348 



MINERALOGY 



s(lll),e(101),m (110) a (100). Twinning plane 101, both geniculate 

 and cyclic; Cleavage, 110 imperfect; Brittle, fracture uneven; 

 H. = 6-7; G. = 6.8-7.1; Luster, splendent adamantine ; Color, 

 various shades of brown, red, and gray to almost black ; Streak, 

 pale; n = 1.997.1 



B.B. Infusible, reduced with soda and borax on coal yields 

 malleable tin buttons. Insoluble in acids. 



General description. Crystals are short stout prisms with the 

 prism faces striated parallel to the c axis, usually terminated with 

 the two unit pyramids. Acicular crystals terminated by the pyra- 

 mids (321) and (521) occur at Cornwall, England, also massive or 



FIG. 417. Cassiterite from Bohemia. The Upper Figures are Stream Tin 

 from Mexico. 



granular. While cassiterite is found as a granular or disseminated 

 primary accessory mineral in some igneous rocks, it is more often 

 connected with the cavities and pegmatitic veins in the region of 

 granitic masses which have been intruded in sedimentary forma- 

 tions. Here its origin is the result of pneumatolytic agencies 

 which have concentrated the tin on the border of the granitic mass, 

 where it has been deposited in the veins, close at hand, of the dis- 

 turbed area. In such veins it is associated with fluorite, tourma- 





