28 GEOLOGICAL SURVEY OF ALABAMA 



Muscovite has a specific gravity of 2.8 and is therefore 

 heavier than its associations of quartz and feldspar. 



In hardness, the various varieties and to some extent 

 the same varieties of mica, vary somewhat, scaling in 

 the Alabama field at approximately 2.5 or about that of 

 copper, although frequently slightly softer. A close ap- 

 proximation in hardness to copper is material to the value 

 of electrical mica in generators, as affording equal wear 

 and the consequent absence of sparking in commutators 

 of direct-current motors and dynamos, built up of copper 

 bars and mica strips, for which purpose, and for the above 

 stated reason, Canadian phlogopite receives the prefer- 

 ence. The hardest mica recovered in Alabama is of the 

 green variety of muscovite. 



The structural characteristics and forms of mica are 

 minutely and exhaustively described and illustrated in 

 Bulletin 430-J of the United States Geological Survey by 

 D. B. Sterritt and H. J. Gale, from which report, the fol- 

 lowing descriptions and explanations are literally quoted. 



Muscovite, like all the micas, belongs to the monoclinic 

 system of crystallization, and has a symmetry approxi- 

 mating the hexagonal. This symmetry is indicated by 

 the nearly hexagonal outline often observed in the prisms 

 by the percussion and pressure figures, and by "ruled" 

 and "A" mica as described below. Mica mined for com- 

 mercial purposes is generally found in rough blocks, some- 

 times with an irregular development of crystal faces. 

 The faces are not usually as many as would be required 

 to complete the simplest figure, and their surfaces are 

 generally very rough. Very commonly a large part, if 

 not all of a block of mica has a ragged outline without 

 plane surfaces. Occasionally fairly well developed hexa- 

 gonal or rhombic prisms are observed in crystals weigh- 

 ing hundreds of pounds. 



Rough crystals, or "books" of mica as they are called 

 in the Western States, do not split perfectly until the 

 outer shell of etched and sometimes partly crushed mica 

 has been removed. This is accomplished by rough split- 

 ting, or cleaving the large book into sheets one-eighth 

 inch thick or less, and trimming the edges with a knife 

 held at a small angle with the cleavage. Further split- 



