Physical Characteristics 

of Commercial 

Sheet Muscovite 

in the Southeastern 

United States

GEOLOGICAL SURVEY PROFESSIONAL PAPER 225

Prepared under a cooperative agreement 

with the State of North Carolina and the 

Tennessee Valley Authority
Physical Characteristics 

of Commercial 

Sheet Muscovite 

in the Southeastern 

United States

By RICHARD H. JAHNS and FORREST W. LANCASTER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 225

Prepared under a cooperative agreement 

with the State of North Carolina and the 

Tennessee Falley Authority

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1950
UNITED STATES DEPARTMENT OF THE INTERIOR

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 31.25 (paper cover)
CONTENTS

Page

Abstract.-_.____— ——— __ —. —. — ----- — ._-__ — __ 1

Introduction. _._—_______-__--____--_--_-_-__-___--_.-— 2

General statement___--__-------_-----_-------__--__- 2

Field and laboratory work.-_.___-._.__.__.__.___. 3

Acknowledgments.-.--. __-._-__-._ ___________________ 4

Commercial sheet muscovite_----_-_--_-__-__---__-_-_____ 4

General properties.__________________________________ 4

Properties affecting value-----------_--__----___-____- 6

Cleavage...__„______.____-_-_.--_-___ 6

Hardness, flexibility, and elasticity._______________» 7

Structural imperfections.__—_-______--_-____--__ 8

Color..______________-._-—-- — „.__ 11

Staining, intergrowths, and inclusions______________ 12

Primary impurities-... ——___________________ 12

Secondary impurities.______.________-.-_.__._ 15

Electrical properties_-__-____-______-_--___--_.-_- 16

Uses— ___--__- — __.-__----_--_--___---____--__-, 16

Grading and classification_---_--_-__---__-----__-_-_-_ 17

Visual methods.._..__-___.________—_________ 17

Electrical methods.___________________________. 22

Combination method of the American Society for Testing Materials.... ______________________________ 24

Preparation and marketing----___--______-_-_-_--_---_ 26

Page 

Sheet muscovite in the Southeastern States.____________ 28

Distribution of deposits———————————_———_—_ 28 

History and production——_—_———___________. _ 29

Occurrence- .___—__—______———__—______ 30

General geologic features—————__—_———___ 30 

Pegmatites———————————————————————— 31 

Mica deposits. ___——_—————_—___—_____ 33

Tests of sheet muscovite from the Southeastern States————_ 34

General statement...————————————————————_ 34

Color tests.—————————————————————————— 42

Methods of testing——————————————————.._ 42

Results...,———————————————————————— 42

Petrographic examination.._—________—_———....._, 43

___ Staining and inclusions-...___-._-__.__--__..... 43

Pinholes and hair cracks____—____.._._._.____ 44

Electrical tests___-_-_——————————————.——... 45 

Apparatus and methods of testing._ _——————__. 45 

General tests..--..—_—_—__________________ 45

Tests of visually qualified mica..———_ ——— _______ 97

Summary and conclusions.———————— ——— ____-_-__-.__ 103

List of references..—„__-__—_.__—__-_______.--_-____._ 106

Index__________----___-.-_---____.__-__.--_ 109

ILLUSTRATIONS

Page

PLATE 1. Books and cleavage pieces of muscovite__-____--_--_-__-_____--_----_-----------------_-------------_---_---_ 8

2. Reeve structures in muscovite-_______________________._____________________-__-__---_-____-___----__.__ 9

3. Wedge structure and deformation in muscovite____-____--_--_--_--___-____----_-----_---_----------__------__ 10

4. Ruling in cleavage pieces of muscovite__.______________________-_____-__-_--_-_----__---____-___--___-__ 11

5. Primary stain in muscovite_______-__-_---_________.____________________-_-_________-___--_----__-_-_ 14

6. Primary hematite stain in muscovite---....-.. ___________________________--___----____-__-----_--__---_---__ 15

7. Color variations and primary stain in muscovite________-_-_______---_-----___-_-_-----_---_---_.----_--_-_.-. 16

8. Mineral inclusions and clay stain in muscovite—_____...__-__-___—-___ ———— __ — _ —— -_-___-__ — __-__ 17

9. Test sets for electrical classification of sheet mica_________—___.__---__---.-_ ——— _____--__--___--__--__ 24

FIGUKE 1. Typical single and twinned crystals of muscovite_______.._————_-.__--_--.-__ —— — -___-_--__--_----._-__ 5

2. Orientation of pressure and percussion figures in muscovite—- ———— ._—______ ———— ___._ — —— __-__-_-____ 6

3. Inclusions of muscovite, quartz, and apatite in muscovite..-._-_.____-__-_-_---_——_-_-_______-_-____-_.____ 7

4. Relation of reeves to crenulations and discontinuities in muscovite laminae_____-_-___--_----_---_ —--------._---_ 8

5. Relation of reeves to crystal directions in muscovite-.______..____.__________-______---_.-_--_-__-___--____-_ 9

6. Deformation of muscovite laminae_--____-____._.______________________-__- — _-__--__--__---__--_____-_ 10

7. Types of muscovite fragments formed by breaking along parting or ruling planes—______-__-__---____-_--_---____ 10

8. Biotite-muscovite intergrowths_______________________________________—_____——_________________________ 15

9. Size-grading chart for domestic block mica______________________--_-_-___ — ——— _-_--__-_-___---___-____-_ 19

10. Chart for grading mica according to Indian standard sizes___.___---_.. ———— ———— __-_.__ — _.__-_____—— 20

11. Simplified circuit of spark-coil set for testing sheet muscovite——___———— ——————— —— _- —. — — ... — -. 23

12. Simplified circuits of equipment for determining power factor of sheet muscovite—————— _____ — _________ — __._ 24

13. Meter scale of the Bell Telephone Laboratories' Q-test set__________-_--_-—_--_---—--—------------.------ —__- 24

14. Outline of operations involved in the extraction and preparation of sheet, punch, and scrap mica——____-___ —______ 26

15. Distribution of principal pegmatite areas and districts in the southeastern United States.__ —— —————— .-_-._____ 29

16. Idealized plans of a pegmatite body, showing typical distribution of pegmatite units———_---_-__,-__——._--__-._ 32

III
IV

TABLES

1. Theoretical composition of muscovite, compared with composition of specimens from several localities.______________ 5

2. Domestic and Indian size groups for sheet mica, including punch and sheets larger than punch_____________________ 18

3. Electrical, physical, and visual quality requirements of natural block mica and mica films for use in capacitors..______ 25

4. Q and power-factor values for electrical-quality groups E-l, E-2, and E-3____-_-_---_-____--_-_-----___-__-__-__ 26

5. Price ranges of clear and stained sheet mica in the Southeastern States during the period 1910-40.-_-__.-___________ 27

6. Price schedules for domestic clear sheet mica during the period December 1941-February 1945—— _________________ 28

7. Summary data on the number of mica deposits in the southeastern United States._.____________-_-__-____-_-___-_ 30

8. Production of sheet and punch mica, 1912-14, in North Carolina, the Southeastern States, and the entire United States. _ 30

9. Occurrence of stain in 2,148 test lots of muscovite from the Southeastern States__________________________________ 44

10. General test data for mica specimens, arranged by lot and by mine or prospect-_-___--_-___-_-.___---___-_____--_ 47

11. Results of electrical tests of mica samples, from specific mines or deposits, that include material of E-2 or E-3 quality. _ 96

12. Test data for mica, taken from the production line of the Colonial Mica Corporation in 1945, previously qualified by

visual means__________________________________________________________________________ — _______________ 98

13. Test data for mica (not identified by mine), taken from the production line of the Colonial Mica Corporation in 1943

and placed in storage, qualified by visual means in 1943________-____________--_-______--_-__--_-___-_-______ 102

14. Results of electrical tests of mica samples, taken from the production line of the Colonial Mica Corporation in 1943,

that include material of E-2 or E-3 quality_____________--__-___---__-_-------_---__---_--__---__---_____-- 103
PHYSICAL CHARACTERISTICS OF COMMERCIAL SHEET MUSCOVITE 

IN THE SOUTHEASTERN UNITED STATES

By RICHARD H. JAHNS and FORREST W. LANCASTER

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
COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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.

INTRODUCTION

GENERAL STATEMENT

Sheet muscovite is used mainly for special electrical 

installations and equipment, both as simple pieces 

split from individual crystals and as composite pieces 

built up from many thin splittings. Most raw mica is 

graded and classified according to quality, size, and 

thickness and is sold according to requirements specified by the purchaser. Quality classification is based 

upon such features as hardness, ease and perfection of 

splitting, flexibility and elasticity, color, reaction to 

heating, electrical properties, and amount and distribution of structural imperfections, stain, and inclusions. Emphasis generally is placed upon features 

subject to qualification by careful visual examination. 

Classification according to size and thickness is based 

upon the dimensions of usable material within a given 

piece of mica.

Specifications vary with individual end uses. It is 

only natural that fabricators and other purchasers of 

raw mica should set high quality requirements for 

specific uses, thereby reducing the proportion of material that conceivably might cause failure in finished 

equipment. Correlations between end uses and specifications have been developed for some physical properties of muscovite through long periods of trial and 

careful study, and tolerances are recognized and understood within narrow limits. Little basic or empirical information is at hand for other properties, 

however, and specifications concerning them appear 

to be founded upon less certain ground. The setting 

and maintenance of high acceptance standards for 

the less fully understood properties of commercial 

sheet muscovite have made it difficult to determine 

whether or not so-called inferior types of material are 

satisfactory for a given use. The effects of color, stain, 

and inclusions on the electrical properties of muscovite are perhaps least known, and there are serious 

gaps in the correlation of these with other properties.

In the absence of data that would permit positive 

assignment of a certain physical feature in a given 

lot of mica as the cause of an undesirable proportion 

of failure in finished electrical equipment, any reluctance to lower consumers' specifications is easily 

understood. On the other hand, the steadily growing 

demand for muscovite of superior grade, especially 

during the past two decades, has forced some use of 

lower-quality mica and even the substitution of other 

materials. Such conservation measures became a 

practical necessity because of the unprecedented de-
INTRODUCTION

mand during the recent wartime period, when the 

entire problem was thrown into sharp relief. The 

best qualities of mica were allocated only for the manufacture of articles in which no other material was 

judged usable, and supplies were further conserved 

by requiring the use of poorer qualities wherever possible, by the use of the smallest possible sizes, by the 

reduction of waste and the reclamation of partly 

spoiled material, and by the substitution of ceramics, treated paper, or other insulating substances 

wherever practicable.

Most of the recent wartime substitution was 

ordered in great haste and under the stress of expediency, so that much of it was necessarily based 

upon incomplete and empirical information. Nevertheless, the program of conservation was markedly 

successful in terms of equipment performance. With 

a return of peacetime conditions, however, there has 

been a partial return of higher specifications for raw 

sheet muscovite. The degree to which this return becomes complete may well depend upon the results of 

extensive fundamental tests on physical properties, 

methods of preparation, and methods of grading, 

classification, and testing carried out by several organizations during the war. Many of these tests were 

parts of longer research programs that are currently 

active, but some of the results already have been released in preliminary form.

The question of the suitability of a given piece of 

muscovite for a given use involves so many variables 

that no single series of tests seems capable of providing a general answer, but the investigations described 

in this report were designed to bring together as many 

types of data as possible. Some significant electrical 

properties of sheet muscovite from Virginia, North 

Carolina, South Carolina, Georgia, and Alabama were 

determined and then were correlated and compared 

with geographic occurrence, geologic occurrence, and 

physical characteristics of direct commercial application. The report contains a general discussion of commercial muscovite in the Southeastern States, descriptions of tests made on thousands of specimens, 

and a discussion of the results of these tests. In the 

compilation and integration of data on physical properties, emphasis was placed upon factors of economic 

rather than theoretical significance.

FIELD AND LABORATORY WORK

During the period 1939-46 the United States Geological Survey carried on extensive field investigations of mica deposits in the Southeastern States. The 

North Carolina Department of Conservation and Development cooperated in this work from July 1941 

through the year 1946. The investigations were intended in part to supplement the results of earlier

studies by Sterrett 1 and in part to provide new and 

more detailed coverage of structural, mineralogic, 

and economic features. The work involved some areal 

geologic mapping and the examination of 1,644 deposits, more than 400 of which were mapped in detail. 

Samples of muscovite were collected from stock piles 

or lots of mine-run material at operating mines, from 

the walls and backs of mine workings, and from 

dumps at mines no longer accessible. Most of the mica 

described in this report was obtained by J. C. Olson 

during the early stages of the work and, during the 

later stages, by W. R. Griffitts, E. W. Heinrich, J. E. 

Husted, W. P. Irwin, R. H. Jahns, P. W. Lancaster, 

R. W. Lemke, J. M. Parker III, and R. A. Swanson. 

Where different types of muscovite were recognized 

in the same pegmatite body, an attempt was made to 

determine their respective spatial and genetic relations.

Mica collected from 124 mines in 1939 and 1940 was 

electrically tested by the National Bureau of Standards, and several samples were tested in greater detail by two large fabricators of electrical equipment.2 

Early in 1942 Jasper L. Stuckey, State Geologist of 

North Carolina, suggested more extensive and systematic investigations that might further define the potential usefulness of Southeastern sheet muscovite 

as a critical wartime material. Plans were made for 

electrical testing of samples from many mines, but it 

was not possible to begin the work at that time.

Late in 1944 the War Production Board issued a 

directive that only ruby sheet mica should be purchased from domestic producers by the Federal Government after January 1, 1945. The directive reflected the increasingly favorable position of the 

United States in terms of sheet-mica stocks, but it 

raised several serious problems. In many instances it 

proved exceedingly difficult to distinguish ruby from 

nonruby micas with assurance, and it was some time 

before a usable definition for "ruby" mica and a procedure for color grading of mica were developed.3 

Moreover, a real need for discrimination between 

types of commercial sheet muscovite on the basis of 

color, as implied by the directive of the War Production Board, does not yet appear to have been satisfactorily demonstrated.

The question raised concerning the quality of socalled nonruby mica, which constitutes a large part 

of the output from the Southeastern States, focused 

attention more strongly than ever on the need for re1 Sterrett, D. B., Mica deposits of western North Carolina: U. S. Geol. 

Survey Bull. 315-M, pp. 400-422, 1907; Mica deposits of North Carolina: 

U. S. Geol. Survey Bull. 430-J, pp. 593-638, 1910; Mica deposits of the 

United States: U. S. Geol. Survey Bull. 740, 1923.

2 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942.

* Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. 

Research, vol. 35, pp. 253-256, 1945.
COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

liable correlation between physical properties and 

commercial classification of sheet material. Plans for 

a program of mica testing by electrical means were 

revived and greatly expanded in March 1945, when 

Jasper L. Stuckey arranged a conference with H. S. 

Rankin and Charles E. Hunter, of the Tennessee 

Valley Authority, and R. A. Laurence and R. H. Jahns, 

of the United States Geological Survey. It was decided at this time that electrical testing of numerous 

samples of mica representing many different deposits 

might well establish within reasonable limits the influence of color, staining, and other properties on the 

electrical behavior of sheet mica.

G. A. Rosselot, of the State Engineering Experiment Station, Georgia School of Technology, was employed as a Consultant to investigate previous work 

of this general nature; he made valuable recommendations based on visits to the Bell Telephone Laboratories in New York City and the laboratories of 

the Battelle Memorial Institute in Columbus, Ohio, 

where he discussed the proposed studies with other 

investigators in the field. Helpful advice and suggestions were also contributed by W. J. Alexander, 

L. A. Norman, Jr., and Bradley Johnson, of the Colonial Mica Corporation. The North Carolina State 

College made available the services of F. W. Lancaster for conducting the electrical testing.

The project was carried out jointly by the State of 

North Carolina, the Tennessee Valley Authority, and 

the United States Geological Survey during the period 

July 1945-April 1946. All samples collected by the 

Geological Survey were studied, and several lots of 

prepared and qualified mica loaned by the Colonial 

Mica Corporation also were investigated in detail. 

The spark-coil test set and the rapid, direct-reading 

Q-meter developed by the Bell Telephone Laboratories were used for the electrical testing, which was 

done in Asheville, N. C., during the summer of 1945 

by F. W. Lancaster. Careful color determinations 

were later made by Frances H. Jahns in Asheville 

and Spruce Pine, N. C., and other physical characteristics were studied by R. H. Jahns. These investigations were completed in the laboratories of the California Institute of Technology, Pasadena, Calif. A 

total of 2,502 lots containing 237,764 pieces of mica 

was examined and tested. This material was obtained 

from at least 850 deposits.

