niCA. 



BY JOSEPH HYDE PRATT. 



rjosrph Ilydo Prntt, Roolopi.sl unci consulting mining (nginccr; born Hartford, Conn. 

 February 3, 1870; graduated from the Sheffield Scientific school of Yale university; 

 iBstructor in mineralogy at Yale and later at the I'niversity of North Carolina; state 

 geologist of North Carolina, 1897-99; assistant geologist of the United States geological 

 survey since 1900. Author of about 125 monographs and other articles on miner- 

 alogy and geology, mostly contributed to scientific periodicals.] 



Mica, in some form or other, is probably familiar to every- 

 body because it is so very widely distributed in nature, being a 

 component part of many of the crystalline and sedimentary 

 rocks. Its commercial value is dependent upon its occurrence 

 in blocks or masses that are capable of being split into sheets a 

 square inch or more in size. Deposits of commercial mica 

 occur for the most part in pegmatitic dikes or veins which are 

 found in granite and hornblende and mica gneisses or schists. 

 These dikes or veins vary in thickness from a few inches to sev- 

 eral hundred feet and are often very irregular, having arms or 

 veujiets branching off in many directions. In character these 

 dikes are similar to granite, and sometimes they are called 

 coarse granite, and if we could conceive of the constituents 

 of granite being magnified a hundred times or more we would 

 have an appearance similar to a pegmatitic dike. 



The mica in these pegmatitic dikes will seldom average 

 over 10 per cent of the contents of the dike and sometimes will 

 be as low as 1 per cent. Often a dike will have the appearance 

 of containing a very high percentage of mica on account of a 

 number of blocks being clustered together in bunches almost 

 touching each other, while in another portion of this same dike 

 the mica will be almost entirely absent for a distance of from 

 5 to 20 feet; thus the general average of mica in the dike will 

 be from 1 to 10 per cent only. Of the mica obtained from 

 these dikes, usually only from 5 to 25 per cent can be cut into 

 sheet or plate mica. The average is probably about 10 per 

 cent. Occasionally large quantities of mica are mined in 

 which there is not over 1 per cent that can be made into sheet 

 mica. These commercial occurrences of mica are not very 



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