416 



UNITED STATES MINERAL RESOURCES 



are classified as punch mica and are included in the 

 general term "sheet mica." Built-up mica made 

 from very thin sheets and reconstituted mica made 

 from scrap can be substituted for larger sheet 

 mica for some uses. 



SHEET MICA 



Sheet mica is an important dielectric and insulat- 

 ing material used in the manufacture of electronic 

 and electrical equipment, such as electronic vacuum 

 tubes for radio and television, capacitors, motors, 

 transformers, generators, and electrical appliances. 

 Nonelectric uses include shields for high-tempera- 

 ture steam-gage glasses, regulator diaphragms, and 

 in years gone by, stove windows and lamp chimneys. 

 The value of sheet mica depends on the color, size, 

 structure, and quality of the natural crystals. Qual- 

 ity of sheet mica varies widely within and between 

 mica districts and even within a single deposit or 

 mica-rich shoot. 



A variety of synthetic materials — such as built-up 

 mica, reconstituted and glass-bonded mica, organic 

 polymers, and synthetic mica — and alternate ma- 

 terials — such as alumina ceramics, fused quartz, and 

 talc — are replacing sheet mica in some uses. Tech- 

 nological developments, such as miniaturization of 

 circuits and solid-state electronics, also reduce the 

 need for sheet mica. 



Muscovite in sheets as thin as paper is generally 

 colorless, but thicker sheets may range from red or 

 yellow brown to greenish brown or green. Reddish 

 brown or ruby mica is generally thought to be of 

 slightly better quality and brings a higher price 

 than the other varieties of mica. 



The best sheet mica is clear, flat, and free of 

 mineral inclusions and structural defects. Mica that 

 contains inclusions or impurities is called stained 

 mica. Primary impurities are air bubbles, mottling, 

 and mineral intergrowths ; secondary impurities are 

 air creep, clay, iron oxide and manganese oxide in- 

 clusions, and organic or vegetable stain. 



Structural defects of the mica crystals are also 

 of primary and secondary origins. The most com- 

 mon primary defects are reeves and wedge struc- 

 ture. Reeves are lines, striations, or shallow corru- 

 gations that lie in the plane of the cleavage. Where 

 two sets of reeves intersect at an angle of nearly 

 60°, the resulting mica is said to have "A" structure. 

 Wedge structure is caused by interlayering of sheets 

 of unequal size. Some muscovite crystals, or books, 

 have discontinuous sheets, partial intergrowth of 

 sheets, or internal distortions and do not split freely. 

 These books, termed "locky," or "tangled," are used 

 only for scrap. Secondary structural defects include 



bending, cracking, twisting, and ruling, all caused 

 by deformation of the mica after crystallization. 

 Ruling is a secondary cleavage that cuts the basal 

 cleavage at an angle of nearly 67°. 



Crude books of mica must be at least 2 inches 

 across to yield trimmed sheet mica. The average 

 commercial book is about 5 inches across and about 

 Vs to 1/2 as thick as it is wide. In some deposits 

 books measuring 8 to 12 inches are common. Some 

 larger books as much as several feet across and 

 weighing 50-300 pounds have been found. 



SCRAP MICA 



Many pegmatite deposits contain only scrap- 

 quality mica, and a large amount of scrap is pro- 

 duced during the mining, trimming, and fabricat- 

 ing of sheet mica. Such scrap is of high quality and 

 is in great demand for making mica paper or recon- 

 stituted mica. Scrap mica obtained from clay or 

 feldspar deposits or from mica schists is generally 

 finely divided and is often called flake mica. 



Scrap mica from sheet operations, from scrap 

 mining, and from clay and feldspar mining may be 

 blended before grinding to either dry-ground or 

 wet-ground mica. Wet-ground mica is produced in 

 chaser mills using wooden grinding surfaces. The 

 finished product is usually white and has a greasy 

 ungent feel and a pronounced sheen (Broadhurst 

 and Hash, 1953, p. 9). Individual particles are 

 highly polished and have smooth edges. Particle 

 size generally ranges from 80 to 325 mesh. Wet- 

 ground mica is used chiefly in the manufacture of 

 wallpaper, rubber goods, and paint. 



Dry-ground mica is produced in rod mills, ham- 

 mer mills, cage disintegrators, and various types of 

 attrition mills (Skow, 1962, p. 89), The product is 

 a flat white powder having little luster; individual 

 particles have rough edges and little polish. Particle 

 size ranges from 20 to 250 mesh. Some special mi- 

 cronized mica has particle sizes of 1,000-3,000 mesh, 

 or 5-20 microns. Dry-ground mica is used chiefly 

 in the manufacture of rolled roofing or asphalt 

 shingles, wallboard or joint cement, oil-well drilling 

 mud, paint, rubber goods, plastic, and welding rods. 

 The value of scrap mica depends on color and free- 

 dom from impurities. Various other mineral fillers 

 can substitute for ground mica in most uses. Syn- 

 thetic fluorophlogopite can substitute for scrap in 

 reconstituted mica (Shell and Ivey, 1969, p. 214). 



EXPLOITATION 



Mica was mined in North Carolina, Virginia, and 

 Alabama by the Indians more than a thousand years 

 before the coming of the white man (Prufer, 1964, 