ACKNOWLEDGMENTS

Many members of the United States Geological 

Survey participated in the collection of samples and 

the accumulation of basic geologic data on mica deposits, and without their help it would have been impossible to obtain a reasonably complete background 

for the testing program. It is a pleasure to acknowledge the contributions of W. B. Alien, H. K. Dupree,

Edward Ellingwood III, V. C. Fryklund, P. W. Gates, 

L. Goldthwait, W. R. Griffitts, J. B. Hadley, E. W. 

Heinrich, F. W. Hinrichs, J. R. Husted, W. P. Irwin, 

T. L. Kesler, M. R. Klepper, D. M. Larrabee, R. W. 

Lemke, Roswell Miller III, J. J. Norton, J. C. Olson, 

J. J. Page, J. M. Parker III, L. C. Pray, L. W. Seegers, 

R. L. Smith, J. H. Stillwell, W. C. Stoll, R. A. Swanson, and J. R. Wolfe, Jr., in this connection. W. J. 

Alexander, southern district manager for the Colonial Mica Corporation, drew generously on his broad 

experience in giving general advice and in commenting upon the results of the tests. He also provided 

facilities and equipment on several occasions and expedited the investigations in many other ways. Helpful suggestions and comments were made from time 

to time by Bradley Johnson, Adrian Newhouse, and 

L. A. Norman, Jr., of the Colonial Mica Corporation.

The chief organizers of the project, J. L. Stuckey 

and H. S. Rankin, retained an active interest from its 

inception to its close and were primarily responsible 

for the planning, financing, and successful completion 

of most phases of the work. C. E. Hunter, of the 

Tennessee Valley Authority, contributed many helpful suggestions; Jane Anderson, of the North Carolina State Department of Conservation and Development, compiled most of the electrical-testing data; 

and L. L. McMurray, superintendent of the Asheville 

laboratories, North Carolina State College, aided 

greatly in compiling and checking the electrical and 

other data for table 10. Frances H. Jahns assisted 

in the sorting and classification of samples, in the 

determination of several physical properties of the 

muscovite, and in the final preparation of the manuscript. The technical assistance of Ida M. Morgan, 

Madeline F. Harding, Joan T. Rounds, and Louis 

Reeder also is gratefully acknowledged. The report 

was critically reviewed by H. S. Rankin and J. L. 

Stuckey and by E. N. Gamer on and L. R. Page, of the 

United States Geological Survey.

Mine owners and operators were uniformly cordial 

and cooperative throughout the investigations. Permission to collect samples for purposes of examination 

and testing was granted freely and consistently. H. 

A. Knight, of High Point, N. C., was particularly 

helpful in lending nearly 200 pounds of prepared electric (stained sheet) mica for detailed testing, and 

he generously permitted the retention of a portion of 

this material as a reference sample. Smaller lots of 

commercial muscovite were loaned or donated by 

other mine operators as well.

COMMERCIAL SHEET MUSCOVITE 

GENERAL PROPERTIES

Minerals of the mica group are complex aluminum 

silicates whose outstanding characteristic is an almost perfect basal cleavage. In the true micas this
COMMERCIAL SHEET MUSCOVITE

TABLE 1.—Theoretical composition of muscovite, compared with composition of specimens from several localities

SiOj.....

TiO2 ._...

AbOs---. 

Fe2O3 __

MmOa...

FeO--.._

MnO....

MgO_._.

CaO.---.

K2O.-..

Na*O._..

LisO._...

H2O— ._

F ___ ...

Total 

O correcCorrected totalTheoretical 

composition

45.2

38.5

11.8

4.5

100.0

Composition of specimens

Hiddenite 

mine, 

Alexander 

County, 

N. C.'

45.40 

1.10 

33.66 

2.36

N.d.

1.86

8.33 

1.41 

Trace 

5.46 

.69

100.27 

.29

99.98

Miask, 

Ural 

Mtns., 

Siberia 2

44.17

37.35 

1.29 

N.d. 

.20 

.10

10.00 

1.14

5.73 

.90

100.88 

.37

100.51

Bengal , 

India'

45.57

36.72 

.95 

N.d. 

1.28 

N.d. 

.38 

.21 

8.81 

.62 

.19 

5.05 

.15

99.93

— — — -

Auburn, 

Maine 4

44.48

35.70 

1.09 

N.d. 

1.07 

N.d. 

Trace 

.10 

9.77 

2.41 

Trace 

5.50 

.72

100.84

Harding 

mine, 

Taos 

County, 

N. Mex. *

44.80

37.72 

.67 

.21 

N.d.

None

10.66 

1.40 

Trace 

4.52 

.20

100.18 

.08

100.10

1 Clarke, F. W., Analyses of rocks and minerals from the laboratory of the 

United States Geological Survey: U. S. Geol. Survey Bull. 591, p. 330, analysis D, 

1915.

2 Idem, analysis E.

3 Dana, E. S., The system of mineralogy of James Dwight Dana, p. 617, analysis 1, 1909.

4 Idem, analysis 6.

5 Schaller, W. T., and Henderson, E. P., Purple muscovite from New Mexico: 

Am. Mineralogist, vol. 11, p. 12, 1926.

cleavage permits splitting into thin sheets, laminae, 

or films that are tough, flexible, elastic, and extremely 

low in electrical and heat conductivity. In contrast, 

the clintonites (or "brittle" micas) and the chlorites 

yield cleavage sheets that are brittle and inelastic. 

The true mica group includes several species, one of 

which is muscovite, or potassium mica. Biotite contains iron and magnesium, and phlogopite contains 

magnesium and a little fluorine. Among the other 

species are lepidolite, a lithium mica; zinnwaldite, a 

lithium-iron mica; and roscoelite, a vanadium-bearing iron-magnesium mica. Sodium, barium, manganese, chromium, titanium, and other elements are 

minor, generally rare constituents in the group. All 

micas yield water upon ignition; ordinarily this 

amounts to 4 or 5 percent of the molecule.

Muscovite is the only true mica that is sufficiently 

abundant in the Southeastern States to be of present 

commercial importance in sheet form, and the discussions of mica in this report are therefore devoted to 

this species alone. In general it is an orthosilicate of 

aluminum, potassium, and hydrogen, with the theoretical composition shown in table 1. Analyses of 

specimens from five localities are included in the table 

for comparison. Muscovite is chemically very stable 

and is little decomposed by weathering or by ordinary 

acids in the laboratory. It is said to yield water, however, when heated to temperatures above 400° C.,4 

and in some varieties this change is accompanied by 

swelling and loss of elasticity and transparency.

Muscovite crystallizes in the monoclinic system but 

is nearly hexagonal in symmetry. Much commercial 

mica occurs in rough crystals or "books," some of 

which are partly or completely bounded by poorly 

developed faces. The books generally are tabular to 

equant, with their shortest dimension perpendicular 

to the cleavage direction. Some are elongated normal 

to the cleavage direction and are characteristically 

tapered; many of these are distorted by slippage along 

the cleavage planes and are known as "step crystals." 

Most well-developed crystals are hexagonal or rhombic in outline, with six or more faces forming the 

margins of cleavage pieces (pi. 1). The simplest forms 

comprise basal, prismatic, and clinopinacoidal faces 

(fig. 1), but modifying faces are common. As obM

M

B

M

M

I

4 Horton, F. W.. Mica: U. S. Bur. Mines Inf. Circ. 6822, p. 6, 1935 (revised 1941).

D

FIGURE 1.—Typical crystals of muscovite, showing the most common faces: 

c, hase ; M, prism; h, clinopinacoid; N, prism. A, Simple crystal; B, C, 

and D, Twinned crystals.
6 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

served on a cleavage surface or basal face of a crystal, 

the traces of the prismatic and clinopinacoidal faces 

meet at angles of almost exactly 120°. Twinning is 

common, chiefly with the crystals united along the 

base or along irregular surfaces nearly perpendicular 

to the base (fig. 1).

The symmetry of muscovite crystals is clearly 

shown by percussion figures. If a cleavage plate of 

sheet mica is struck sharply by a blunt punch or large 

needle with a dulled point, a partial or complete sixrayed pattern of cracks is developed. Individual rays 

of this percussion figure extend outward from the 

point of impact, and their orientation is constant regardless of where the mica piece is struck (pi. 1). 

Two of the cracks, generally deeper and longer than 

the other four, intersect the cleavage surface to form 

a single line parallel to the trace of the clinopinacoidal 

face (fig. 2). The others form two lines that meet

FIGURE 2.—Pressure and percussion figures in muscovite, showing orientation with respect to the prismatic (M) and clinopinacoidal (b) faces.

at angles ranging from less than 53° to nearly 56° and 

are nearly parallel with the trace of the prismatic 

faces.5

A second type of figure can be developed by firmly 

pressing a dulled point against the piece of mica, 

particularly if the mica rests on a flat but elastic surface. Like the percussion figure, this pressure figure 

is six-rayed wherever complete, but only two or three 

rays are commonly developed. Some pressure figures 

are formed simultaneously with percussion figures, 

and the two have common centers. One set of cracks 

is perpendicular to the trace of the clinopinacoid and

6 Walker, T. L., Observation's on percussion figures on cleavage plates of 

mica: Am. Jour. Sci., 4th ser., vol. 2, pp. 6-7, 1896; Sterrett, D. B., Mica 

deposits of North Carolina: U. S. Geol. Survey Bull. 740, p. 12, 1923.

hence perpendicular to the principal direction of the 

percussion figure (fig. 2). The others are perpendicular to the prism faces and therefore do not bisect the 

angles between the rays of the percussion figure. The 

cracks of the pressure figure coincide with distinct 

glide or parting planes in the crystal and meet the 

cleavage planes at an angle of about 67°. Both percussion and pressure figures are very useful in determining the crystallographic orientation of mica 

books, especially where no crystal faces are present. 

In this report the orientations of other features are 

generally described in terms of percussion- and pressure-figure directions, which serve as convenient references.

Muscovite is transparent and nearly colorless when 

split into thin sheets, but most thick plates are distinctly colored in shades of green, brown, yellow, 

orange, and pink. Phlogopite, the other mica commonly used as an electrical insulator, is yellow, golden 

brown, dark brown, or black; much of it has a coppery, 

translucent appearance, and in general it is less transparent than muscovite. Freshly cleaved fragments of 

muscovite have a hard, brilliant luster, in contrast to 

the dull, rough surfaces of most books. The mineral 

ranges in hardness from 2.0 to 2.5 and is easily cut 

by a knife. In general the hardest books are the least 

flexible; commonly they can be split only with difficulty. The specific gravity of muscovite ranges from 

2.76 to 3.00.

Many muscovite books are intergrown with biotite 

or enclose smaller biotite crystals, and some muscovite occurs as well-crystallized inclusions in books of 

biotite. The cleavages of the two micas generally are 

parallel. Inclusions of magnetite, hematite, and 

goethite occur in the muscovite of some deposits, commonly as latticelike crystals or crystal aggregates 

that are distributed in accordance with the crystal 

directions of the host mineral. Such inclusions are so 

abundant in some books that thin cleavage pieces of 

the mica are opaque or nearly so. Apatite, garnet, 

quartz, zircon, tourmaline, and many other minerals 

also occur as inclusions within or between the mica 

laminae.

PROPERTIES AFFECTING VALUE 

CLEAVAGE

The edges of most crystals or books of muscovite 

are twisted, crushed, tangled, and irregularly intergrown with other minerals and hence must be cut 

away before the remaining material can be split into 

films or thin sheets. Owing to their elasticity, strength, 

and almost perfect basal cleavage, undistorted books 

or parts of books readily yield films less than onethousandth of an inch (1 mil) thick. Films that are 

uniform in thickness and have fiat or very nearly flat 

surfaces are commercially the most desirable. Their
COMMERCIAL SHEET MUSCOVITE

value increases with their size, and the rate of increase is greatest in the largest sizes. Book mica that 

yields such films is said to be free splitting. The films 

can be easily tested for constancy of thickness by 

means of machinist's or dial micrometers or by placing them between crossed polaroids and observing 

whether uniform interference colors are obtained.

Some mica does not split uniformly, but tears into 

irregular partial films. Such material may split evenly 

and easily in some places but very imperfectly in 

others. It is known in the Southeastern States as

'locky," "gummy," "tangled," "tanglesheet,' 'tanglefoot," or "tacky." The designation "tanglesheet" is 

also applied to coarse aggregates of irregularly intergrown books, which commonly form masses of "bull" 

mica several feet in maximum dimension. Discontinuity of cleavage, or lockiness, generally is caused 

by a partial intergrowth of books or of laminae in a 

single book (fig. 3), by internal distortion of the book, 

by finely divided inclusions, or—rarely—by twinning 

with composition planes nearly normal to the base 

(fig. 1). Many locky books do not differ markedly in 

appearance from those that split freely and evenly. 

The hardest varieties of muscovite commonly are 

more difficult to split than softer varieties, but numerous exceptions are known. Many very dark varieties 

also are not free splitting, but others yield large, uniform sheets. There appears to be some correlation 

between the lockiness of Southeastern micas and the 

presence of disseminated flakes, shreds, and very 

thin plates of biotite and chlorite—especially in the 

hard brown, brownish-olive, or buff micas—but more 

detailed microscopic study is needed to demonstrate 

the mechanism and consistency of this relation. The 

effects of larger inclusions are clearer. Prismatic 

crystals of apatite and quartz, for example, commonly 

lie nearly normal to the mica cleavage and hence are 

like nails driven through a series of thin boards (fig. 

3). Such mica is said to be "tied," "nailed," or "naillocked." Similar tying is accomplished by small elongate crystals of muscovite that are oriented obliquely 

to the host crystal (fig. 3).

HARDNESS, FLEXIBILITY, AND ELASTICITY

The hardness of Southeastern mica varies from one 

deposit to another, less commonly from one book to 

another, and in some instances within a single book. 

In general the brown, cinnamon-brown, and buff 

micas are harder than the greenish-olive and green 

varieties in the Southeastern States. Where other 

factors are equal, the hardest varieties are the least 

flexible and consequently are among the most difficult 

to split. Relative degrees of hardness are easily determined by judging the ease with which sheets of 

known thickness can be cut, by bending them slightly,

or by tapping them against a thick piece of wood or a 

knuckle of the hand. Pieces of very hard mica sound 

like glass when shaken together.

Flexibility and elasticity are important properties, 

especially in mica that must be bent sharply without 

breaking or must be exposed to unusual jarring or vibration. To meet most commercial specifications a 

sheet of mica 0.005 inch (5 mils) thick must return

FIGURE 3.—Inclusions in muscovite. A, Intergrowth of muscovite in muscovite. Orientations of sheets are shown by percussion figures. B, Inclusions 

of apatite (ap) and quartz (qtz) in muscovite. C, Locking inclusions of 

muscovite in muscovite. The cleavage planes of the inclusions are not 

parallel with that of the host crystal.

promptly to its normal planar condition after being 

wrapped around an ordinary lead pencil and then released. Flexibility and elasticity are most seriously 

affected by cracks, holes, and other structural defects. 

Most of these are easily recognized, but some extremely thin and fine cracks commonly escape detection. These are known as "hair cracks" or "hair 

lines." Films of mica in which some laminae are haircracked generally fail when bent, especially if the 

flaws are abundant or if they extend through considerable thicknesses of the films. Such mica is termed 

"brittle."

Where exposed to the weather for long periods, 

muscovite loses its luster and gradually becomes soft, 

pliable, and "punky." This weathering is chiefly a 

mechanical process involving the separation and 

splitting off of cleavage laminae by moisture, temperature changes, and vegetation, acting either singly or 

in combination. Some chemical attack by organic 

acids and possibly by other acids as well reduces many 

of the separate laminae to lusterless, opaque, crumbly
8 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

flakes. Any inclusions of magnetite or hematite that 

are present in such mica generally are altered to hydrous iron oxides.

STRUCTURAL IMPERFECTIONS

Perfection of cleavage is of fundamental importance in governing the usefulness of sheet muscovite, 

but the size and flatness of sheets obtainable from 

mica books are limited by several other structural 

features as well. Chief among these are "reeves," 

"wedging," "warping," and "ruling."

"Reeves," or "cross grains," are lines, striations, 

shallow corrugations, or small, narrow folds that lie 

in the plane of cleavage. Some reeves are simple, 

closely spaced flexures or crenulations, presumably 

caused by stress during or after crystallization of the 

mica. Others, however, are formed by discontinuities 

in incomplete sheets or laminae. As traced across a 

mica book, such laminae die out abruptly along 

straight lines but commonly reappear along parallel 

straight lines beyond. Where the distance between 

lines is very small and the space from which the laminae are missing is accordingly narrow, the reeves appear only as fine lines, but where laminae are missing 

over greater distances the spaces are occupied by adjacent laminae warped downward from above and upward from below. Such reeves are thus a combination of corrugations and the edges of incomplete laminae (fig. 4). Their depth is a function of the number 

of missing laminae, and their spacing is a function of 

the distribution of discontinuities in the laminae. 

The edges of some incomplete sheets are flanked by 

series of narrow ribbons or strips, and the reeves in 

such mica are very closely spaced (fig. 4).

Reeves are specifically oriented with respect to 

crystallographic directions in the mica. They are perpendicular to the trace of the prismatic and clinopinacoidal faces and hence are parallel to the rays of 

the pressure figure. "A" ("housetop," "fishtail," "Vridge," or "spearhead" ) mica is distinguished by two 

series of reeves that intersect at an angle of about 60°. 

The third series necessary to complete the "A" is not 

present, so that the structure actually resembles the 

letter "V." Typically a single pair of "A" reeve 

groups extends across an entire mica book, with the 

point of the "A" very near one edge (pi. 2). Such 

books rarely show well-developed crystal faces.

Three directions of reeves are present in "doubleA" mica, and their pattern suggests two pairs of "A" 

reeve groups with a common point and a common side 

(pi. 2, no. 6). The relation of "A" structure to crystal 

directions is best shown in books with reeves that extend in six directions from a common center. In describing such a book from the Marie mine, Spruce 

Pine district, N. C., Kesler and Olson 6 state that "a 

complete six-sided crystal, 2.5 inches in diameter, was 

divided at intervals of 60° by typical 'A' reeves into 

six parts like the pieces of a pie, the reeves diverging 

from the center, and each set bisecting the face of the 

mica crystal." Books with only one pair of "A" reeve 

groups thus can be considered as highly distorted 

crystals representing little more than one-sixth of a 

perfect crystal of flat mica (fig. 5).

