258 



MINERALOGY 



Fracture is where the specimen breaks, not along a smooth plane 

 face, but irregularly in no definite direction. The appearance of 

 the uneven surf ace of fracture is also characteristic, and the follow- 

 ing terms are used in describing the fracture of minerals: Conchoi- 

 dal, when the break results in curved or warped surfaces, as in glass, 

 chrysocolla, or flint, Fig. 372. Subconchoidal, when the curves are 

 not well marked, are uneven, and the surfaces are slightly rough, as 

 in most minerals. Hackly, when the roughness consists of sharp 

 points, as in copper and most metals. Splintery, when the fracture 



FIG. 372. Obsidian, showing a Conchoidal Fracture. 



shows a fibrous structure, as in some steatites. Scaly, where the 

 mineral is formed of fine crystal scales, as in lepidolite. 



Tenacity and hardness both depend upon cohesion. The force 

 necessary to overcome this attraction of one molecule for its 

 neighbor will vary with the molecule and the direction in the crys- 

 tal. 



A sectile mineral may be cut with a knife, and the shavings 

 remain whole and possibly curl like the shaving of a quill or horn, 

 as graphite, molybdenite, and most micas and metals. 



A mineral is malleable when it flattens on hammering and 

 spreads out, increasing in area without cracking, as lead, silver, or 

 tin ; ductile when it may be drawn out in wire, as copper, silver, 

 iron ; brittle when on hammering it breaks down in a powder as do 

 most minerals, though the ease with which this occurs is modified 

 by such terms as tough, as rhodonite; soft, as wad; friable, as 

 kaolinite. 



