418 



UNITED STATES MINERAL RESOURCES 



1900 1910 1920 1930 1940 



1960 1970 



Figure 51. — Production of scrap mica in the United States, 

 1900-70, compared with that in the rest of the world, 

 1925-70. Data from U.S. Geological Survey and U.S. 

 Bureau of Mines, Mineral Resources of the United States 

 and Minerals Yearbook, 1900-70. 



Details of the preparation and classification of 

 mica and trade practices of the industry are too 

 elaborate to discuss here, but they have been de- 

 scribed well by Rajgarhia (1951), Skow (1962), 

 Montague (1960), Jahns and Lancaster (1950), 

 and Wierum and others (1938). 



Most deposits of scrap mica must be deeply weath- 

 ered so that they can be mined in bulk with power 

 equipment and the ore can be washed and screened 

 to recover the mica. The content of recoverable 

 mica ranges from 3 to 20 percent. In simple wash- 

 ing plant operations, mica smaller than 8 mesh is 

 not recovered readily ; but through the use of spiral 

 methods of concentration, better recovery is pos- 

 sible (Broadhurst and Hash, 1953, p. 6; Skow, 1962, 

 p. 86). 



PRICES 



The selling price of mica ranges from only a few 

 cents per pound for punch or scrap to many dollars 

 a pound for large sheets of the best quality. In 1970, 

 prices for sheet mica ranged from $0.85 a pound 

 for sheets IV2 inches across to $8.00 a pound for 

 sheets 8 inches or more across. Punch mica brought 

 $0.07-$0.12 a pound. Scrap mica generally brought 

 $30-$40 per short ton. Ground mica prices were 



).02-$0.04 a pound for dry-ground mica 

 ).075-$0.095 a pound for wet-ground mica. 



and 



GEOLOGIC FEATURES OF MICA DEPOSITS 



Sheet-muscovite deposits occur in coarse-grained 

 igneous rocks called pegmatites; scrap-mica de- 

 posits are found in pegmatites, granite, and mica- 

 rich metamorphic rocks. Phlogopite deposits are in 

 pyroxenite layers in marble or gneiss. 



PEGMATITES 



Most pegmatites are light-colored coarsely crystal- 

 line igneous rocks, commonly found as dikes or sills 

 in metamorphic rocks or in large granitic intru- 

 sions. Individual mineral grains and crystals range 

 in length from less than 1 inch to many feet, and 

 a large variation in grain size within a single peg- 

 matite body is common. 



Pegmatites range from small pods and thin seams 

 to large masses hundreds of feet thick and thou- 

 sands of feet long. In general, they are tabular or 

 lenticular bodies. Some pinch and swell along strike 

 or downdip, or both. Most lenticular bodies are dis- 

 coidal, but some bodies are best described as tongue- 

 like or pipelike. In some pegmatite districts, most 

 of the deposits are concordant with the foliation of 

 the enclosing country rock; in others, most are dis- 

 cordant bodies in joints or faults. Branching, folded, 

 or irregular bodies may be present in areas of com- 

 plex structure. Most elongate bodies and minor 

 bulges or rolls in concordant pegmatites are gen- 

 erally parallel to the plunge of minor structural 

 features in the country rock. 



Pegmatites are composed mostly of feldspar, 

 quartz, and mica but may contain accessory min- 

 erals such as garnet, tourmaline, apatite, and beryl 

 and unusual or rare minerals of many types. The 

 many and unusual minerals found in some pegma- 

 tites make pegmatites favorite collecting sites for 

 mineralogists and rockhounds. 



Plagioclase is generally the most common feld- 

 spar in muscovite-rich pegmatite. Perthite or per- 

 thitic microcline, an intergrowth of plagioclase and 

 microcline feldspars, is generally present and may 

 be abundant in some pegmatites. The feldspars are 

 discussed in greater detail in a separate chapter. 



Book mica in commercially mined pegmatite gen- 

 erally constitutes 2-6 percent of the rock. Local 

 rich shoots or pockets might have as much as 40 

 percent muscovite, but large volumes of pegmatite 

 have no more than 2 percent. 



The minerals in many pegmatites are almost 