Some "A" books in which the imperfections are 

shallow, widely spaced, or otherwise not seriously developed can be split into sheets of commercial value. 

"A" structure is developed only near the edges of other 

books (pi. 2, no. 3), and flat sheets of good quality can 

be split from their inner parts after the reeved material is trimmed away. Very large trimmed sheets 

have been obtained from such "flat-A" mica in many 

Southeastern mines. In still other mica the reeves are 

confined to certain sheets or groups of sheets, so that

FIGURE 4.—Diagrammatic sections, showing the relation of distribution and 

depth of reeves to crenulations and discontinuities in muscovite laminae.

6 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, p. 14, 1942.

1. Cluster of books with poorly developed crystal faces. The muscovite is 

locally intergrown with white albite.

2. Parallel growth of small, well-developed crystals.

3. Single wedged book and associated irregular books of much smaller size 

in plagioclase^quartz pegmatite. One-fourth natural size.

EXPLANATION OF PLATE 1

4. Books with somewhat irregular crystal faces. One-inch square gives 

scale.

5. Percussion figures in trimmed sheet. One-half natural size.

6. Books with smooth to very rough crystal faces. One-half natural 

size.
OEOIXXMCAI. Sl'HVEY PROFESSIONAL PAPER 22.5 PLATE 1

BOOKS AM) CLEAVAGE PIKCES OF MUSCOMTE


GEOLOGICAL STRVEY PROFESSIONAL PAPER 22.1 PLATE 2

REEVE STRUCTLBES liN MUSCOVITE
COMMERCIAL SHEET MUSCOVITE 9

imperfect material can be split out and good sheets 

obtained from the remainder of the book. Much "A" 

mica, however, is so seriously marred that it must be 

classed as scrap.

"Herringbone" ("fishbone," "fishback," "feather," 

or "horsetail") mica is marked by reeves and hence is 

somewhat similar in appearance to "A" mica; howFIGURE 5.—Relation of reeves to crystal directions in muscovite. Orientations 

are shown by percussion figures. A, Complete directional development of 

"A" reeves in crystal; B, Typical development of "A" reeves; C, Typical 

herringbone reeves.

ever, the angle of intersection of the reeve groups is 

about 120°, rather than 60°. The reeves characteristically flank a central line or strip of reeves to form a 

pattern resembling that of a feather or the skeleton 

of a fish (pi. 2, no. 7; fig. 5). The central line or strip 

generally is perpendicular to the trace of the clinopinacoidal crystal face. Few herringbone books from 

deposits in the Southeastern States contain flat material; hence nearly all are processed as scrap.

Combinations of "A" and herringbone structures 

occur in some books, particularly those of the flat-A 

type. The herringbone reeves generally are discontinuous and irregularly distributed in such mica. 

Books not marred by "A" or herringbone structure

commonly contain a single set of reeves. If the reeves 

are thin and fine, the material is sometimes referred 

to as "hair-lined." Such reeves are not to be confused, 

however, with the true hair lines and hair cracks previously discussed.

"Wedge" structure, or "wedging," is caused by the 

interlayering of sheets of unequal size. Some extend 

entirely across the book, whereas others taper out at 

intermediate points. Books in which incomplete laminae extend inward from all edges can be externally 

regular in shape, but owing to their complex internal 

wedging they yield no sheet mica. Books in which 

a preponderance of incomplete laminae extends inward from one edge are markedly thicker on one side 

than on the other (pi. 3). Wedge structure is common 

in herringbone and "A" micas, and the term "wedgeA" is used in contradistinction to "flat-A." Herringbone and most wedge-A books consist almost wholly 

of scrap, whereas many flat-A books contain appreciable quantities of sheet material.

Not all wedged mica is marked by reeves, and 

Sterrett 7 states that in plate mica the difference in 

thickness on opposite edges may be greater than half 

an inch in a crystal 3 inches in diameter. The amount 

of wedging generally is greater in reeved than in unreeved mica, however, and wedge angles of 25° or 

more are common in "A" books. Small, thickly 

wedged "A" books are known as "chub-A" (pi. 3, no. 

5).

"Ribbed," "rippled," "ridged," or "creped" mica is 

marked by waves or ridges, generally shallow, that 

are not assignable to "A" structure or to other reeve 

groups. Unlike reeving, most ribbing and rippling 

appear to be the result of deformation of the mica 

after, rather than during, crystallization. Some ripples are traceable along their strike into broad warps, 

some may be traced into fractures or partings, and 

others die out abruptly (pi. 3). Still others extend 

across entire cleavage surfaces without marked 

change in size. In general these minor warps or 

crenulations are spaced much farther apart than 

typical reeves, and good sheet material can be recovered from parts of many rippled books. Sheets that 

are only slightly affected are termed "wavy," and 

mica that is bent on a broad scale is said to be "buckled," "warped," or "cupped." Many types of deformation occur, and all gradations are known between flat

7 Sterret, D. B., Mica deposits of the United States: U. S. Geol. Survey Bull. 

740, p. 16, 1923.

EXPLANATION OF PLATE 2

1. Flat-A mica. One-fourth natural size.

2. Severely reeved mica, split from same book.

3. Flat-A or broad-A mica, with reeves near edges only. One-fourth natural 

size.

4. "A" mica with widespread development of reeves. One-fourth natural size.

6. Partial intergrowth of two books of "A" mica. One-half natural size.

6. Double-A structure, with flat material between reeve groups. One-half 

natural size.

7. Herringbone structure in somewhat clay-stained mica. Two-thirds natural 

size.
10 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

and severely buckled mica (fig. 6). An extreme ex* 

ample of deformation is the chevronlike folding of 

some mica books in the Meadow deposit, Spruce Pine 

district, N. C. (pi. 3, no. 10). Another type of folding, 

known as "cleavage stepping," comprises small, sharp 

monoclinal flexures. These are subpiarallel, and they 

typically distort the cleavage faces into series of 

broad, low steps (pi. 3).

D

E

FIGURE 6.—Diagrammatic sections, showing deformation of Muscovite 

laminae. A, Broadly -warped, or -wavy, sheets; B, Warped or rippled sheets ; 

C, Gradation of -warped mica into buckled or folded mica; D, Mica cut by 

parting planes (ruling); E, Cleavage-stepped mica.

One of the commonest structures in mica books is 

"ruling," or "secondary cleavage," which occurs as 

regular, sharply defined parting planes that intersect 

the basal cleavage plane at an angle of nearly 67°. 

They are parallel to the pressure figure. Only one set 

of these partings is present in many books, but two 

or all three sets occur in others. Their traces on cleavage surfaces intersect at angles of about 60°, and 

where all three sets are present they commonly separate the sheets into triangular or hexagonal fragments (fig. 7). Many sheets ruled in two directions 

are similarly separated into rhombic or diamondshaped fragments or into strips and laths.

The structure generally continues through the entire thickness of severely ruled books, but in others it 

is confined to certain layers in which it may extend 

partly or entirely across the cleavage faces (pi. 4). 

Where one set of ruling planes is well developed, the 

mica is thereby separated into strips or ribbons that 

commonly are less than an inch wide. The ruling in 

some mica is so closely spaced that individual ribbons 

are sliver like or hair like (pi. 4, no. 3). Accumulations 

of such slivers are termed "hair mica." Ribbons in 

some large books, on the other hand, are as much as 4 

or 5 inches wide and hence yield satisfactory sheets if 

free from other defects. Ruling appears in much "A" 

and herringbone mica, where it either coincides in direction with the striations and corrugations or forms 

the cross bar of the "A" (pi. 4, no. 4). It is more common, however, in unreeved books, where its distribution is of prime importance in determining the sizes 

of sheets that can be trimmed out.

Ruling, warping, buckling, and rippling are most 

intense in books that occur near faults or slip joints 

and plainly are the result of distortion from postcrystallization movements. Both percussion figures 

and ruling are strongly developed in some books that 

lie near blast holes, where they appear to have been 

formed by the shock of the blasting. Such ruling generally occurs in small hexagonal or rhombic patterns 

(fig. 7).

FIGURE 7.—Types of fragments formed by breaking along parting or ruling 

planes in a muscovite crystal. Orientation is shown -with respect to percussion-figure directions and crystal faces.

EXPLANATION OF PLATE 3

1. Wedge-A book. One-half natural size.

2. Edgewise view of same book. Approximately three-fifths natural size.

8. Curved -wedge-A book. One-half natural size.

4. Edgewise view of same book. Approximately three-fifths natural size.

5. Chub-A book. Approximately natural size.

6. 7. Ribbed, or cleavage-stepped, mica. Two-fifths natural size. 

8, 9. Buckled, or cupped, mica. Two-fifths natural size. 

10. Severely folded mica. One-half natural size.
GEOLOGICAL STRVEY PROFKSSIONAL PAPER 225 PLATE 3

WEDGE STRUCTURE AM) DEFORMATION 11N MUSCOVITE


GEOLOGICAL SURVEY PROFESSIONAL PAPER 225 PLATE 4

RILING J]\ CLEAVAGE PIECES OF MUSCOVITE
COMMERCIAL SHEET MUSCOVITE 11

COLOR

Most mica is distinctly colored, especially in sheets 

one-sixteenth of an inch or more thick. Thick sheets 

and plates of commercial muscovite have been variously described as "red," "ruby," "rum," "red rum," 

"green," "amber," "yellow," "gray," "white," "watercolored," and "black," with or without modifying 

terms indicating tints, shades, or intermediate hues. 

Much confusion has arisen from duplication and inconsistencies in the usage of these terms. "Amber" 

mica, for example, is the common trade name for 

phlogopite; therefore the term "amber" is not wholly 

desirable for muscovite. Deep-brownish to greenisholive mica is termed "water-colored" in North Carolina, whereas the same designation is applied to very 

light green mica in northern New Mexico. "White" 

and "gray" are used either to describe mica that is 

very pale in color or to distinguish any thinly split 

muscovite from the amber phlogopite. Some micas 

with many air bubbles or clouds of minute lightcolored inclusions also are referred to as "gray." 

"Black" is applied to muscovite with abundant inclusions of magnetite or hematite and also to sheets of 

dark-brown, greenish-brown, or black biotite ("blackjack"). "Red" mica either contains numerous inclusions of brightly colored goethite and turgite or is interlayered with iron-stained clay.

In general the color of commercial muscovite, as 

viewed by light transmitted through cleavage pieces, 

ranges from drab and pinkish buff through reddish 

brown and shades of brown and green to pale yellowish green and yellow. The color of muscovite is chemico-compositional,8 as it depends upon composition for 

pieces of a given thickness. Studies in the biotite 

group have indicated that iron causes a green color, 

titanium produces brown and red, and magnesium 

dilutes or masks the effects of titanium,9 but no such 

studies appear to have been made in the muscovite 

group. Systematic and detailed tests for color-composition relations in several muscovites from the Southeastern States are being made at the present time, and 

those thus far completed indicate that variations in 

iron content are accompanied by appreciable variations in color.

Muscovite is distinctly pleochroic, with the greatest

* Kennard, T. 6., and Howell, D. H., Types of coloring in minerals: Am. 

Mineralogist, vol. 26, p. 418, 1941.

9 Hall, A. J., The relation between colour and chemical composition in the 

biotites: Am. Mineralogist, vol. 26, p. 29, 1941.

color differences in the so-called ruby and rum varieties. The colors of smooth-faced crystals, viewed 

parallel to their cleavage, generally are pale tints of 

yellow, yellowish green, green, or yellowish brown, 

whereas the colors as seen perpendicular to the cleavage direction are darker and commonly are more 

brownish or reddish. Muscovite is much more transparent to light rays parallel to the cleavage than to 

those normal to it; thus some books of cinnamonbrown mica 3 inches in diameter and a quarter of an 

inch thick are fully as transparent when viewed edgewise as when viewed from top or bottom.

Few mica books are colored uniformly throughout. 

Some are crystallographically zoned and yield cleavage plates with well-defined color bands that are called 

"mine markings" in several Southeastern districts. 

The bands are parallel to crystal outlines and hence 

are generally hexagonal or rhombic in plan (pi. 7). 

Alternating narrow bands of slightly but distinctly 

differing shades of brown or green characterize the 

books of some deposits. They are most abundant near 

the rims of most zoned books but are concentrated 

near the centers or are scattered throughout others. 

Broader color bands are known from many deposits 

but in general are not so common. Color differences 

between adjoining bands generally are greater in 

broadly zoned books than in those with more abundant 

but narrower bands. Crystallographic zoning in 

many books appears only as narrow rims or as small 

centers or cores. Some pinkish-buff and cinnamonbrown micas are marked by distinctive color patterns 

of a grating or gridiron type (pi. 7). The distribution 

of pattern areas in otherwise uniformly colored sheets 

is related to the crystal directions of the mica.

The margins of nearly all books are distinctly 

lighter in color than their inner parts. This appears 

to be a bleaching effect and is recognizably later 

than true Crystallographic color zoning. Similar 

bleached areas flank cracks and surround holes and 

many inclusions. The boundaries between the bleached 

and the unaffected parts of the books are gradational 

and irregular. Some mica is mottled, with very irregular splotches of one color in a background of a 

slightly different color. Most of this mica is clear, 

transparent, and otherwise of sound appearance.

Color has affected the merchantability of muscovite since the days when its chief uses were based 

largely upon transparency. The light-brown and 

light-green micas, which yield more transparent

1. Part of thick book, bounded by two well-developed ruling planes. Approximately natural size.

2. Deeply ruled pieces from large, flat book, with typical strip mica, or 

ribbon mica, at bottom. One-third natural size.

3. Deeply ruled and cracked pieces from slightly warped book. Ruling interrupted by inclusion of quartz elongated parallel to the ruling plane in

EXPLANATION OF PLATE 4

piece at top. Hair mica, developed by very closely spaced ruling along 

edge of broken fragment, appears in two pieces at bottom. One-half 

natural size.

4. Large piece of flat-A mica, showing relation of ruling to reeve directions. 

Approximately one-half natural size.
12 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

sheets of a given thickness, commanded a higher 

price than the darker-colored micas. This was carried 

over into periods of increasing electrical uses, the 

dark-brown and brownish-green micas being "classed 

as 'No. 2,' even when flawless and clear." 10 Later, 

when the scrap-mica industry first expanded into 

prominence, it was found that the lighter-colored 

micas yield a whiter ground product; hence such 

varieties of scrap were sold for higher prices.

In selecting sheet mica for electrical uses during 

recent years, purchasers have placed varying degrees 

of emphasis upon the desirability of the so-called 

ruby micas as contrasted with the green and especially the dark-green varieties. It has been stated 

repeatedly that dark or green muscovites "usually 

are poorer dielectrics," but the basis for such statements is not entirely clear. At best, they appear to 

be generalizations extrapolated from scattered and 

unsystematic data. That they are open to serious 

question has been plainly indicated by several series 

of tests made during recent years, and their acceptance therefore might well be deferred until the effects 

of color have been more fully investigated.

STAINING, INTERGROWTHS, AND INCLUSIONS

The presence of visible mineral or vegetable impurities in book mica generally reduces its value. 

Some impurities occur as crystals or crystal groups 

that extend through considerable thicknesses of the 

mica; they must be removed and the immediately 

surrounding mica trimmed away before the remainder of the book can be split into sheets. Others are 

finely divided or thinly flattened between the mica 

laminae, and the part of the book in which they occur 

generally is trimmed away and either discarded as 

scrap or prepared as sheet stock of inferior grade. 

The proportion, distribution, and type of inclusions, 

intergrowths, and stain lead to such designations as 

"mottled," "specked," "spotted," "freckled," "lined," 

"black," "black-stained," "black-spotted," "lightly 

stained," "heavily stained," "dotted," "powderspecked," "blotched," and "curdy."

Two general types of impurities occur in commercial muscovite. These are most simply designated as primary and secondary and can be subdivided as follows: 

Primary impurities:

Air stain.

Mottling and "vegetable stain" (inorganic).

Mineral intergrowths and inclusions. 

Secondary impurities:

Air creep.

Clay stain.

10 Sterrett, D. B., Mica deposits of the United States: U. S. Geol. Survey 

Bull. 740, p. 17. 1923.

Iron stain.

Manganese stain.

Vegetable stain (organic).

The primary stains were formed during or soon after 

crystallization of the mica and hence are distributed 

without regard to the position of the deposit with respect to the present land surface. These blemishes 

are as likely to increase with depth as to decrease. 

Secondary stains, in contrast, occur only at or near 

the surface and characteristically are absent from 

those parts of the deposit beneath the oxidized zone.

PRIMARY IMPURITIES

"Air-stained" mica contains flattened pockets, tiny 

bubbles, or groups of closely spaced gas bubbles (pi. 

5, no. 1). Clusters of very small bubbles are known 

as "silver spots" in the Southeastern States. Other 

types of air stain include grayish to silvery spots, 

specks, and lines, many of which are so closely spaced 

that they destroy all transparency of the mica. Where 

they are confined to certain sheets they can be removed by careful splitting, but they are scattered 

through all parts of many books. In general the effects of air inclusions on the splitting and electrical 

qualities of muscovite are not so serious as the effects 

of most mineral stains, and moderately to heavily 

air-stained mica is unsatisfactory only for certain 

types of specialized electrical equipment.

