GUIDE TO THE MIXERALOGIC COLLECTION'S 



75 



Cassiterite (stream tin) Sn0 2 



Cassiterite is the dioxid of tin containing 21.4$ oxygen and 

 78.6$ tin. 



Tetragonal crystals of the general type shown in fig. 192 are 

 terminated with a low pyramid. Forms of prismatic habit 

 with steeper and more complicated term- 

 inations are also characteristic, and twins 

 similar to the form shown in pi. 22 x are 

 quite common. Reniform masses and rounded 

 pebbles with fibrous radiated structure 

 (stream tin) are of common occurrence. The 

 luster of cassiterite is adamantine and in the 

 case of crystallized varieties is usually splend- 

 ent: the color is brown or black, sometimes 

 red, grey or yellow, and the streak is brown. 



Cassiterite occurs in veins traversing granite, gneiss and other 

 igneous and metamorphic rocks. It is found abundantly in Corn- 

 wall and other parts of England, in Bohemia, Saxony, East 

 Indies, Australia, Bolivia and Mexico; also in the United States 

 in South Dakota, California and other states. 



Cassiterite is the sole source of tin. 



Rutile (nigrin) Ti0 2 



Rutile is the dioxid of titanium and contains 4(K oxygen and 



60$ titanium. 



In crystallization rutile closely resembles cassiterite (fig. 103). 



The crystals are prismatic in habit, often passing into acicular 

 and hairlike forms which are vertically striated 

 and are sometimes included in quartz and other 

 minerals. Twinning, resulting in knee-jointed 

 crystals and rosettes (pi. 22 2 ), is quite common. 

 Rutile is occasionally found in compact masses 

 which carry some iron. The luster of rutile is 

 rather more brilliant and metallic than cassiter- 

 ite and may be described as metallic-adamantine; 



in color it varies from brownish red to nearly black and when 



seen by transmit ted light in t ransparent varieties ii is deep red. 

 Rutile occurs in granite, gneiss, syenite, mica, slate and some 



times in the limestones; it is frequently embedded in ether 



