420 



UNITED STATES MINERAL RESOURCES 



Many other minerals are commonly present, includ- 

 ing quartz, feldspar, garnet, kyanite, and graphite. 

 Most mica schist was originally fine-grained silt- 

 stone or shale, which was subjected to at least 

 medium-grade regional metamorphism. 



Where slightly weathered, mica schist can be 

 easily mined. Recent work by the U.S. Bureau of 

 Mines (Browning and Bennett, 1965; Adair and 

 Browning, 1969) has developed methods of con- 

 centrating good-quality flake mica from mica schist. 

 Preliminary studies in North Carolina indicate that 

 the percentage of mica in schist ore is about four 

 times as high as that of mica ores now being mined, 

 and a flow sheet was developed that would make 

 mica-schist products competitive in the present 

 market (Lewis and others, 1971). 



Another type of flake-mica deposit mined in Lan- 

 caster County, S.C, is very fine grained and clay- 

 like. The original rock, apparently a volcanic ash 

 or tuff, has been altered to a mixture of sericite 

 (fine-grained muscovite) and quartz. The rock is 

 easily mined and requires minimum preparation for 

 sale as a mineral filler. 



Muscovite is one of the more stable rock-forming 

 minerals and under ideal conditions will be con- 

 centrated in sediments. Such a deposit of detrital 

 mica in Holocene sediments was mined in Davy 

 Crockett Lake, Greene County, Tenn., from 1956 

 to 1960. The conditions of formation of this deposit 

 are unusual ; the lake is downstream from the wash- 

 ing and grinding plants of the Spruce Pine pegma- 

 tite district, and fine mica, which was lost in min- 

 ing and processing of clay, feldspar, and scrap mica, 

 collected in the headwaters of the lake. Similar de- 

 posits may occur elsewhere in the world where 

 weathered metamorphic and igneous rocks are being 

 eroded. 



RESOURCES 

 IDENTIFIED AND HYPOTHETICAL RESOURCES 



Few quantitative data are available to estimate 

 reserves or resources of sheet mica. Most of the 

 deposits mined to date were found by testing sur- 

 face outcrops. Some mines have reached depths of 

 300—400 feet in the United States and as much as 

 800 feet in India. If the present surface is consid- 

 ered a random sample of what occurs at depth, then 

 many pegmatites in the world do not crop out but 

 are still within mining depth (less than 500 ft) . On 

 this basis, conditional and hypothetical resources of 

 sheet mica in the United States probably are at 

 least equal to the amount already mined — that is, 

 about 75 million pounds of sheet mica in deposits 

 similar to those mined but not exposed at the sur- 



face in the principal mica pegmatite districts men- 

 tioned previously. Unless the economics of sheet- 

 mica mining change radically, little of this hypo- 

 thetical mica will ever be looked for or mined. In 

 case of a national emergency, initial demand for 

 sheet mica will be filled by the Government stock 

 piles (table 84). 



Table 84. — Sheet mica on hand in all U.S. Government in- 

 ventories, ds reported by General Services Administration, 

 March SI, 1972 



[Classification: block mica, minimum thickness of 0.007 in. and minimum 

 usable area of 1 sq in.; film. mica, variable thicknesses rangrinsT from 

 0.0012 to 0.004 in. and minimum usable area same as block mica; 

 splittings, maximum thickness of 0.0012 in. and minimum usable area, 

 for top qualities, of 0.075 sq in. (Skow, 1962, p. 4)] 

 Classification Weight (pounds) 



Muscovite block, stained and better 10,648,007 



Muscovite block, nonstockpUe grade 3,603,422 



Muscovite film, 1st and 2d qualities 1,468,340 



Muscovite film, nonstockpUe grade 640 



Muscovite splittings 37,630,606 



Phlogopite block 16.721 



Phlogopite block, nonstockpile grade 137,217 



Phlogopite splittings 4,669,274 



Resources of sheet mica also occur similarly in 

 many countries throughout the world. Some of these 

 are Canada, Guatemala, Argentina, Bolivia, Colom- 

 bia, Austria, France, Norway, Rumania, the Soviet 

 Union, Burma, China, Australia, and several small 

 districts in Africa. The major resources of sheet 

 muscovite, however, are in India and Brazil, and 

 the major resources of phlogopite are in the Mala- 

 gasy Republic. No quantitative data are available 

 to estimate reserves or resources of sheet mica in 

 these countries, but the resources in the Bihar and 

 Madras areas of India are probably very large. 

 Resources in Brazil, West Africa, Rajasthan area 

 of India, and the Lake Baikal and Sayan Upland 

 areas of the Soviet Union are large (Skow, 1962, 

 p. 42). Resources in the other countries are small 

 to moderate. 



A few data are available to estimate resources of 

 scrap mica. Although mica-rich zones in some peg- 

 matite bodies may contain as much as 40 percent 

 mica, the largest resources of scrap mica are in 

 mica schists that may contain as much as 90 per- 

 cent mica and in muscovite granites that contain 

 10-20 percent. Using a conservative figure of 40 

 percent mica, the average mica schist would contain 

 1,450 tons of mica per acre-foot of outcrop area. 

 Mica schist formations are typically measured in 

 tens or hundreds of feet in outcrop width and miles 

 in outcrop length. Muscovite granites like the alas- 

 kite of Spruce Pine, N.C., or the granite of Stone 

 Mountain, Ga., are principally a source of feldspar 

 but contain 350-700 tons of byproduct or coproduct 

 mica per acre-foot and have outcrops measured in 

 many acres or square miles. Assuming a mining 

 depth of 100 feet, the resources of scrap mica per 