Some mica is marked by a pale-green, yellowish, 

or greenish-brown discoloration that is termed 

"vegetable stain." Where essentially primary, such 

stain consists of minute scales or finely divided aggregates of chlorite, biotite, or material rich in ferrous iron. Individual crystals and mineral masses 

cannot be recognized megascopically or even with low 

magnifications under the microscope. Most vegetable 

stain is evenly distributed as extremely thin, curdy 

aggregates. Similar material that occurs as separate 

clumps is generally referred to as "mottling." Specks, 

spots, and lines of such stain also are known (pi. 5). 

Vegetable stain and mottling rarely are so dense that 

they seriously affect the transparency of mica that 

otherwise is of good quality. Like air stain, they 

generally are significant defects in terms of the most 

exacting end uses only.

Deep grassy-green specks, spots, and lines are 

sparsely scattered through much book mica. These 

are like typical vegetable stain in general curdy appearance, but individual masses are darker and more 

clearly defined and appear to be somewhat denser 

than the scales and aggregates that compose most 

vegetable stain. Some mica contains scattered green, 

brown, or reddish-brown spots and "bursts" of curdy 

stain. Where they are a quarter of an inch or more 

in diameter, the brown spots are commonly termed 

"cigarette burns." In some districts the occurrence
COMMERCIAL SHEET MUSCOVITE 13

of brown and green spots with fuzzy edges and dark, 

well-defined centers leads to the designation "frogeye" mica. Some spots and bursts are haloes of discoloration that surround tiny inclusions of zircon 

and allanite, but others contain no recognizable cores 

of foreign material.

Mineral stain, which comprises intergrowths and 

inclusions of recognizable crystals, is the most serious of the primary impurities. Among the minerals 

that occur within books of muscovite are actinolite, 

albite, allanite, apatite, beryl, biotite, brookite, chlorite, columbite, dumortierite, epidote, fluorite, garnet, 

hematite, kyanite, magnetite, manganese oxides, 

marcasite, microcline, pyrite, pyrrhotite, quartz, 

rutile, sillimanite, sphene, staurolite, thulite, topaz, 

tourmaline, vermiculite, zircon, and zoisite. The distribution and shape of many of these minerals are 

influenced or controlled by crystal directions in the 

host mica. This is especially clear in the case of magnetite, hematite, and some manganese oxides but can 

be demonstrated for other minerals by means of 

statistical studies only.11 Probably few of the included 

minerals are unoriented in the strictest sense.

Magnetite and hematite are the most common inclusion minerals. Magnetite occurs as laths, needles, 

skeletal forms, and flattened crystals that are sixsided in plan (pi. 5). In general, these last are of an 

octahedral-dodecahedral combination form 12 and are 

characterized by prominent octahedral parting 

cracks. Some of the inclusions, gray to bluish gray or 

violet, are so thin that they are transparent. Crystal 

outlines and parting directions are oriented in accordance with the pressure and percussion figures. 

The six-sided crystals are about 9 millimeters in 

maximum diameter, with an average of less than 1 

millimeter. The markedly elongated crystals are at 

least 4 millimeters in average length, with maximum 

recorded lengths of 10 centimeters.13 Most magnetite inclusions are less than 0.01 millimeter thick, 

but some are 0.1 millimeter and a few nearly 1 millimeter thick.

The inclusions in much of the so-called specked 

and lightly specked muscovite are magnetite, characteristically scattered through the books. In some 

books, however, they are confined to certain sheets 

or groups of sheets, and in others they occur in welldefined belts parallel to "A" reeves or in concentric 

zones parallel to crystal faces of the host mica. The 

zonal arrangements appear to reflect the stages of 

mica growth, and some very narrow individual zones

of tiny inclusions lead to recognition of phantom 

mica crystals that probably are analogous to the wellknown phantom quartz crystals. The thinnest inclusions of magnetite do not affect the general splitting quality of the muscovite, and sheets and films 

that enclose such plates, laths, and needles are easily 

split. The thicker inclusions, in contrast, tie the mica 

on a small scale and thus seriously impair its filming 

properties. Such mica is sometimes referred to as 

"spot-welded," "spot-locked," or "black-pitted."

Hematite, the most abundant and widespread inclusion mineral in commercial muscovite, occurs as 

flattened skeletal crystals with a hexagonal outline, 

laths and flattened needles, simple and complex dendritic forms, and latticelike forms that are extreme 

developments of skeletal crystals (pi. 6). The lattices are characteristically triangular, with the three 

elements parallel to rays of the percussion figure or, 

much less commonly, to the rays of the pressure figure in the enclosing mica. Other elements lie perpendicular to the three principal directions of some 

lattices, and the symmetry of the whole is hexagonal. 

The hematite inclusions are black, dark brown, reddish, smoky brown, and buff (pi. 7), and none show 

the bluish shades of most very thin magnetite inclusions. In general they are more transparent than the 

magnetite crystals.

The dendritic and latticelike growths have been 

identified as magnetite by many investigators 14 and 

as hematite by others.15 It can be demonstrated that 

those in micas from the southeastern and southwestern United States are hematite, and those in micas 

from other localities probably are hematite as well. 

Frondel and Ashby 16 have summarized the principal 

differences between magnetite and hematite inclusions in muscovite, chiefly on the basis of detailed 

studies of collections from the northeastern United 

States.

Some hematite laths and plates are bounded by 

smooth and regular crystal faces, but the edges of 

most are so irregular that they create a feathered or 

dendritic appearance. All are extremely thin, especially as compared with their areal extent, and they 

are not separable from the enclosing mica by ordinary mechanical means. Their maximum thickness is 

considerably less than 0.01 millimeter, whereas most 

occupy areas of several square centimeters. Individual lattices and skeletal crystals more than 3 feet 

long occur in some large books. Where the hematite

11 Frondel", Clifford, Oriented- inclusions of tourmaline in muscovite: Am. 

Mineralogist, vol. 21, pp. 777-799, 1936; Oriented inclusions of staurolite, zircon, and garnet in muscovite; skating crystals and their significance: Am. 

Mineralogist, vol. 25, pp. 69-87, 1940.

12 Frondel, Clifford, Oriented inclusions of tourmaline in muscovite: Am. 

Mineralogist, vol. 21, p. 107, 1936. 

18 Idem.

862911°—60—2

14 Dana, J. D., and Brush, G. J., On the magnetite in the mica of Pennsbury, Pa.: Am. Jour. Sci., 2d ser., vol. 48, pp. 360-362, 1869; Sterrett, D. B., 

Mica deposits of the United States: U. S. Geol. Survey Bull. 740, pp. 17-18, 

1923.

15 Rose, G., Aaterism: Preuss. Akad. Wiss. Monatsber., pp. 614 et seq., 

1862; Frondel, -Clifford, and Ashby, G. E., Oriented inclusions of magnetite 

and hematite in muscovite: Am. Mineralogist, vol. 22, pp. 104-121, 1937.

16 Frondel, Clifford, and Ashby, 6. E., op. cit.
14 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

stain is dense, it consists of many superposed crystals 

that are separated by thin films of mica, and where 

it is less dense, correspondingly fewer crystals or 

lattices are present. The stained parts of some books 

can be split out, leaving clear sheets of high quality. 

The stain is confined to the centers or to one or more 

sides of other books, in which the hematite-bearing 

mica can be trimmed away from the clear material.

The outer parts of hematite-stained books are characteristically free from inclusions, even where the remainder of the mica is very heavily stained. Clear 

mica commonly flanks cracks and parting planes and 

surrounds holes in books that elsewhere contain hematite inclusions. The distribution of such stains also is 

influenced by some warping, rippling, and other secondary structures in the mica. Zonal concentrations 

of hematite comparable to those of magnetite have 

not been observed, and in general the hematite appears to have developed after crystallization of the 

muscovite. On the other hand, its distribution within 

individual unaltered mica books and its occurrence in 

deposits at considerable depths beneath the oxidized 

zone preclude its explanation as an ordinary alteration product of the mica or as material introduced 

from sources outside the mica books. It appears rather 

to have crystallized from solid solution in the muscovite, as suggested by Frondel and Ashby,17 and as such 

is here considered "primary" in the same sense as 

analogous exsolution growths in metallic ore minerals. 

Iron oxide films and spots, presumably of exsolution 

origin, can be developed experimentally by heating 

some varieties of clear muscovite to temperatures of 

550° C. or more, but typical hematite dendrites and 

triangular lattices are not so simply reproduced.

Inclusions of hematite appear to have little effect 

upon the splitting qualities of muscovite, at least as 

far as its commercial preparation is concerned. Ordinarily the stained parts of books with hematite lattices or spots split as easily and uniformly as the unstained parts, and there is no perceptible tying of 

sheets. Both hematite and magnetite, however, seriously increase the conductance of the mica in which 

they occur and hence lower its value. The amount of 

lowering depends upon the thickness, abundance, and 

distribution of the inclusions.

Goethite occurs in sheet mica as yellow, orange, 

red, reddish-brown, or brown scales, stains, and

pseudomorphs of other iron oxide minerals (pi. 7). 

Most appear to have been formed by alteration of 

hematite and magnetite inclusions, but some of the 

most finely divided scales may well have been developed directly by precipitation from solid solution 

in the mica.

Quartz and albite are interlayered with some muscovite plates to form composite masses of little or no 

economic value. The edges of other books are intergrown with these minerals. Books in which quartz, 

apatite, or tourmaline spindles are present are known 

as "gritty," "sandy," "stony," or "sand-pitted." 

Where the axes of such spindles are oblique or perpendicular to the cleavage, the impurities effectively 

tie the books. Actinolite, allanite, beryl, kyanite, rutile, tourmaline, zoisite, and other species of elongate 

habit commonly occur as individual crystals, bundles, 

and parallel groups of crystals, sprays, and rosettes 

(pi. 8). Although they lie parallel to the cleavage 

surfaces, they generally penetrate enough laminae of 

the mica to affect its splitting properties seriously. 

Many of the crystals are oriented parallel to rays of 

the pressure or percussion figures. Others appear to 

be distributed at random, although preferred orientations for several minerals have been demonstrated by 

statistical studies.18

Fluorite, garnet, pyrite, and other minerals occur 

as inclusions that are much flattened parallel to the 

plane of cleavage in the mica. They are characteristically equant in that plane. The flattening is extreme 

in many books, but in others the inclusions are much 

thicker and they tie considerable numbers of mica 

laminae. The average diameter of garnet inclusions 

probably is less than 5 millimeters, although flattened 

tablets an inch or more in diameter are known. Most 

are less than 0.3 millimeter in thickness, and many 

are as thin as 0.05 millimeter. These dimensions also 

apply to other inclusion minerals of similar habit. 

The prismatic and other elongated inclusions are of 

comparable thickness but commonly reach lengths of 

several inches.

Biotite is intergrown with muscovite in many deposits. Inclusions of biotite in muscovite are common 

(pi. 8), but inclusions of muscovite in biotite are 

sparse or rare. In some districts the muscovite is in17 Frondel, Clifford, and Ashby, G. E., op. cit.

18 Frondel, Clifford, Oriented inclusions of tourmaline in muscovite: Am. 

Mineralogist, vol. 21, pp. 777-799, 1936; Oriented inclusions of staurolite, 

zircon, and garnet in muscovite; skating crystals and their significance: Am. 

Mineralogist, vol. 25. pp. 69-87, 1940.

EXPLANATION OF PLATE 5 

[Figures approximately three-quarters natural size]

1. Air stain. 6. Flattened crystals of magnetite elongated parallel to pressure-figure direc2. Dense green mottling and vegetable stain. tions. Note thin, fine color strips and air creep, which appears as dark3. Grassy-green stain in form of rectangular lattice. gt&y areaa fa ^ mica shee^

4. Curdy green stain with little systematic arrangement.

5. Dense lattice of green stain and abundant vegetable stain, with black

spots of hematite. 7. Scattered specks and small spots of hematite. Widespread air creep.
GEOLOGICAL STRVEY PROFESSIONAL PAPER 225 PLATE 5

PRIMARY STAIN J1N MUSCOVITE


GEOLOGICAL PURVEY PROFESSIONAL PAPER 225 PLATE 6

PRIMARY HEMATITE STA^ IN MUSCOVITE
COMMERCIAL SHEET MUSCOVITE 15

tergrown with vermiculite, which presumably was 

derived from biotite by alteration. Where biotite and 

muscovite are intergrown, the cleavages of the two 

minerals generally are parallel, but the pressure and 

percussion figures of the inclusions are commonly 

oriented perpendicular to those of the host (fig. 8). A 

few inclusions of biotite are elongated normal to their 

cleavage direction and lie oblique to the enclosing 

muscovite, thus tying the sheets of the host book. Most 

of the inclusions are bounded by well-formed prism 

and pinacoid faces, and the contacts between the two 

micas are tight. Other biotite crystals, however, are 

markedly pyramidal in habit, and their contacts with 

the muscovite intersect the mutual cleavage surface 

at low to moderate angles. Such inclusions are less 

firmly joined to the host mineral and tend to pop out 

when the mica book is split.

FIGURE 8.—Biotite-muscovite intergrowths. A, Biotite in muscovite, with 

orientation shown by percussion figures; B, Muscovite in an inclusion of 

biotite.

A few inclusions of biotite in muscovite themselves 

contain inclusions of muscovite (fig. 8), and the relations of some of these "multiple crystals" are somewhat similar to those of crystal zoning. The biotite 

inclusions in some books are merely rinds around 

smaller muscovite crystals, and in others they surround muscovite that forms the core of the book. 

Other composite books are composed of muscovite

with muscovite inclusions or of muscovite cores with 

successive rims of muscovite. These are difficult to 

recognize but probably are not common.

SECONDAKY IMPUEITIES

"Air creep," a secondary stain, is similar in appearance to some types of air stain. It is merely air that 

enters the mica sheets through their edges and penetrates them along cleavage planes (pi. 5). In general 

it is caused by rough handling during preparation of 

the mica, especially by trimming with shears or a 

dull knife. This type of stain is easily distinguished 

from primary air stain, which consists of many small 

bubbles or larger air pockets that are fully and firmly 

enclosed. The creep pockets, in contrast, are either 

connected with the trimmed or natural edge of the 

sheet or can be so connected by pressing the mica between the thumb and forefinger. They do not constitute a significant defect unless they occupy so many 

cleavage openings that the transparency of the mica 

is materially reduced.

Most books that have been exposed to weathering 

and the action of downward-percolating surface 

waters are coated with calcite, chalcedony, clay minerals, hydrous iron oxides, manganese oxides, or other 

secondary minerals. Where these have been deposited 

by waters that penetrated between the laminae of the 

mica, they must be removed by careful splitting and 

trimming (pi. 8). The value of the books is thus materially reduced. Books marred chiefly by silica, calcite, and clay minerals are termed "clay-stained," 

whereas those that are strongly colored yellowish, 

reddish, or brownish by iron oxides are referred to as 

"iron-stained." The term "manganese-stained" has a 

similar basis.

The organic type of vegetable stain, a truly secondary feature characteristic of the weathered zone in 

mica deposits, consists of plant material that coats 

the outer surfaces and some of the cleavage laminae 

of mica books. Some of the material is carried into the 

mica by waters that penetrate fractures and cleavage 

cracks, and some forces .its way into the books 

through the action of growing plants. The near-surface mica of many deposits is veined by the roots of 

grasses, bushes, and even trees. Most organic vegetable stain is accompanied by heavy clay and iron 

staining, and books so affected yield little usable sheet 

material. On the other hand, appreciable quantities

EXPLANATION OF PLATE 6 

[Figures approximately three-fifths natural size]

1. Dense aggregates of spots and triangular lattices confined to sectors of 

a book.

2. Coarse spots and thin-bladed, latticelike crystals.

3. Blotches, small spots, and pale-green vegetable stain.

4. Lathlike intergrowths, showing relation to crystal faces. Note absence 

of stain near edges of book.

5. Thin triangular lattices, with simpler rows of spots at top.

6. Coarse blades in lattice arrangement, with rows of dendrite spots.
16 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

of clear sheets can be obtained from many near-surface books that are marred by only one of these stains.

ELECTRICAL- PROPERTIES

The extremely low electrical conductivity of muscovite is a very important property as far as most of its 

commercial uses are concerned. Unstained mica is 

the least conductive and is therefore suited for electrical equipment of the best quality. Many mineral 

inclusions, particularly magnetite and hematite, increase the conductivity; hence stained mica generally 

is used in articles that do not require the most effective insulation. Conducting impurities in sheet muscovite can be detected by means of a high-voltage 

spark, which causes glowing or small-scale arcing as 

it passes through the mica at or near the inclusions.

The dielectric constant (K), or specific inductive 

capacity, of muscovite is the ratio of the capacitance 

of a condenser in which the muscovite is the nonconducting substance to the capacitance of a condenser 

in which air (or, more exactly, a vacuum) is the dielectric. Values for this property generally are given 

in terms of specific conditions of applied voltage, 

frequency of applied current, and temperature, moisture content, and other features of the mica in question. K for sheet mica ranges from 2.0 to about 8.5 but 

generally is more than 6.5, with an average of about 

7.2. It is a property of great significance for many 

electrical uses but is so uniformly satisfactory in 

micas that are otherwise of good quality that limiting 

values are rarely specified by purchasers.

