42 



NEW YORK STATE MUSEUM 



faces of the cube, octahedron or dodecahedron. A good exam- 

 ple of cubic cleavage is presented by the specimen of galena 

 shown in pi. ll lm 



Tetragonal and hexagonal minerals show basal, prismatic or 

 more rarely pyramidal cleavage. Rhombohedral cleavage is 

 common among minerals crystallizing in the rhombohedral divi- 

 sion of the hexagonal system. The rhombohedral cleavage of 

 calcite is shown in pi. 11 2 . 



In the orthorhombic system cleavage parallel to one or more 

 pinacoids is common, also prismatic cleavage. Clinodiagonal 

 cleavage, parallel to the clinopinacoid, is found in many mono- 

 clinic species; also basal and prismatic cleavages and occasion- 

 ally cleavage parallel to the hemipyramids as with gypsum. 



In the triclinic system it is customary to select the axes so as 

 to make the cleavage directions parallel to the pinacoids. 



The parallel planes produced by cleavage may sometimes be 

 advantageously observed by holding the specimen so as to reflect 

 the light from a prominent face, and noting how the cleavage 

 faces, previously hidden by the rough surface of the specimen, 

 catch and reflect- back the light. Cleavage is also evidenced by 

 reflections from the interior of the crystal, incipient cracks and 

 many other traces which appeal to the eye of a trained observer. 



The fracture of a mineral is observed on a broken surface 

 other than a cleavage plane. It may be: 



1 conchoidal, with a smooth, curved surface like broken glass 

 or porcelain; 



2 even, with more or less regular depressions and elevations; 



3 uneven, with a rough, irregular surface; 



4 hackly, with sharp, jagged elevations like broken iron. 



The degree of resistance offered by the smooth surface of a 

 mineral to abrasion is known as hardness. A relative scale of 

 hardness of 10 common minerals is arranged as follows: 1 



^his scale of hardness was introduced by Mohs and is now generally 

 accepted. 



Fracture 



Hardness 



1 talc 



2 gypsum 



3 calcite (crystallized) 



4 fluorite 



