19 14-] CERTAiisr Mineral Resources 99 



principal mineral constituents of these dikes are quartz, feld- 

 spar, and muscovite mica, wliich occur in varying proportions. 

 In examining these dikes it will be found that sometimes the 

 quartz and feldspar are nearly equally distributed throughout 

 a certain part of the vein, while in other parts sometimes one 

 and again the other will predominate. Feldspar has been ob- 

 served that has crystallized out in enormous masses of more 

 than a ton in weight, and in one instance, at the Irby mine, 

 near Spruce Pine, Mitchell County, North Carolina, a well- 

 developed cr)'stal of feldspar was observed that measured 3 

 by 11 1-2 feet. Occasionally feldspar, quartz, and mica have 

 separated out in rather small masses, giving the vein the ap- 

 pearance of containing an equal quantity of each. In such 

 cases the three minerals are so intimately associated with each 

 other that the mica is of little or no commercial value and the 

 feldspar is also of no commercial value. Judging from obser- 

 vations made at a great many mica mines the pegmatitic dikes 

 that yield the best commercial mica are those in which the 

 three minerals have had a tendency to crystallize out in large 

 masses. Thus, where feldspar and quartz are in small crystals 

 or fragments, the mica is also apt to be small. Those dikes 

 that are two feet or less in width very seldom contain mica hav- 

 ing any commercial value beyond what could be obtained for 

 it as scrap mica, and hence little or no attention should be 

 paid to such dikes as a source of mica. IN'ot all of the wide 

 dikes carry mica of the right quality or in sufficient quantity 

 to afford profitable mining, for in some the mica has been 

 observed to occur in such small crystals and blocks that no 

 sheets could be obtained over an inch or two in diameter. 



The muscovite mica occurs in these dikes usually in rough 

 crystals (called blocks or books), which are sometimes distrib- 

 uted nearly evenly throughout the dike and at other times 

 nearer the contact of the dike with the country rock. These 

 blocks of mica are occasionally nearly perfect in their crystal- 

 line form, which is monoclinic, but imitating rhombic or hex- 

 agonal symmetry. The commercial blocks of mica usually 

 vary in thickness from 6 to 18 inches and from 3 to 15 inches 

 in diameter, although some blocks have been found as much as 