The dielectric strength of muscovite is its ability to 

resist break-down or rupture under conditions of high 

voltage or electric field strength. It is defined in terms 

of the maximum potential gradient that material of a 

given thickness can withstand and generally is expressed in kilovolts per centimeter or volts per mil 

of thickness. It is tested by means of the high-voltage 

spark (concurrently with testing for conducting impurities in the mica) or—more commonly—by applying high voltages through spherical or platelike 

contact electrodes. Dielectric weakness is caused by 

pinholes, cracks, tears, and other inhomogeneities or 

discontinuities in the mica sheets. Many of these are 

recognizable in ordinary visual examination, but

others are so small or inconspicuous that they are 

easily overlooked. High dielectric strength and heat 

resistance are particularly desirable in mica used for 

electrical generating equipment.

A very significant electrical property of mica used 

in condensers is its power factor (PF), which is a 

measure (expressed in percent) of the loss of electrical energy in a condenser in which the mica is the 

dielectric. Such loss is due chiefly to dielectric absorption, which produces a component of the applied voltage in phase with the current as though an additional 

resistance were placed in series with the condenser. 

Excessive overheating and damage result from high 

power losses; hence good condenser mica must have a 

power factor of less than 0.04 percent at a frequency 

of 1 megacycle. The Q value, a factor more commonly 

used in recent years, is the reciprocal of the power 

factor, so that Q = 1/PF. The Q value of good condenser mica therefore should be at least 2,500.

USES

The uses of mica are based upon its perfect cleavage, exceptionally low conductivity of heat and electricity, high dielectric constant and dielectric 

strength, low dielectric loss, heat resistance, noninflammability, mechanical strength, flexibility, elasticity, transparency, luster, and lubricating properties and the ease with which it can be worked into 

final form. The degree of emphasis placed upon given 

properties by the purchaser depends upon the specific 

end use involved.19 Flexibility is particularly important, for example, in the "cigarette" mica used in 

spark plugs for aircraft engines. This material, in 

films 0.0012 inch or less thick, is wrapped around rodlike spindles a little more than an eighth of an inch in 

diameter. In condenser mica dielectric properties are 

most desirable, and—in contrast—the use of mica for 

windows in furnace walls and doors is founded upon 

its transparency, heat resistance, and mechanical 

strength.

A very high proportion of all sheet mica is used as 

electrical insulating material. Washers, disks, and

19 Wierum, H. F., and others, The mica industry: U. S. TarifE Gomm. Rept. 

130, 2d ser., pp. 11-26, 1938 ; Spence, H. S., Mica: Canada Dept. Mines, 

Mines Branch, Pub. 701, pp. 102-120, 1929.

EXPLANATION OF PLATE 7

Crystallographic color zoning. One-third natural size.

3. Major color change, with green-brown boundary parallel to crystal 

faces. One-half natural size.

Rude crystallographic control of hematite stain. One-half natural size.

Alined spots and specks of iron oxides, with faint vegetable stain. Onehalf natural size.

Relations of iron oxides and other stains to crystal directions in a colorzoned sheet. One-half natural size.

Color zoning apparently caused by marginal bleaching. One-half natural 

size.

Greenish stripes and gridiron color patterns. One-half natural size.

9, 10. Hematite specks, spots, and blotches In yellowish-olive to yellowishgreen mica, with local vegetable stain. Two-fifths natural size.

11, 12, 13. Lattices and spots of hematite in yellowish-olive to brownisholive mica. Two-fifths natural size.

14. Rectangular lattices of curdy brown stain in yellowish-olive mica. Twofifths natural size.

15. Hematite spots, blotches, and trigonal lattices in yellowish-olive mica. 

Two-fifths natural size.

16. 17. Hematite specks and spots, with pale vegetable stain, in yellowisholive mica. Two-fifths natural size.
flKOLOCICAI. SIKVKY 1'KOKKSSJONAl, 1'AI'KR 223 IM.ATK 7

17

OOI.OK \AKIVno\S \M) I'HIMAin S'I'MIS LN MVSCOMTK

GEOLOGICAL SURVEY PROFKSSIONAL PAPER 225 PLATE 8

MINERAL INCLUSIONS AND CL4Y STAIN IIN MUSCOVITE

COMMERCIAL SHEET MUSCOVITE 17

other small trimmed or stamped forms are not only 

employed as such, but can be built up into rods, tubes, 

or other articles that are bonded with a suitable cementing material. Simple and composite pieces are 

used, for example, as tubes, sleeves, studs, washers, 

bushings, laminations, and thin perforated plates in 

condensers, transformers, small heating elements, 

rheostats, fuses, incandescent bulbs, radio and electronic tubes, various types of coils, and acoustic, Xray, and other specialized equipment. Thin splittings 

are built up into mica board or are applied as facing 

on paper, cloth, and other materials used in the manufacture of heater elements; commutators; boards, 

panels, and other mounting forms; parts of condensers; and many other electrical devices. In addition, 

coarsely and finely ground micas have a wide variety 

of uses.

The unusual conditions of wartime periods create 

heavy demands for so-called "strategic" mica, which 

is used as splittings in the form of built-up mica commutator segments and coil insulation in motors and 

generators and in transformers, switchboards, blasting apparatus, and aircraft generators and spark 

plugs.20 Some of the greatest demands and most exacting requirements during World War II were outgrowths of developments in the field of military radio 

and electronic equipment. When it became evident that 

the need for dielectric materials of high quality would 

increase without concomitant assurance of adequate 

supplies of sheet muscovite from India, the electrical 

industry was strongly encouraged to develop satisfactory substitutes for mica. Plain or treated papers, 

manufactured to meet precise standards of homogeneity, thickness, porosity, strength, chemical composition, and freedom from conducting particles, 

proved satisfactory for many condensers, particularly those not designed to operate at the highest voltages. Some ceramic dielectrics also gave excellent results when used in capacitors, even at very high voltages.

Substitution of other materials, though successful 

for some types of condensers, was by no means satisfactory for general application. Mica remained the 

only suitable dielectric for many varieties of capacitors, particularly those for military use at radio frequencies. The consumption of condenser-quality

mica therefore was sharply increased, and as a result 

many problems were raised in connection with increasing its supply.21 To stimulate domestic production of such mica, the Metals Reserve Company, a 

subsidiary of the Reconstruction Finance Corporation, designated the Colonial Mica Corporation as its 

agent, with authority to purchase mica of certain 

types and to assist the operators of mines with equipment leases, development loans, and consulting services on problems of mica mining and preparation.

GRADING AND CLASSIFICATION

VISUAL, METHODS

As taken from the mine, the mica crystals or books 

are designated as "mine-run," "run-of-mine," "book," 

or "block" mica. The term "block" mica is more commonly used for partially prepared stock that will yield 

sheet material and hence might well be dropped in 

favor of "book" mica. Commercial muscovite is 

broadly classified by visual methods as "sheet," 

"punch," "circle," "washer," or "scrap," depending 

upon the type of material obtained from the mine-run 

books. "Scrap" comprises books, flakes, and fragments 

that are too small or too marred by inclusions, cracks, 

holes, or other imperfections to yield acceptable sheet 

or punch stock, as well as the waste from those books 

that do yield punchings and trimmed sheets. The material removed from the mine-run mica within or near 

the mine portal is known as "rough" or "cobbed" 

scrap, or more commonly as "mine" scrap. Ordinarily 

it never reaches the shops where the better mica is 

split and trimmed. "Bench" scrap is the mica obtained 

as discarded splittings and trimmings in the preparation of sheet and punch goods and in general is of 

better quality than mine scrap. Bench scrap generally 

amounts to more than 90 percent of the mica that is 

not classified as scrap at the mine.

"Sheet" muscovite, in the broadest sense, is any 

material other than scrap. It is flat, or nearly so, and 

is sufficiently free from structural defects to be manufactured into products of certain shapes that are used

20 Wayland, R. G., Mica in war: Am. Inst. Min. Met. Eng. Tech. Pub. 1749, 

pp. 1-2, 1944.

21 Billings, M. H., and Montague, S. A., The wartime problem of mica supply: Eng. and Min. Jour., vol. 145, no. 8, pp. 92-95, 1944; Burgess, B. C., 

Mica mining and preparation cost: Am. Inst. Min. Met. Eng., preprint of 

paper presented at New York meetings, February 1944, pp. 1-17, 1944; 

Lintner, E. J., Mica, a war essential mineral: Rock Products, vol. 47, no. 5, 

pp. 48-50, 92-93; no. 6, pp. 74-76, 114-116, 1944 ; Wayland, R. G., Mica in 

war: Am. Inst. Min. Met. Eng. Tech. Pub. 1749, 7 pp., 1944 ; Spence, H. S., 

Mica as a critical war mineral: Canadian Min. Jour., vol. 67, pp. 611-617, 

710-717, 1946; War Production Board, Mica releases 1 and 2, 1942.

EXPLANATION OF PLATE 8

1, 2, 3. Single crystals and sprays of black tourmaline. Three-fifths natural 

size.

4. Flattened crystals of pyrite. Three-fifths natural size.

5. Rosettes of zoisite in clay-stained mica. One-half natural size.

6. 7, 8. Euhedral crystals of biotite. Three-fifths natural size. 

9. Magnetite specks and two brown "cigarette burns" in clay-stained mica, 

with thin color stripes parallel to pressure-figure directions. Approximately one-half natural size. Clay stain, as viewed here in transmitted 

light, appears dark.

10. Flattened garnet crystals in hematite-stained mica. Three-fifths natural 

size. Note absence of iron oxide spots from immediate vicinity of garnet inclusions.

11. 12. Color zoning partly obscured by clay stains. Approximately one-half 

natural size.
18 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

in electrical equipment, stoves, lamps, and other appliances. "Uncut" sheet mica is partly prepared stock 

that has been freed of obvious scrap, split or "rifted" 

into plates three-eighths of an inch or less thick, and 

trimmed by any of several methods. If it will yield 

sheets or "patterns" of regular shape that are li/_» by 

2 inches in minimum size, it is specifically known as 

"sheet" or "pattern" mica in the New England and 

Southeastern States and as "plate" mica in the Southwestern States. Gwinn 22 has pointed out that the general term "block" mica is most suitable for such material and has defined it as prepared stock of "random 

thickness i/8 inch to less than 1/100 inch (125 to 10 

mils), which contains a usable area of l1/^ by 2 inches 

minimum."

"Punch" mica is difficult to define rigorously, owing 

chiefly to inconsistencies in the usage of the term. In 

general, however, it is uncut material capable of yielding punched or trimmed sheets that are free of cracks 

and other defects, are at least l1/^ inches in diameter, 

and are less than I 1/! inches wide and 2 inches long. 

"Circle" mica will yield prepared sheets 2 inches in 

diameter, and "small punch," "washer," or "washerpunch" mica will yield 1-inch sheets. Some washer 

mica is little more than scrap, and many users do 

not recognize it as a separate class, especially during 

periods of low prices. "Punch" mica is sometimes 

used as a general term, including circle, punch (in the 

strict sense), and some small punch or washer. The 

terms "uncut punch" and "uncut circle" are synonymous with "punch" and "circle" as generally used but 

may be helpful in distinguishing such material from 

the prepared sheets or punchings. "Trimmed punch," 

or small sheet material, is prepared from ordinary 

punch stock, generally by knife trimming.

Size grading of sheet mica is based upon the area 

and minimum width of the largest rectangle of a 

given quality that can be obtained from-the block. In 

general, the rectangle must meet the quality specifications set by the purchaser. A size-grading chart of 

the type adopted by the American Society for Testing 

Materials for domestic block mica and mica splittings 

is reproduced in figure 9. The method outlined by the 

Society 23 is as follows :

* * * The specimen to be graded shall be laid upon the chart 

so that it covers point O and has its maximum and minimum 

dimensions extending along and covering the lines AO and OB, 

respectively. The specimen should be shifted until its usable 

area completely covers the largest rectangle determined by a 

diagonal extending from point O to or beyond a point on any 

of the curves, Nos. 6, 5%, 5, 4, 3, 2, 1, or A-l (Special). The 

number of the curve at greatest distance from O cut by the 

diagonal of the rectangle designates the grade of the specimen.

22 Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, p. 18, 

1943.

28 American Society for Testing Materials, Standard methods of testing, 

grading, and classifying natural mica, A. S. T. M. Designation: D 351-46: 

1946 Book of A. S. T. M. Standards, pt. 3-B, p. 144, 1947.

The size groups for domestic and Indian sheet 

mica are summarized in table 2, and a chart for grading mica according to Indian standard sizes is shown 

in figure 10. A discussion of size grading and simplified charts for grading are included in a report issued 

by the United States Bureau of Mines.24

Thickness of mica is determined by means of a 

machinist's micrometer or a rapid-reading dial gage, 

and specifications for standard methods of determination have been outlined by the American Society 

for Testing Materials.25 The minimum acceptable 

thickness for sheet muscovite is 0.007 inch, but thinner material is sometimes sold under specific agreements. Uniformity of thickness, a function of splitting quality, is judged by viewing the split-out films 

between crossed polaroid sheets.

TABLE 2.—Domestic and Indian size groups for sheet mica, including 

punch and sheets larger than punch

^Adapted from charts issued by the Colonial Mica Corporation. Applies to sheets 

not less than 0.007 inch in thickness]

Usual 

domestic 

grades

Small punch '_...

1 Vz by 2 inches _ 

2 by 2 inches _ .. 

2 by 3 inches _____ 

3 by 3 inches _____ 

3 by 4 inches __ . 

3 by 5 inches. _ .

4 by 6 inches _____

6 by 8 inches __ _ 

8 by 8 inches _____ 

8 by 10 inches ____ 

Larger than 8 by 

10 inches.

Usable area in 

single rectangle, 

in square inches

Minimum

1

3 

4 

6 

10 

12 

15 

f 24 

\ 36 

48 

60 

80 

100+

Maximum

3 

4 

6 

10 

12 

15 

24 

36 

48 

60 

80 

100

Minimum 

dimension 

of one side, 

in inches

1

} 2 2 

3 

4 

4 

4 

4 

4

Standard 

India

grades

6 small. 

6.

5. 

4. 

3.

2. 

1.

A-l (Special). 

Extra special. 

Extra extra special. 

Over extra extra special. 

Over over extra extra 

special.

1 Ordinarily included under general designation of "punch," which applies to 

mica yielding usable sheets less than 1 J_ inches wide and 2 inches long but not less 

than 1 square inch in area.

NOTE.—In a new list of tentative specifications, proposed for adoption by the 

A. S. T. M. in 1949, the limiting dimensions for grade 5}^ are altered_sligntly, 

grade 6 small is changed to grade 6, and the old grade 6 is eliminated.

Quality designations for sheet mica vary according 

to the visual classification used, and a combined visual 

and electrical classification recently adopted by the 

American Society for Testing Materials yields still 

another set of terms. Interpretations of visual standards differ from one observer to another, but attempts 

have been made to define the standards and to describe 

the methods of determination in such exact terms that 

inconsistencies are reduced to a minimum. The Indian 

standard groups, beginning with the best quality, are 

clear, clear and slightly stained, slightly stained, fair 

stained, good stained, stained, heavy-stained, lightdotted, black-spotted, and black-stained. In 1938 the

24 Gwinn, G. R., op. cit., pp. 7-10.

25 American Society for Testing Materials, Standard methods of test for 

thickness of solid electrical insulation, A. S. T. M. Designation: D 874-42: 

1946 Book of A. S. T. M. Standards, pt. 3-B, pp. 47-51, 1947.
COMMERCIAL SHEET MUSCOVITE 19

S3HONI 

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i I?

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20 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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Q.

tn

<

6

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6

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Z

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INCHES

according FIGURE; Chart for grading Indian standard 10. mica sizes. to — 
COMMERCIAL SHEET MUSCOVITE 21

American Society for Testing Materials 26 set up 

Indian grading as the American standard and designated the seven principal qualities here listed. Several 

of these groups are sometimes subdivided into more 

specific categories.

Quality classification of sheet muscovite

A.S.T.M.

designation

Clear...___

Clear and slightly 

stained.

A. S. T. M. 

description

Free of all mineral 

and vegetable inclusions, stains, air 

inclusions, waves, 

or buckles. Hard, 

transparent 

sheets.

Free of all mineral 

and vegetable inclusions, cracks, 

waves, and buckles, but may contain slight stains 

and air inclusions.

Fair stained. Free of mineral and 

vegetable inclusions and cracks. 

Hard. Contains 

slight air inclusions and is slightly wavy.

Good stained..._

Supplementary 

description !

Hard and substantially 

flat. Ten percent of 

largest rectangle may 

contain small air spots, 

and the remainder may 

contain a few small air 

specks.

Hard, but may be slightly 

wavy. The largest rectangle may contain 

small air specks, and 

one-third of it may 

contain small air spots. 

One-tenth of the rectangle may contain 

small, light air stains, 

light vegetable stains, 

or a few mineral specks.

Hard, but may be slightly 

wavy. One-fourth of 

the largest rectangle 

must be free of all 

stains except small air 

specks, and threefourths may contain 

small air spots. Onefourth of the rectangle 

may contain light air 

and light vegetable 

stains or small mineral 

specks.

Hard, but may be wavy 

though not buckled. 

The largest rectangle 

may contain small air 

specks and small air 

spots over the entire 

area. Half of the rectangle may contain 

light air stains and 

light vegetable stains, 

but not more than onetenth may contain 

dense air stains or 

mineral spots and lines.

