30 GEOLOGICAL SURVEY OF ALABAMA 



in the Alabama mica field, but is found in some of the 

 present operating mines. 



In respect to color, muscovite is broadly termed "white" 

 mica, although in the thin sheet it may present wide 

 variations in shade, ranging from almost clear white to 

 amber, yellow, green, brown and brownish-red, and dark- 

 er red or "ruby." 



Phlogopite, in the thin sheet, is of a yellowish or brown- 

 ish color, deepening in the thicker sheet to yellow, brown, 

 copper colored and verging on black. Color variations 

 appear to be closely related to variations in hardness, 

 elasticity, and dielectric quality. 



In the matter of flexibility, a sheet of good muscovite 

 a thousandth of an inch in thickness, should be capable of 

 being bent into cylindrical form of % inch diameter with- 

 out evidence of cracking. 



USES. 



*The uses of mica may be summarized as follows, the 

 classification being based on the physical property of 

 greatest importance. 



Electric insulation: Sheets, films, mica board, wash- 

 ers, mica cloth, and tape; for dynamos, condensers, tele- 

 phones, light sockets, spark plugs, and innumerable other 

 electric appliances. Ground mica, mixed with shellac, is 

 molded into various insulating forms. 



Heat insulation: Screens in front of highly heated 

 material, as retarder of heat waves in optical lanterns, 

 and in electrical heating devices. Ground mica is used 

 as pipe and boiler coverings, in annealing steel, and in 

 fire-proof paints and coverings. 



Transparency : Glazing the fronts of stoves, for fur- 

 nace sight holes, lamp chimneys, lamp shades, military 

 lanterns, lantern slides, sight holes of diver's helmets, 

 compass covers, gage fronts, in windows where glass 

 would be broken by heavy shocks or vibrations, and as 

 coverings for wounds. 



Resonance: Phonograph diaphragms and various 

 sounding devices (submarine detectors). 



*U. S. Geological Survey Mica in 1918. 



