tarr] some com M ox MINERALS 319 



i\\'hile it is well to have a scale of hardness, for ordinary pur- 

 poses the following method of determining the hardness will be 

 found sufficient. 



\'ery soft (below 2.5) ; can be scratched by the finger nail 

 or very easily with a knife. 



Soft (2.5-4); not scratched with the finger nail, but easi- 

 ly with a knife. 



Hard (4-6) ; can be scratched with a knife, but not easi- 

 ly. Those from 5.5-6 only with a good knife. 



\'ery hard (6-7) ; not scratched by a knife, but can be 

 scratched by quartz. 



Above 7 ; cannot be scratched by quartz. These miner- 

 als are comparatively rare. 



A word of caution is necessary in regard to obtaining the 

 hardness. Always break the mineral and get a fresh, unaltered 

 surface. The outside of a specimen may be decomposed and 

 will not give the true hardness. 



Cleavage. — Some minerals have a tendency to part or 

 split along certain lines. This is called cleavage. It is generally par- 

 allel to a crystal face. Galena exhibits this very well and the 

 resulting form is a cube, hence it is called cubic cleavage (fig. 21). 

 Calcite also exhibits this property and always cleaves into 

 rhombs (fig. 16. This figure illustrates also the propertv of 

 double refraction by calcite.). 



Fracture. — This is the kind of surface produced when a 

 mineral breaks in any direction other than that of cleavage. 

 It may be smooth or shell shaped (conchoidal) like flint or chert; 

 or uneven ; or hackly, when the elevations on the surface of the 

 fracture are sharp and jagged as in broken iron. 



Specific Gravity. — This is the ratio of the density of 

 the mineral to water. Thus galena has a specific gravity of 7.5, 

 which means that a cubic foot of galena will weigh 7.5 times 

 as much as a cubic foot of water or 468.75 pounds. Minerals 

 which consist largely of iron, copper, lead or other element with 

 a high atomic weight, in general have high specific gravities. 

 When no apparatus is at hand to obtain the specific gravity an 

 estimate can be made by comparing it with a piece of quartz 

 or something similar. 



Color. — The colors of minerals include every color of the 

 spectrum. In one group of minerals the smallest particle ob- 



iThis is the scale of hardness as used by mineralogists: 1.— Talc 

 2.— Gypsum. 3. — Calcite. 4.— Fluorite. 5.— Apatite. 6.— Orthoclase. 7.— 

 Quartz. 8.— .Topaz. 9. — Corundum. 10.— Diamond. 