May be soft or hard (approximately 75 percent hard and 25 percent medium soft) and 

may be slightly buckled. The entire area 

of the largest rectangle 

may contain air specks 

and stains, but not so 

densely concentrated as 

to give a silvery appearance to more than 

one-third of the rectangle. Light vegetable stains or some 

small mineral specks 

may be present over 

the entire area, but 

not more than onethird of the rectangle 

may contain dense vegetable stains, lines, or 

__ __ spots.

24 American Society for Testing Materials, Standard methods of testing, 

grading, and classifying natural mica, A. S. T. M. Designation: D 351-46: 

1946 Book of A. S. T. M. Standards, pt. 3-B, p. 145, 1947.

Free of mineral inclusions and 

cracks, but contains air inclusions 

and some vegetable inclusions 

and may be somewhat wavy.

Stained. Free of mineral inclusions and cracks, 

but many contain 

considerable clay 

and vegetable 

stains and may be 

more wavy and 

softer than the 

better qualities.

Quality classification of sheet muscovite—Continued

A.S. T. M.

designation

Heavy-stained. _.

Supplementary 

description 1

May be soft or hard (ap - 

proximately 50 percent 

hard and 50 percent 

soft) and may be wavy 

and slightly buckled 

(about 10 percent may 

be buckled). The 

largest rectangle may 

contain dense air stains, 

and two-thirds of the 

area may contain dense 

vegetable stains, spots, 

or lines.

Black-stained 

and spotted.

A.S.T.M. 

description

Free of mineral inclusions, but contains more clay 

and vegetable 

stains than stained 

quality and is distinctly inferior as 

regards rigidity 

and toughness.

Apt to contain some 

mineral inclusions 

consisting of magnetite (black), 

specularite (red), 

and hydrous iron 

oxide (yellow).

1 Specific quality-classification data were kindly supplied by Bradley Johnson 

shop_ superintendent for the Colonial Mica Corporation, Asheville, N. C. The 

specifications were applied to sheet muscovite purchased by the Colonial Mica 

Corporation and in general are slightly less rigorous than those set up by the 

American Society for Testing Materials. Terms of degree are based on the following descriptions:

Small specks: Size of small pin or needle holes. 

Specks: Twice the size of small specks or about the size of the periods made by a

typewriter. 

Small spots: About three times the size of small specks or about one-sixteenth of an

inch in diameter.

Spots: Any spots larger than small spots. 

Lines: Series of specks or small specks, forming continuous rows. Individual

specks may be in contact or slightly apart. 

Light stains: Stains not sufficiently dense to impair the transparency of block mica

when ordinary print is read through the sheet. 

Dense stains: Stains so dense that they distinctly impair the transparency of the

mica, as roughly determined by the reading test.

Degree of flatness is determined by observation of light reflected from the mica 

sheet as the sheet is rotated back and forth: 

Flat: No wave nor ripple of light. 

Substantially flat: Only a slight wave or ripple of light. 

Slightly wavy: Light appears to rise and fall evenly, without jerkiness, like waves

on a pool of water. 

Slightly buckled: Light appears to rise and fall jerkily, but the rise and fall from

different parts of the sheet do not overlap. 

Buckled: Light appears to rise and fall jerkily, with overlap and confusion of

motion.

Much mica was purchased in this country according to the so-called domestic classification during 

World War II. This fourfold classification and its 

correlation with the A. S. T. M. categories are shown 

in the following tabulation. Mica sold for use in stove 

manufacture is generally graded as A-No. 1, No. 1, 

A-No. 2, and No. 2, in order of decreasing quality. 

However, a general twofold classification is most commonly used for all domestic sheet mica: Black-stained 

and spotted material is referred to as "stained" or 

"electric," and the other types are grouped under the 

general term "clear." Clear micas, according to this 

usage, include the No. 1, No. 2, and No. 2 inferior 

categories listed below:

Domestic quality

classification A. S. T. M. designation

No: 1_____._____--__ 20 percent clear and slightly stained;

80 percent fair stained.

No. 2______________ Good stained.

No. 2 inferior._______ 50 percent stained; 50 percent heavystained.

No. 3_______________ Black-stained and spotted.

During the recent wartime period, particular attention was given to "strategic" mica, which was defined in War Production Board Conservation Order
22 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

M-101 as reasonably flat block and sheet mica of 

heavy-stained quality or better that is free from 

cracks and comparable imperfections. Excluded from 

this category are scrap mica, block mica that will trim 

to a size less than 1 by 1 inch, splittings used in making built-up mica, and the so-called "electric" mica. 

The definition of strategic mica, however, is by no 

means fixed, and the meaning of the term varies with 

changes in military needs, anticipated future requirements, and conditions of supply. Most strategic mica 

is simply sheet material of superior quality. Gwinn 27 

suggests the alternative term "mica of military 

grade," which would include all sizes and qualities of 

mica used in the manufacture of equipment for the 

armed forces. He further suggests that "strategic" 

mica then would be the quality or sizes in short supply 

at a particular time. In 1944 the Army and Navy 

Munitions Board approved the definition of strategic 

materials (including mica) as those "required for 

essential uses in a war emergency, the procurement 

of which in adequate quantities, quality, and time is 

sufficiently uncertain for any reason to require prior 

provision for the supply thereof."

EIjECTRICAL METHODS

Owing to increasing demands for high-quality mica 

in electrical equipment, attention has been focused 

during recent years upon grading and classification 

of sheet material in terms of electrical properties. It 

has been noted that the dielectric constant of muscovite that is otherwise of good quality is uniformly 

within the required limits prescribed for condenser 

use and that consequently there is little need for 

further systematic determination of this property. 

However, such other properties as dielectric strength, 

electrical conductivity, and power factor are of considerable significance in predicting how well given 

sheets or lots of mica will serve in finished condensers.

The American Society for Testing Materials 28 has 

adopted standard methods of test for dielectric 

strength of electrical insulating materials at commercial power frequencies. In the testing of muscovite, high voltages are applied to cleavage plates of 

measured thickness by means of metal electrodes. The 

electrode contact surfaces can be either rounded or 

flat, but one type or the other should be used in any 

tests where direct comparison of results is desired. 

The A. S. T. M. standards ^ specify metal disks 1 inch 

thick and 2 inches in diameter, with edges rounded, to 

a radius of a quarter of an inch. The tests are made 

under conditions of temperature and humidity closely

approximating those under which the particular lot of 

mica is to be ultimately used. Similarly, the mica is 

tested in air or oil, depending upon the type of immersion to be employed in the finished electrical 

equipment.

The magnitude of the break-down or rupture voltage ordinarily varies with the period of time during 

which the voltage is applied, and most electrical insulating materials are more resistant to voltages applied briefly than to comparable voltages applied over 

longer periods. For this reason two types of test are 

commonly used. For short-time determination of 

dielectric strength, the voltage is increased from zero 

to the break-down value at a uniform rate, generally 

0.5 or 1.0 kilovolt per second.30 In the long-time or 

step-by-step test, the initial applied voltage is equal 

to about half the break-down voltage as determined 

by the short-time test. This voltage is subsequently 

increased by equal increments, the duration of each 

"voltage step" being determined from specifications 

for the material involved.

The break-down voltage of an insulating material 

decreases in value with increase in frequency of the 

applied current, so that the dielectric strength generally is lower at radio frequencies than at audio frequencies. At high frequencies the currents flowing in 

a dielectric material produce heat, corona, flash-over, 

blistering, or other deterioration, with a subsequent 

change in values of current and voltage as application 

continues. Application by means of different types of 

electrodes, as well as other variations in test procedure, are briefly discussed by Horton.31

Mica sheets or films can be tested for electrically 

conducting impurities according to the standard 

methods of test, established by the American Society 

for Testing Materials, for conducting paths in electrical slate.32 The mica is laid on a metal plate, and its 

upper surface is then explored by means of a pointed, 

probelike electrode. The electrode and plate are connected to the output terminals of a vibration-type 

spark coil operating on an input voltage of 6. The 

secondary windings should be capable of forming a 

%- to 1-inch spark between two needle points. A 

battery-operated spark test set, developed during the 

recent wartime period by the Bell Telephone Laboratories, is a compact instrument that permits rapid

37 Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, p. 3, 1943.

28 American Society for Testing Materials, Standard methods of test for 

dielectric strength of electrical insulating materials at commercial power 

frequencies, A. S. T. M. Designation: D 149-44: 1946 Book of A. S. T. M. 

Standards, pt. 3-B. pp. 124-130, 1947.

89 Idem, p. 126.

»° American Society for Testing Materials, Standard methods of test for 

dielectric strength of electrical insulating materials at commercial power 

frequencies, A. S. T. M. Designation: D 149-44: 1946 Book of A. S. T. M. 

Standards, pt. 3-B, p. 129, 1947.

' » Horton, F. W., Mica: U. S. Bur. Mines Inf. Circ. 6822, pp. 32-33, 1935 

(revised 1941).

82 American Society for Testing Materials, Standard methods of test for 

conducting paths in electrical slate, A. S. T. M. Designation: D 273-40: 1946 

Book of A. S. T. M. Standards, pt. 3-B, pp. 131-134, 1947; Tentative specifications for natural block mica and mica films suitable for use in fixed micadielectric capacitors, A. S. T. M. Designation: D 748-46T: 1946 Book of 

A. S. T. M. Standards, pt. 3-B, pp. 705-717, 1947.
COMMERCIAL SHEET MUSCOVITE 23

exploration of block and film mica for conducting impurities (pi. 9; fig. 11).

Such mechanical imperfections as pinholes, hair 

cracks, and tears in the mica permit ready passage of 

the spark from electrode to plate during a test, and 

those that do not extend entirely through the sheet 

generally cause puncture or rupture. Similar failure 

is not ordinarily caused by conducting impurities, 

however. These are revealed instead by a distinct 

sparking or glowing in the mica, observation of which 

is best made in a room with subdued lighting.

TO CASE

TO CASE

FIGURE 11.—Simplified circuit of spark-coil set for testing sheet muscovite. 

Courtesy of the Bell Telephone Laboratories, Inc.

Determination of power loss of muscovite is of 

great value in estimating the quality of condensergrade material, particularly if it is to be used in highfrequency circuits. Such loss is best expressed in 

terms of power factor, Q value (reciprocal of power 

factor), or heat equivalent. Systematic measurements 

of power factor have been made in steadily increasing numbers during recent years and have yielded 

much useful information. The National Bureau of 

Standards has been particularly active in this connection.

A resistance-variation method was used in a series 

of power-factor tests by E. L. Hall and others 33 on 

mica specimens submitted by the Federal Bureau of 

Mines, and the results of these tests were first reported in 1931.34 Subsequent tests in the laboratories 

of the Bureau of Standards were made by a much 

more rapid bridge method, in which a preliminary 

capacitance measurement of a given specimen is made 

with a radio-frequency bridge, followed by (1) an adjustment of the bridge power-factor dial to zero with 

respect to a standard air condenser and (2) measurement of the power factor of the specimen by means of 

a power-factor dial on the bridge. Each test specimen 

is made into a simple condenser by means of metalfoil electrodes fastened with a little petroleum jelly. 

Full descriptions of this method have been made by 

Hall.35

A method of test tentatively adopted by the American Society for Testing Materials 36 is similar to the 

above method but specifies cleaning, heating, and 

drying of the samples before testing, use of steel or 

mercury electrodes arranged in such a way that two 

pieces of mica are required for each test, and enough 

determinations of power factor for different electrode 

pressures to obtain a pressure-power factor curve. 

The Bureau of Standards tests ordinarily are made 

on single pieces of mica, without any special procedure in pretreatment of the samples and without 

any attempt to determine changes in power factor 

with changes in electrode pressure.37

The time required for an ordinary power-factor 

determination of mica has been reduced during recent years to about 3 minutes, but prospects for further quickening of the usual method do not appear to 

be good. Thus it remains too slow for commercial use, 

as far as its inclusion in any combined visual-electrical testing procedure is concerned. More rapid 

techniques for estimating power loss in mica have 

been described,38 and a very useful instrument was 

developed by the Bell Telephone Laboratories in response to an appeal from the War Production Board 

in 1942. This instrument, a resonant-circuit test set 

known as a Q-meter (pi. 9), is used for direct determination of Q value, or reciprocal of the power factor. 

As shown in figure 12, it consists essentially of a highfrequency oscillator, a vacuum-tube voltmeter, an 

inductance with a Q value of at least 300 at 1 megacycle, an adjustable capacitor with a maximum of 

about 10 micromicrofarads, and a fixed low-loss air 

capacitor of about 20 micromicrofarads. When used, 

the circuit is first tuned to the applied frequency, and 

a full-scale voltmeter reading is obtained across the 

air condenser by appropriate adjustment of the input. 

When the test piece of mica is inserted in series with 

the fixed air condenser, the adjustable condenser is

83 Horton, F. W., op. cit., pp. 41-46.

34 Lewis, A. A., Hall, E. L., and Caldwell, F. R., Some electrical properties 

of foreign and domestic micas and the effect of elevated temperatures on 

micas: Nat. Bur. Standards Jour. Research, vol. 7, research paper 347, 1931.

85 Hall, E. L., Report on measurement of power factor in muscovite, in 

Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, N. C.: 

U. S. Geol. Survey Bull. 936-A, pp. 18-24, 1942 ; Equipment and method for 

measurement of power factor of mica: Inst. Radio Eng. Proc., vol. 32, pp. 

394-395, 1944; Power factors, in Hidnert, Peter, and Dickson, George, Some 

physical properties of mica: Nat. Bur. Standards Jour. Research, vol. 36, pp. 

342-344, 1945.

84 American Society for Testing Materials, Tentative methods of test for 

power factor and dielectric constant of electrical insulating materials, A. S. 

T. M. Designation: D 150-46T: 1946 Book of A. S. T. M. Standards, pt. 3-B, 

pp. 647-681, 1947.

87 Hall, E. L., Equipment and method for measurement of power factor of 

mica: Inst. Radio Eng. Proc., vol. 32, p. 395, 1944.

38 Barber, A. W., Simplified dielectric loss measurements: Inst. Radio Eng. 

Proc., vol. 17, p. 26, 1937; Snow, H. A., Losses in mica and simple test procedure: Inst. Radio Eng. Proc., vol. 17, p. 19, 1937.
24 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

used to reestablish resonance, and the voltmeter reading is then compared with the original full-scale reading. The series resistance causing the voltage drop is 

the measure of the power loss in the mica. More detailed descriptions and analyses of Q-meter operations appear in the published record.39

TO OSCILLATOR 

CIRCUIT I

FIGURE 12.—Simplified circuits of equipment for determining power factor 

of sheet muscovite. Courtesy of the Bell Telephone Laboratories, Inc. A, 

Circuit indicating general method of testing: L, inductance; Cv, adjustable 

capacitance; Ca, fixed air condenser; Cs , condenser with test piece of mica 

as the dielectric; V, voltmeter. B, Simplified circuit of rapid-reading Qmeter: Vlf tube of oscillator circuit; Ct, adjustable capacitance; CT, adjustable capacitance ; PJt P2 , variable resistances; V, voltmeter; V2, tube 

of vacuum-tube voltmeter; M, meter of vacuum-tube voltmeter; Ca, fixed 

air condenser.

The Q-meter of the Bell Telephone Laboratories 

was first tested for calibration on small lots of sheet 

mica, and since that time it has been employed in 

numerous types of testing programs. It has been 

shown that its usefulness can be extended, through 

proper calibration, to permit direct reading of capacitance, dielectric constant, and absolute value of Q or 

power factor.40 The thickness of each test specimen 

must be dealt with in this connection (fig. 13), although the calibration for Q value is based fundamentally on the probable Q values of capacitors made 

from splittings of the mica.41 The meter was designed

89 Coutlee, K. G., Saving mica by testing: Bell Lab. Rec., vol. 22, pp. 509- 

513, 1944; Progress report on A. S. T. M. round robin power factor tests of 

block mica, Am. Soc. for Testing Materials, sec. 3, subcommittee 9, and 

Bell Telephone Lab., Inc., New York, 8 pp., 1945; Judging mica quality 

electrically: Am. Inst. Electrical Eng. Trans., vol. 64, pp. 1-7, 1945.

40 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 

Eng. Trans., vol. 64, pp. 2-3, 1945.

41 American Society for Testing Materials, Tentative specifications for 

natural block mica and mica films suitable for use in fixed mica-dielectric 

capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. 

Standards, pt. 3-B, pp. 705-717, 1947.

more as a fast and simple means for electrical grading 

of mica than as a precision instrument; nevertheless, 

it yields Q values that agree closely with determinations made by means of other, more exact, methods.

COMBINATION METHOD OF THE AMERICAN SOCIETY FOR 

TESTING MATERIALS

Tests aimed at correlation of electrical properties 

of sheet muscovite with its behavior under actual use 

in condensers have been made from time to time by 

manufacturers of electrical equipment. Most of these 

tests have been used to solve specific problems encountered during production, rather than to establish basic generalizations; hence few have provided 

comprehensive information or have involved material 

of specifically known source. In response to a growing need for more systematic data, the United States 

Geological Survey, the National Bureau of Standards, 

the R. C. A. Manufacturing Co., and the Scintilla 

Magneto Division of the Bendix Aviation Corp. determined the relations between power factor and 

visual characteristics and between these properties 

and utility in built-up condensers for several samples 

of muscovite from mines in the southeastern United 

States. This work was done in 1940 and 1941, and 

the results were summarized and discussed by Kesler 

and Olson in 1942.42 The investigations were limited 

in scope by the small quantities of raw mica available 

for testing, but both the manufacturing organizations 

concluded that most of this mica compared favorably 

with India mica in dielectric strength and power 

factor. It was pointed out, however, that the proportionate yield of finished films was inferior to that 

from India mica, owing mainly to relatively poor 

preparation of the domestic material.

