tarr] some common MINERALS 321 



tained by crushing has the same color as the original. These 

 minerals generally have a metallic luster, that is, they reflect 

 light like a metal. In the other group of minerals the powder 

 may have a different color from the larger mass. These miner- 

 als have an unmetallic luster. The colors of a single mineral 

 are almost unlimited although in many cases there is a charac- 

 teristic color which is fairly well adhered to by the mineral. In 

 the table of minerals given the dominant color is given first and 

 the less important ones follow. 



Streak. — This is the color of the powder of the mineral. 

 It is usually obtained by rubbing the mineral on a piece of un- 

 glazed porcelain or by scratching it. This is a great aid in de- 

 termining some minerals. 



Luster. — This is the property of reflecting light. It is 

 possessed by all minerals but varies with the nature of the re- 

 flecting surface. Metallic luster has been mentioned above. Un- 

 metallic luster is of several kinds, as follows. Vitreous, like 

 broken glass and subvitreous. not so bright as vitreous. The 

 luster is called resinous when it is like resin. Pearly when like 

 pearl, as talc. Silky luster is the result of a fibrous structure. 

 Admantine luster is a brilliant reflecting of the light as in the 

 diamond. 



Degree of Transparency. — A mineral is transparent 

 when an object can be distinctly seen through it (fig. 16). 

 Translucent, when light is transmitted but objects not seen. 

 Opaque when no light is transmitted. All minerals exhibit these 

 properties in some degree. 



Fusibility. — Some minerals when heated will melt ; 

 while others will not. This can be determined sometimes by put- 

 ting a small splinter of the mineral in a candle or alcohol flame. 



Taste. — This is a property of soluble minerals. The 

 commonest is the salty taste of the mineral salt or halite (it's 

 mineralogical name). Soda has an alkaline taste, and epsom 

 salts a bitter taste. 



Odor. — Most minerals do not have an odor naturally but 

 under certain conditions will give off an odor. The odor pro- 

 duced when pyrite is struck is sulphurous. The odor of mois- 

 tened clay is argillaceous. Some limestones when struck give 

 off a fetid odor. 



Feel. — Some minerals feel smooth to the touch and oth- 

 ers, such as talc, feel greasy. 



There are over a thousand minerals and as many more 

 species and varieties, but onlv about two hundred are well 



