THE MINING AND QUARRY INDUSTRY I9I3 47 



the dark color renders it unsuitable for many purposes. Phlogopite 

 is a magnesium variety, containing no iron, but less transparent 

 usually than the best muscovite. Its color is amber or yellow, 

 sometimes red or of a greenish tinge. It is employed for the same 

 purposes as muscovite, but seems to be even preferred to the latter 

 in electrical work. The distinction of the different varieties of mica 

 when not apparent from outward appearance requires the use of 

 a polarizing microscope, supplemented possibly by chemical tests 

 to determine the nature of the basic elements. 



Muscovite and biotite are allied in their occurrence, both being 

 important ingredients of the crystalline silicate rocks such as gran- 

 ite, and many gneisses and schists. Typical granite contains both 

 varieties. The commercial sources of the two minerals, however, 

 are limited to pegmatites, those modifications of granite in which 

 the minerals are coarsely crystallized and irregularly distributed. 

 Pegmatite is found in rather limited bodies, usually in dikes or 

 lenses, which have intrusive relations with the country formations, 

 more rarely as irregular masses within normal granites. The dikes 

 or lenses range from very small examples, a few inches or a foot 

 or two thick, up to bodies several hundred feet in diameter and of 

 much greater length. They afford feldspar, quartz and other com- 

 mercial materials in addition to mica. 



Phlogopite is seldom if ever found in granite pegmatites, but its 

 occurrence in New York is practically restricted to crystalline lime- 

 stones where it appears to represent a secondary product of meta- 

 morphism. probably in most cases as a result of contact influences 

 exerted by igneous intrusions. It is associated with such other min- 

 erals as amphibole, pyroxene, wernerite, tourmaline, fluorite, titan- 

 ite and apatite. The mineral association varies from place to place 

 and the occurrence of phlogopite is quite irregular or bunchy, or 

 else restricted to a definite part of the contact zone. According to 

 the report of Cirkel S the commercial phlogopite deposits of Canada 

 are associated with pyroxene which penetrates country gneisses and 

 limestones in the form of dikes, the pyroxene being regarded as an 

 igneous rock. There is no resemblance to such conditions in the 

 Adirondack occurrences, though pyroxene is a frequent accompani- 

 ment of the mica. The minerals rather have resulted from a con- 

 version of the limestones by mineralizing solutions and vapors given 

 oft" bv granite intrusions and they gradually disappear with increas- 



1 " Mica, Its Occurrence and Uses." Mines Branch, Department of the 

 Interior, Ottawa, 1905. 