40

20 80

100-

FIGURE 13.—Meter scale of the Bell Telephone Laboratories' Q-test set, showing the relation between power-factor groups (E—1, E-2, and E-3) and 

thickness of test pieces (0.010-, 0.020-, and 0.030-inch controls).

A much more extensive series of investigations was 

begun in 1942 by the Bell Telephone Laboratories at 

the request of the War Production Board. Block mica 

from seven domestic mines was classified according 

to Q value by the rapid method and was tested for conducting impurities with the spark set. This electrically selected material, much of which was marred by

42 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 225 PLATE 9

A. SPARK-COIL SET FOR RECOGNITION OF PFNHOLES. CRACKS, AND CONDUCTING IMPURITIES

B. RAPID, DIRECT-RE DING Q-METER

TESTS SETS FOR ELECTRICAL CLASSIFICATION OF SHEET MICA 

Courtesy of the Bell Telephone Laboratories, Inc.

COMMERCIAL SHEET MUSCOVITE 25

air stain and inclusions of iron oxide, was then split, 

punched, and built up into molded 1,000- and 4,000- 

micromicrofarad capacitors by the Western Electric 

Co. In subsequent performance tests these capacitors, 

as noted by Coutlee,43

satisfactorily met Q, insulation-resistance, dielectric-strength, 

corona-voltage and life requirements. ... As a result of these 

tests, it was concluded that block mica could be selected dependably for use in capacitors, some for the better grades and 

the rest for less critical types, and that the test methods appeared entirely adequate for the electric control of capacitor 

mica. Even the two lots of capacitors made with block mica in 

which conducting inclusions were detected by the spark test 

and low Q by the block mica Q tests gave excellent dielectric 

strength and life performance.

A new set of specifications for condenser-quality 

mica, based upon electrical, physical, and visual requirements, was then developed in terms of all available testing data, and checking of these specifications 

on a commercial scale was started immediately thereafter. This involved electrical classification of more 

than a ton and a half of block mica from Brazil, 

Canada, India, and the United States. Twenty-two 

different types of material were included. All the mica 

had been previously qualified, by visual means alone, 

as stained or poorer according to the India scale or as 

No. 2, No. 2 inferior, or No. 3 according to the domestic scale. The blocks were split, punched, and 

made up into more than 47,000 capacitors of various 

types by eight manufacturers. Results of performance tests on these capacitors, which have been recorded and discussed in detail elsewhere,44 led to the 

conclusion that electrical-visual methods of selection 

can be satisfactorily applied to capacitor-grade mica 

required for given end uses.

A set of specifications for commercial sheet mica, 

based upon a combination of visual and electrical 

tests, was subsequently proposed by the American 

Society for Testing Materials.45 The system of classification based upon these specifications has been tested 

commercially and "found to be both practicable and 

reliable." It

does not discriminate against the presence of spots or stains 

in even first quality electrically selected mica providing the 

mica conforms to specific electrical and physical requirements.

48 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 

Eng. Trans., vol. 64, p. 6, 1945.

** Townsend, J. R., Final report on commercialization of mica testing 

equipment: War Production Board, War Metallurgy Comm. Rept. W-1B1, 

1944 ; Townsend, J. R., Addendum to War Metallurgy Cbmm. Rept. W-1B1: 

War Production Board Rept. W-170, 1944; Coutlee, K. G., Saving mica by 

testing: Bell Lab. Rec., vol. 22, pp. 509-513, 1944; Coutlee, K. G., Progress 

report on A. S. T. M. round robin power factor tests of block mica. Am. 

Soc. for Testing Materials, sec. 3, subcommittee 9, and Bell Telephone Lab., 

Inc., New York, 8 pp., 1945 ; Coutlee, K. G., Judging mica quality electrically: 

Am. Inst. Electrical Eng. Trans., vol. 64, pp. 1-7, 1945.

45 American Society for Testing Materials, Tentative specifications for 

natural block mica and mica films suitable for use in fixed mica-dielectric 

capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. 

Standards, pt. 3-B, pp. 705-717, 1947.

Appreciable amounts of air inclusions and waviness also are 

permitted in all electrical quality classes providing the mica 

meets specific electrical and physical requirements. However, 

such mica is not considered generally as desirable as mica 

having lesser amounts of such defects. Mica capable of meeting specific requirements also may be of any basic color such 

as white, ruby, light green, dark green, brownish green, rum, 

or other colors derived from any source.

In this latest A. S. T. M. method of classification 

the mica is graded for size according to the so-called 

India scale (figs. 9, 10) and is qualified according to 

certain electrical, visual, and other physical features 

by means of carefully specified methods. Division 

into three general classes is accomplished by evaluation of several properties as shown in table 3. Standard procedures are recommended for determination 

of visual properties, dielectric strength, and other 

characteristics, and two methods for determining Q 

value or power factor are recognized as satisfactory.

TABLE 3.—Electrical, physical, and visual quality requirements of

natural block mica and mica films for use in capacitors

[Adapted from A.S.T.M. specifications]

Electrical properties:

Q value or power 

factor at 1

Dielectric strength 

(volts per mil 

at 60 cycles per 

second), minimum:

Single. __-- . 

Dielectric conPhysical properties: 

Weight loss on 

heating (5 minutesat600°C.)t 

maximum perThickness uniformity (mica 

films)3

Temperature coefficient of capacitance and

Visual qualities: 

Air inclusions: 5 

A __ . ..... 

B_. --.....-

C.-.-. _ ..

Waves: • 

A..... — .-

B _ . ......

C— — ....

Cracks __ ------

Pinholes. ------- 

Stones __ ------ 

Buckles— —— -_

C-l

iE-1.... ___ ...

1,000..... .......

850 ___ .. ......

(')

0.2 ______ . ...

Best... .........

(4)

None to slight _ _

Flat to slight ....

None ———— .....

None —— -____-_- 

None __ ........ 

None —— ———— .

C-2

iE-2— ---------

1,000 .._.._...._.

850 ____ ......

(2)

0.2 __ . _ . ___

Best... _ ..._..

(4)

None to slight ....

None _ ---------

None _ ------ ... 

None —————— ... 

None.. ...._..__.

C-3

"E-3.

1,000.

850. 

«

(«>

None to slight.

None

None 

None 

None

1 See table 4 for corresponding values of Q and power factor.

2 No specified requirement is needed.

3 Until definite values can be specified, the permissible amounts of such defects 

shall be agreed upon by the purchaser and the manufacturer.

4 It has been found that the temperature coefficient of capacitance and retrace of 

capacitors made with classes C-l, C-2, and C-3 are more dependent on such 

factors as electrical and mechanical design and manufacturing technique than any 

differences that may be attributed to the mica itself.

6 The amount of air inclusions shall not exceed the specified limits for each 

subclass as judged by the photographic reference standards given in figures 1-3 

on pages 708-713 of the 1946 Book of A. S. T. M. Standards. The permissible 

amount of air inclusions shall be stated by suffixing the letter A, B, or C, as the 

case may be, to the required electrical-quality class.

8 Until definite values can be specified, the permissible amount of waves, 

buckles, and ridges shall be agreed upon by the purchaser and the manufacturer. 

The permissible amount of waves shall be stated by suffixing the letter A, B, or C, 

as the case may be, to the letter denoting the amount of permissible air inclusions. 

For example, class C-l B A block mica films are of the best electrical quality with 

a medium amount of air inclusions and an absence, or only a slight amount, of 

waves.
26 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

On the basis of its Q value, mica can be divided into 

the three groups shown in table 4. These are designated E-1, E-2, and E-3. E-1 mica that is flat to only 

slightly wavy, contains no conducting impurities and 

little or no air stain, and is free from cracks, tears, 

pinholes, stones, buckles, and ripples corresponds to 

fair stained or better material in the older, strictly 

visual, classifications and yields films of top quality, as 

may be seen in the following tabulation of approximate equivalents between electrical quality and visual 

quality based on an A. S. T. M. table:

Electrical quality 

(A. S. T. M. Specifications D 748)

Visual quality 

(A. S. T. M. Methods D SS1)

Block mica and mica films Block mica

Class C-l AA_____ Fair stained (min.)

Class C-l BB _____ Good stained

Class C-l CC_ _ _ Stained

Mica films1

First quality 

Second quality 

Third quality

i Mica-film quality is not specified in A. S. T. M. Methods D 351. .The qualities 

given are in commercial use, and the equivalent block-mica quality is assumed.

TABLE 4.—Q and power-factor values for electricaJrquality groups E-1, E-2, and E-S 

[Adapted from A. 8. T. M. specifications]

rn 

Q or power-factor 

group

E-1. __._._„__

E-2. _ ........

E-3 __ ........

Form of mica

/Block... .._.---

\Films_.--._: _

/Block..... _ ..

/Block.. ________

Q value

2500 (min.) » ________

2500 (min.) * ________

350-2500 i— _„ __._

1500 (min.) *__ _____

50-3501 ____________ 

200-15002 __________

Power factor

0.00285-0.0004.... .___.. ———————— .-

0.00066 (max.)--- ———————--

0.02-0.00285.....-.- ——————————— - 

0.005-0.00066——-- — ------ ——————

Rapid-method meter reading 

(thickness of block mica or stacked mica films, in inches)

0.010 (0.007-0.015)

95-100

87-95

50-87

0.020 (0.015-0.025)

95-100

77-95

32-77

0.030 (0.025-0.035)

95-100

71-95

24-71

i Extensive commercial tests have verified the validity of the Q values of capacitors made with group E-1 block mica to a satisfactory degree. However, the ranges 

for groups E-2 and E-3 are tentative and subject to further verification. •>•*» <* (*»»i«a.^-»_», •<•_»•

* Minimum Q values for material purchased as films and tested in stacks of films 0.010 to 0.030 in. in thickness and probable minimunvQ values of.molded-type, 

1,000-mmf., stacked-foil, and silvered capacitors. These will apply when all factors that would adversely influence the Q value are under control.j_______^.

These equivalents are valid only if the Q value of 

the mica is E-1. Such mica "is suitable for use in all 

sizes and types of silver and foil electrode molded and 

clamped unit capacitors, including the most critical 

types, for use in high stability tuned circuits, as well 

as high current radio frequency capacitors used in 

radio transmitter circuits." 46 Mica that is graded 

as E-2 and has the other characteristics of class C-2 

is also suitable for such condensers, although "a certain percentage of capacitors made with class C-2 

block mica and films may show a somewhat higher 

temperature rise in transmitter types than capacitors 

made with class C-l block mica or mica films."47 Mica 

of class C-3 (E-3 in terms of Q value), though not of 

top quality, is none the less suitable for use in condensers where high Q value, high stability, and low 

temperature coefficient are not required.

PREPARATION AND MARKETING

The first rough separation of mica generally is 

made at the mine—at the face or portal, in a nearby 

shed, or on the dump at a later time (fig. 14). Obvious 

mine scrap is separated from the better books, from 

which adhering fragments of quartz, feldspar, and 

other foreign material are then cobbed. Some of this 

rough-cobbed or selected mine-run mica is sold as 

such to jobbers or manufacturers, but at many mines 

it is prepared further. The books are split or rifted by

means of a blade into plates that generally are less 

than three-sixteenths of an inch thick. Through skilled 

handling of the rifting knife, defective laminae are

Rough-cobbed of 

selected mine-run 

mica (incl 

thumb-fiorid rou 

materia 

sheds)

Cut,punched,and stamped 

sheets and splittings

Mica plate, builtup mico board, 

and other composite forms

Coarse-and fineground mica

*• American Society for Testing Materials, Tentative specifications for 

natural block mica and mica films suitable for use in fixed mica-dielectric 

capacitors, A. S. T. M. Designation: D 74&-46T: 1946 Book of A. S. T. M. 

Standards, pt. 3-B, p. 706, 1947.

* Idem, p. 707.

FIGURE; 14.—Outline of operations involved in the extraction and preparation 

of sheet, punch, and scrap mica.

removed with a minimum waste of higher-quality material, and block mica, punch and washer stock, and 

bench or shop scrap are thereby obtained. In some
COMMERCIAL SHEET MUSCOVITE 27

districts the cobbed mica is commonly split into plates 

more than three-sixteenths of an inch thick, but both 

these and the thinner riftings are generally known as 

"plate" mica. As such, they constitute a specially selected form of mine-run material.

After rifting, the ragged and broken edges of many 

plates are removed with the fingers, a process known 

as "thumb trimming." This is an especially common 

practice in districts where much of the mica is severely ruled or marked by "A" structure. Some 

thumb-trimmed material may be sold to manufacturers as such, or it may be further trimmed with a 

knife and its value thereby increased. Some mica is 

trimmed with shears, but most is cut by a sickle or 

hand knife. As pointed out by Spence,48 material from 

which waste has been removed by a mechanical knife, 

guillotine, or shears is known in the trade as "knifetrimmed" or "shear-trimmed." It has unbeveled edges 

that generally are straight or nearly straight. Mica 

trimmed by hand, with beveled and less regular edges, 

is termed "sickle-trimmed." This distinction is not 

often made, however, in the preparation of domestic 

mica, and material trimmed with a hand knife is commonly referred to as "knife-trimmed." During recent 

years attempts have been made to employ several 

forms of blades and saws for mica trimming in the 

United States, but without much success.

Most India mica is knife-trimmed or sickletrimmed free of cracks and flaws, but domestic procedure is somewhat different. Half-trimmed mica, for 

example, is cut on two adjacent sides with no cracks, 

reeves, cross grains, nor ribs extending from those 

sides. Three-quarter-trimmed mica is cut on all sides, 

with no cracks nor comparable flaws extending from 

two adjacent sides or into the final pattern area. Only 

full-trimmed mica is comparable to India-trimmed 

material in that it is cut on all sides and contains none 

of the flaws noted above. Moreover, upper limits generally are set on the number and size of "V" or figure 

cuts on any one piece of mica, as well as on the proportion of pieces with such cuts in a given lot of mica.

Careful rifting and trimming are very important 

in the preparation of sheet mica, not only because 

waste is thereby held to a minimum, but because wellprepared mica yields a higher proportion of usable 

material in the later stages of manufacture. The mica 

may be rifted, or it may be trimmed to the required 

degree of quality, and the process adopted depends 

chiefly upon the kind and distribution of impurities 

and structural imperfections that the mica contains.

A large proportion of sheet mica is consumed in the 

form of splittings. These are films 0.0007 to 0.001 

inch thick that generally are cleaved from punch and 

the smaller sizes of sheet stock. Some are derived,

also, from thin films or skimmings that are a byproduct of the rifting of larger sheet material. Splittings 

are used in the manufacture of built-up mica board 

and other forms of electrical insulation. Although 

many mechanical devices have been tested for the 

preparation of these films, practically all are still split 

outside the United States by hand methods, generally 

in places where labor costs are very low.

The cut mica blocks that represent punch, circle, 

and larger sheet stock are processed into disks, washers, and thin plates of various sizes and1 shapes. This 

generally involves additional splitting, followed by 

trimming, cutting, or stamping into more or less 

standardized patterns. Most of this material is then 

cut to final form, if necessary, by the manufacturers 

of the devices in which the mica is to be used. Composite forms can be built up to any desired thickness 

by the cementing of individual pieces with shellac, 

glyptol, or a similar bonding medium. In general only 

a small proportion of the prepared block material is 

represented in the finished product. The bulk of such 

material is skimmed or cut away as waste, which is 

marketed as scrap of superior grade.

Most scrap mica, including material derived from 

nonpegmatitic sources, is processed by grinding. It is 

classified on the basis of its freedom from quartz, 

feldspar, and other gritty impurities and on the basis 

of its color when ground.

Prices for sheet mica not only fluctuate widely in 

response to variations in demand, but vary at any 

given time according to the size and quality of the 

material. The general ranges for clear and stained 

trimmed sheet mica in the Southeastern States 

during a 30-year period are shown in table 5. The 

value of punch or untrimmed small sheet mica ranged 

from 21/2 cents to about 15 cents per pound during 

the same period. Trimmed electric (stained sheet) 

mica is sold according to a sliding price scale, with 

values consistently lower than those for clear material of comparable size. Electric mica also is sold 

as thumb-trimmed block, generally at prices that vary 

according to the estimated proportion of waste. Many

TABLE 5.—Price ranges of dear and stained sheet mica in the Sovthk eastern States during the period 1910-JfO

48 Spence, H. S., Mica: Canada Dept. Mines, Mines Branch, Pub. 701, p; 

54. 1929.

Size (inches)

1H by 2-_. — . ————— ------- ——— . ———— .

