Physical characteristics of commercial sheet muscovite in the southeastern United States

Abstract

Most raw muscovite is sold according to requirements specified by the purchaser, and for sheet material emphasis generally is placed on features subject to grading by careful visual examination. Specifications vary with individual end uses, and the chief users of high-quality sheet muscovite ordinarily set very exacting requirements. Correlations between end uses and specifications have not been fully developed for some physical properties of mica, particularly color and clarity. The investigations described in this report were undertaken to provide further basic data that bear on this problem, chiefly by correlation of significant electrical properties of sheet muscovite from the Southeastern States with geographic occurrence, geologic occurrence, and physical characteristics of direct commercial application. The present project, carried out jointly in 1945 and 1946 by the United States Geological Survey, the State of North Carolina, and the Tennessee Valley Authority, involved the preparation, electrical testing, color identification, and petrographic examination of 2,507 lots containing 237,764 pieces of mica. The test specimens were obtained from at least 850 deposits in the States of Alabama, Georgia, North Carolina, South Carolina, and Virginia. This report includes a general discussion of commercial muscovite in the Southeastern States, together with compilations, descriptions, and discussions of the tests and examinations. The uses of sheet mica are based upon its perfect cleavage, exceptionally low conductivity of heat and electricity, high dielectric constant and dielectric strength, heat resistance, noninflammability, low dielectric loss, mechanical strength, flexibility, elasticity, transparency, and ease with which it can be worked into final form. A very high proportion is used as electrical insulating material, and during the recent wartime period exceptionally heavy demands for sheet material of superior quality were based upon uses in radio and electronic equipment, aircraft generators and spark plugs, and other electrical apparatus. Such mica must have a power factor (a measure of the loss of electrical energy in a condenser in which the mica is the dielectric) of less than 0.04 percent at a frequency of 1 megacycle. The preliminary preparation of sheet mica involves separation from other minerals, rough splitting, rough trimming with the fingers, further splitting, and extensive trimming with a knife or other blade. This yields block mica, which is then classified according to size, degree of trimming, and quality. Quality can be classified by visual or electrical means, or by a combination thereof; specifications and methods of test have been published by the American Society for Testing Materials. Block mica generally is further prepared by additional splitting, trimming, and punching or stamping. The Southeastern States constitute the chief mica-producing area in the United States. Fourteen mining districts have yielded much sheet material, but the bulk of production is derived from deposits in western North Carolina. During the period 1912-44 the Southeastern States accounted for 25,028,404 pounds, or about 54 percent of the total United States production of sheet and punch mica, and its value of $9,720,273 amounted to 62 percent of the total. The average value of the output to January 1945 was about 39 cents per pound, in contrast to the value of 89 cents per pound during the wartime period 1942-44. The mica-bearing pegmatites range from lenses and stringers less than an inch thick to dikes, sills, and irregular masses several hundred feet long and 200 feet or more thick. In general, the distribution of book mica within a given deposit reflects the shape of the containing pegmatite body, but there appears to be little correlation between the size of the body and the quantity and coarseness of the mica that it contains. Most of the mica concentrations are confined to specific parts, or zones, of the enclosing pegmatite bodies and occur within many of these zones like the shoots of ore minerals in metalliferous deposits. The position and distribution of such shoots commonly can be correlated with the overall shape of the containing zones or with rolls, bends, bulges, constrictions, or other irregularities in these zones. The results of the color tests of block mica show that material from the Southeastern States covers essentially the entire range known for commercial muscovite. Drab, buff, and brownish ("ruby") mica is locally abundant, but in general the bulk of the sheet material produced from the entire region is green, yellowish olive, or brownish olive. Systematic color variations occur within some mica shoots, within single pegmatite bodies, and within large groups of pegmatite bodies. Variations in individual mica books and within single mica shoots are most pronounced where the muscovite is green or yellowish olive. On a larger scale, brownish books generally are nearer the walls of a given pegmatite body than green ones, and few brownish books occur in those pegmatites with green mica near the walls. Green books are rare in pegmatites with interior concentrations of brownish mica. A substantial proportion of Southeastern muscovite is mineral-stained, and little of this stain can be effectively removed by even the most careful splitting. In contrast, thicker and larger euhedral inclusions of biotite, apatite, garnet, pyrite and other sulfides, quartz, tourmaline, zircon, zoisite, and other minerals can be removed with little difficulty, leaving holes or distinct depressions in the host muscovite. Various types of curdy greenish or brownish stains and supergene clay, iron, and manganese stains are locally abundant, but air-stained books are very rare. Iron oxide stains are common in green, yellowish-olive, and brownish-olive micas but are rare in those that are buff and brown. Thin shreds and wisps of biotite are common in muscovite of nearly all colors. In general, mineral-stained mica is confined to the outer parts of those pegmatite bodies that also contain clear green books and to the inner parts of those that also contain clear buff or brown books. "Pinholes" are common in most districts but are abundant in the mica from only a few mines. Most of them occur in the brown and buff, or ruby, micas in which zircon and apatite inclusions are most numerous. Hair cracks, said to develop in part after rifting and trimming of sheet mica from certain deposits, are much more abundant in green micas than in the brown and buff varieties. The results of the electrical tests indicate a general correlation between power factor and physical appearance, but evidently there are numerous exceptions, particularly among stained micas. Nearly all these exceptions are consistent, however, in that the power factor of such mica is lower than that expectable on the basis of visual appearance alone. There appears to be a partial correlation between power factor and the amount of hematite-magnetite stain in sheet mica. On the other hand, many pieces of heavily stained mica evidently possess excellent electrical characteristics, whereas some perfectly clear pieces are inferior in this respect. As far as electrical characteristics are concerned, the best quality of stained mica cannot be distinguished from the poorest quality by ordinary visual means, but much muscovite that contains substantial quantities of iron—either as inclusions, as exsolved intergrowths, or in solid solution—has distinctly less desirable electrical properties than the iron-poor varieties. Green and brown mottling, "vegetable stain," wispy to platy inclusions of green and brown biotite, and some clay stains that do not transgress the laminae of the host mica appear to have little adverse effect upon its power factor. Neither the color nor the depth of color in clear muscovite appears to have any definite relation to its power factor, and clear green, yellowish-olive, and brownish-olive micas selected by visual means should prove as satisfactory for condenser use as clear buff and light-brown micas. The electrical tests have shown that there is no intrinsic difference, grade for grade, between clear ruby and clear nonruby micas from the Southeastern States and little discernible difference between clear domestic micas and clear material from India. The use of electrical testing and adoption of the combined visual-electrical system of mica classification proposed by the American Society for Testing Materials should result in upward qualification of much sheet material from the Southeastern States and might well go even farther toward improving the currently inferior position of the abundant greenish varieties in the trade. Electrical testing might also force rejection of some sheets of apparently high quality material, owing to pinholes or other defects difficult to recognize. Determination of power factor alone cannot suffice for the proper classification of sheet mica; hence it should be combined with the results of spark testing and careful visual appraisal. Electrical testing of much visually classified mica, especially mineral-stained sheets, should result in considerable conservation of condenser-grade material. This may well amount to 50 percent or more. Although such a procedure might be very important and even vital during periods of unusual demand or restricted supply, the increased cost of piece-bypiece or even lot-by-lot electrical testing might not permit competition with other methods of classification during ordinary times. Competition with foreign sources of supply, with ceramic, glass, treated paper, and plastic substitutes, and even with synthetic mica probably will increase as time goes on. On the other hand, improvements in the design of the electrical testing equipment are to be expected, and these devices probably will be adapted to continuous semiautomatic or automatic operation.

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