ; cal- 

 cite, rhombohedral ; spodumene, prismatic, etc. A perfect cleav- 

 age is one in which the plane face obtained is smooth, even, and 

 polished and is generally easy to obtain but not necessarily so; 



256 