2 by 2 _____ „— — . ——— . ——————————

2 by 3 _____ „ — . ———————————————

3 by 3 ___ -. ————— - ———— - ——————— -

3 by 4 __ —————— -. ————————— - ——— —

3 by 5--_ —————— ----- ———— . ————— —

4 by 6 ___ —— . — - ———————————————

6 by 8 __ ————— - ——— - —————————— --

8 by 10_ —————— . ——— . ———— - ——————

Range in price per pound 

(dollars and cents)

Clear

0.12-0.60 

.22-1.05 

.38-1.45 

.58-2.00 

.78-2.30 

.95-2.70 

1.75-3.65 

2.25-7.25 

3.50-11.50

Stained

0) 

0.06-0.40 

.10- .60 

.15-1.25 

.30-1.50 

.48-1.75 

1.70-2.25 

1.25-2.50 

2.00-3.00

1 Under ordinary conditions the smallest size of stained mica purchased as sheet 

material in the Southeastern States is 2 by 2 inches.
28 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

jobbers purchase selected mine-run material (either 

clear or stained), rift and trim it, and sell the prepared mica to manufacturers. Others purchase punch 

and washer stock from which they recover and trim 

sheet material, and still others have recovered sheet 

and punch mica from mine scrap. Buyers of mica are 

listed in United States Bureau of Mines Information 

Circular 7258.

During war periods, when the demand for sheet 

mica is greatly increased and the problems of supply 

are often complex, prices reach very high levels. The 

rising trend during the period December 1941-December 1944 and the subsequent sharp drop in prices 

are shown in table 6. Through parts of 1942 and 1945

strategic-quality mica was purchased by private individuals and organizations and by the Colonial Mica 

Corporation as well, but during most of the wartime 

period purchases were made solely by the Colonial 

Mica Corporation. With the end of hostilities and the 

trend toward peacetime economy, most of the subsidies for mica production were removed and the 

prices for sheet material dropped sharply. The attendant closing of many mines and general diminution of sheet-mica production curtailed the supply of 

punch and washer material, so that prices for such 

mica actually remained at wartime levels or even 

rose. During 1945, for example, some lots of punch 

mica were sold at prices of 45 to 50 cents per pound.

TABLE 6.—Price schedules for domestic clear sheet mica during the period December 1941—February 1945 

[Adapted in part from Billings and Montague, The wartime problem of mica supply: Eng. and Min. Jour., vol. 145, no. 8, p. 94, 1944]

Size

IJiby 2 inches. 

2 by 2 inches . 

2 by 3 inches. 

3 by 3 inches . 

3 by 4 inches. 

3 by 5 inches. 

4 by 6 inches. 

6 by 8 inches.

Price per pound, in dollars and cents

Private purchasers

^December 

1941

0.10-0.15 

.45- .65 

.60- .85 

1.30-1.50 

1.90-2.05 

2.15-2.25 

2.60-2.75 

3.60-3.70 

5.25-5.50

April-May 

1942

0.12-0.16 

.50- .65 

.95-1.10 

1.50-1.85 

2.00-2.35 

2.25-2.60 

2.75-3.00 

3.75-4.00 

5.50-6.00

Colonial Mica Corporation

June 

19421

0.22 

1.10 

1.75 

2.75 

3.50 

4.25 

5.00 

6.25 

8.00

November 

1942 i

0.30 

2.40 

3.52 

4.64 

5.12 

6.08 

7.04 

8.00 

9.12

May 

1943 «

5.00

February 

1944 * 3

6.00

August 

1944*

6.00 & 8.00

February 

1945*

2.25

February 1945 «

No. 1

quality

1.70-3.50 

5.00

7.80 

9.10

10.50 

13.30 

20.10

No. 2 

quality

1.25-2.50 

3.50

5.50 

6.50

7.10 

8.30 

12.70

No. 2 inf. 

quality

0.50-1.10 

1.60

2.60 

3.40

4.65 

6.20 

9.20

Private 

purchasers, 

February 

1945'

0.08-0.15 

1.00 

1.40 

2.00 

2.55 

3.00 

3.45 

4.30 

6.50

1 Punch, material required to yield 20 percent or more of trimmed pieces 1 by 1 

i nch or larger; price scale for larger mica based on No. 1 quality and half trim, with 

maximum bonuses of 30 and 40 percent for three-quarter trim and full trim, respectively.

^ Uniform price per pound established regardless of size or quality within 

strategic range. Punch material required to be full-trimmed; sheet mica, threequarter-trimmed .

3 Includes $1 bonus.

SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 

DISTRIBUTION OF DEPOSITS

The Southeastern States constitute the chief micaproducing area in the United States. The bulk of this 

production is obtained from mines in western North 

Carolina, but important deposits also are worked in 

parts of Virginia, South Carolina, Georgia, and Alabama. Fourteen well-defined mica-mining districts 

occupy a belt nearly 600 miles long and about 60 miles 

in average width. It extends from east-central Virginia southwestward through the western Carolinas 

and north Georgia to central Alabama (fig. 15). This 

general mica-producing region can be divided into 

two smaller belts: the Blue Ridge belt of North Carolina and Georgia on the northwest and the longer and 

broader Piedmont belt on the southeast. Within the 

Blue Ridge belt are the Jefferson-Boone, Wilkes, 

Spruce Pine, and Buncombe districts of North Carolina; the Franklin-Sylva district of North Carolina 

and Georgia; and the North Georgia district. The 

Piedmont belt comprises the Amelia district of Vir4 Premium price of 88 paid for full-trimmed sheet mica 2 by 2 inches in size 

and larger; $6 for full-trimmed punch material and full-trimmed sheet mica smaller 

than 2 by 2 inches and for three-quarter-trimmed sheet in the larger sizes.

6 Blanket price for full-trimmed sheet and punch mica regardless of size or quality 

within strategic range; drop in price reflects general subsidy-removing policy with 

respect to production of strategic mica.

• Alternative price schedule based on size and quality of three-quarter-trimmed 

mica.

7Price based on untrimmed punch and three-quarter-trimmed sheet mica.

ginia; the Ridgeway-Sandy Ridge district of Virginia 

and North Carolina; the Shelby-Hickory district of 

North Carolina; the Hartwell district of Georgia and 

South Carolina; the Thomaston-Barnesville district 

of Georgia; and three districts in east-central Alabama. Many small deposits are scattered through the 

areas between and around individual districts.

The Spruce Pine district, which occupies parts of 

Avery, Mitchell, and Yancey Counties in the Blue 

Ridge province of western North Carolina, is the 

largest in North America, both in annual and total 

production of mica and in the number of mines and 

prospects that lie within its limits. Since 1900 the 

output from at least 800 mines has been marketed 

and hundreds of other deposits have been prospected. 

The number of mica deposits that have been mined or 

prospected throughout the entire southeastern part of 

the United States probably amounts to 4,900 or more. 

Table 7 is a tabulation, by districts, of the number of 

deposits from which strategic-quality mica was obtained during the period of World War II, the number
SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 29

of deposits studied in detail by the United States Geological Survey during the period 1939-46, and the 

estimated minimum number of deposits worked since 

1880. There is little correlation between the number 

of mica mines and prospects in a district and its areal

extent (fig. 15). The Spruce Pine district, for example, occupies an area of 250 square miles or less, 

whereas the much more extensive Buncombe district 

to the south probably contains less than one-twentieth 

as many deposits.

89° 87

' Pegmatite 

area or district

33

31

33

87

FIGURE 15.—Distribution of principal pegmatite areas and districts in the southeastern United States.

HISTORY AND PRODUCTION

The pegmatites of the Southeastern States have 

yielded large quantities of sheet and scrap mica, feldspar, quartz, and kaolin, as well as some beryl, cassiterite, garnet, monazite, specimen material, spodumene, tantalum-columbium minerals, topaz, uranium 

minerals, vermiculite, and zircon. Feldspar has been 

an important byproduct of some of the mica mining, 

and much mica has been produced as a result of widespread operations for feldspar and kaolin. Many of 

the minor minerals have been produced solely as byproducts.

852911°—50—3

Mining operations for sheet mica date back at least 

to the fourteenth century, and the remnants of many 

ancient trenches, pits, and cuts are recorded from 

several districts.49 Evidently these were made by 

Indians, who probably used the mica for ornamental 

purposes. Their work was confined to the soft, 

kaolinized parts of the deposits, but a little of the 

mining was done underground. Modern mining was

49 Kerr, W. C., The mica veins of North Carolina: Am. Inst. Min. Eng. 

Trans., vol. 8, pp. 457-462, 1880 ; Phillips, W. B., Mica mining in North Carolina: Eng. and Min. Jour., vol. 45, pp. 286, 306-307, 322, 324, 382-883, 398. 

418, 436, 1888; Smith, C. D., Ancient mica mine in North Carolina: Smithsonian Inst. Kept., pp. 441-443, 1876; Sterrett, D. B., Mica deposits of the 

United States: U. S. Geol. Survey Bull. 740, pp. 28, 167, 224-225, 1923.
30 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

started shortly after the Civil War, chiefly in the Virginia Piedmont and in the Shelby-Hickory, Spruce 

Pine, and Franklin-Sylva districts of North Carolina. 

Many mines were opened after 1875, and large quantities of stove mica were produced from 1885 to 1900. 

Increasing demands from the electrical industry 

stimulated later production, especially after 1910, and 

the favorable price schedules of World War I brought 

mining and prospecting activities to the highest levels 

known up to that time. A severe but brief postwar 

slump was followed by a 9-year period of prosperity in 

the industry, during which the average annual production of sheet and punch mica in the Southeastern 

States was about 760,000 pounds. The output fell 

sharply in 1931 and 1932 but later slowly rose until it 

reached the million-pound-per-year level. The unprecedented demands during World War II led to 

even greater production, and in 1943 a total of 2,132,- 

826 pounds of sheet and punch mica was obtained. 

This figure is all the more remarkable, as compared 

with previous records, when the superior degree of 

preparation of the recent output is taken into account. 

Much mica that would ordinarily have been sold as 

half-trim or punch was prepared as large full- and 

three-quarter-trimmed sheets and as small fulltrimmed sheets; hence bulk was materially reduced.

TABLE 7.—Summary data on the number of mica deposits in the southeastern United States

District or area

Wilkes.. ___ . _____ .

Outlying North Carolina

Ridgeway-Sandy Ridge.. 

Outlying Virginia _ .

ShelBy-Hickory. — _ . _ 

Outlying North Carolina

Outlying South Carolina. 

Hartwefl __________

Thomaston-Baraesville _ _

Alabama „_

Total

Number of deposits yielding strategic 

mica during 

World War II

28

*>*>

708

47

411

19

14

13

34 

24

263 

37

27 

76

59 

34

41

1,847

Number of 'deposits studied 

in detail by 

Survey during 

1939-46

12

7

594

21

433

7

3

83

30 

29

168 

6

13 

33

69 

11

126

1,645

Estimated minimum number of 

mines and prospects

en

50

2,000

sn

onn

iwi

Rfl

120

80 

200

360 

100

60 

150

160 

170

250

A Q7O

The yearly production of sheet and punch mica 

from North Carolina, from the Southeastern States, 

and from the entire United States during the period 

1912-44 is shown in table 8. Nearly 97 percent of the 

Southeastern production has been derived from deposits in North Carolina, and this mica represents 

about 95 percent of the total value. Most of the remaining 3 percent of the production has been obtained 

from Georgia and Virginia. Through the same period 

the Southeastern States have 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 has amounted to 62 percent of the 

total. In general these proportions have been rising 

slightly during recent years. The average value of the 

output from the Southeastern States has been about 

39 cents per pound. In contrast, it was 50 cents per 

pound during the wartime period 1917-19 and 89 

cents per pound during the period 1942-44.

TABLE 8.—Production of sheet and punch mica, 1912-44, in North 

Carolina, the Southeastern States, and the entire United States

[Based on data from U.S. Geological Survey and U.S. Bureau of Mines, Mineral 

Resources of the United States and Minerals Yearbook, 1912-44]

Year

1912 __ .

1913 .....

1914 .....

1915 .....

1916 __ .

1917 .....

1918 .....

1919 __ .

1920.....

1921 .....

1922 .....

1923 .....

1924 _ ..

1925 .....

1926 .....

1927 __ .

1928.-—

1929 .....

1930 .....

1931 .....

1932 .....

1933 .....

1934 __ .

1935 .....

1936 .....

1937 .....

1938 __ .

1939..—

1940———

1941 » ....

1942 » _ .

1943 » ....

1944 » __

Total.

North Carolina

Amount

(pounds)

489,599

803,462

274,121

281,074

546,553

643,476

941,200

1,021,306

1,084,946

230,632

544,495

1,130,283

597,385

592,478

700,313

665,360

777,395

894,200

749,074

389,426

127,696

162,672

293,381

512,590

730,446

1,044,328

632,646

401,170

1,002,646

1,614,863

1,654,895

1,901,120

814,874

24,250,005

Value

(dollars)

219,874

230,674

171,370

266,660

380,700

543,207

460,450

331,498

405,654

51,851

119,767

188,317

108,656

105,376

150,362

114,514

129,706

150,293

112,451

51,657

18,322

21,107

38,671

77,598

119,653

218,176

87,879

69,344

218,154

318,783

505,634

1,772,324

1,530,625

9,286,297

Southeastern States 

(Va., N. C., S. C.,

Ga., and Ala.)

Amount

(pounds)

?>

0)

308,121

294,376

616,700

761,044

1,239,700

1,092,152

1,395,838

260,084

598,321

1,194,628

682,961

622,421

742,345

713,554

804,457

936,524

776,075

405,234

139,863

162,731

295,994

517,157

783,768

1,057,320

655,866

419,210

1,046,535

1,707,488

1,783,646

2,132,826

891,465

25,028,404

Value

(dollars)

• 0)

0)

175,704

272,830

406,000

581,707

587,100

385,312

461,936

55,245

130,601

194,652

116,475

111,328

157,576

127,755

133,492

155,649

117,573

53,475

19,554

21,113

38,788

78,052

124,115

219,247

89,706

71,317

223,817

332,050

555,475

2,007,583

1,715,046

9,720,273

United States

(all)

Amount

(pounds)

845,483

1,700,677

566,933

563,821

865,863

1,276,533

1,644,200

1,545,709

1,683,480

741,845

1,077,968

2,063,179

1,460,897

1,793,865

2,172,159

1,512,492

1,681,777

2,035,128

1,465,485

962,953

338,997

364,540

583,528

936,633

1,319,233

1,694,538

939,507

813,708

1,625,437

2,666,453

2,761,844

3,448,199

1,523,313

46,656,377

Value

(dollars)

282,823

353,715

278,540

378,259

524,485

753,874

731,810

483,567

546,972

118,513

194,301

311,180

212,035

321,962

400,184

212,482

230,956

286,321

177,307

111,830

45,882

53,179

90,268

191,160

203,879.

285,244

139,333

138,963

291,685

566,858

725,030

3,228,742

3,262,711

16,134,040

1 No complete data available. 

* Includes splittings.

OCCURRENCE 

GENERAL. GEOLOGIC FEATURES

Most of the mica-bearing pegmatites occur in 

metamorphic rocks, chiefly mica schist and gneiss, 

impure quartzite, and hornblende schist and gneiss, 

with minor interlayered kyanite gneiss, sillimanite 

gneiss, graphitic schist, recrystallized limestone and 

dolomite, and various types of chloritic rocks. The 

foliated micaceous rocks have been included in the 

Wissahickon schist of Virginia, the Carolina gneiss 

of Virginia, Georgia, and the Carolinas, and the Ashland mica schist of Alabama. They form the bulk of 

the country rock in most of the pegmatite districts. 

The hornblendic rocks, which generally are interlayered with the micaceous types, have been grouped 

with the Roan gneiss in Georgia and the Carolinas 

and have been termed metagabbro in parts of Virginia. They are abundant in the Jefferson-Boone, 

Spruce Pine, Franklin-Sylva, Shelby-Hickory, and
SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 31

Ridgeway-Sandy Ridge districts but are not common, 

for example, hr the Amelia, Hartwell, and Alabama 

districts. In general these hornblendic rocks appear 

to be metamorphosed sills, dikes, and flows of intermediate and basic composition; probably they are appreciably younger than the micaceous schists and 

gneisses. Both rock types may be pre-Cambrian in 

age.

Large masses of silicic intrusive rocks, probably 

late Paleozoic in age, are exposed in many districts. 

They are fine to very coarse grained and range in composition from quartz monzonite to quartz diorite. 

Some, like those in the Spruce Pine district, are pegmatitic in texture and are so leucocratic that they 

have been termed alaskite.50 Others contain more 

biotite or other mafic minerals and have a typical 

"salt-and-pepper" texture. Associated with most of 

these intrusive masses are finer-grained satellitic sills 

and dikes, as well as many types of hybrid or migmatitic rocks.

Most of the commercial mica-bearing pegmatites 

occur in metamorphic rocks. In general they are 

similar in composition to nearby intrusive masses and 

in some districts are demonstrably related to them. 

Some pegmatites in Spruce Pine and other districts 

occur within, rather than adjacent to, stocks and 

large sill-like bodies of quartz diorite and granodiorite, but these are in the minority and account for only 

a small proportion of the mica produced from these 

districts. Many geologic features of mica-bearing 

pegmatites in the Southeastern States have been described by Sterrett.51 Those in Virginia are discussed 

by Pegau,52 C. B. Brown,58 and W. R. Brown;54 those 

in North Carolina by Sterrett,55 Maurice,56 Kesler and 

Olson,57 and Olson; 58 those in Georgia by Galpin 59 

and by Furcron and Teague; 60 and those in Alabama 

by Clark.61 General geologic information concerning

M Hunter, G. E., Residual alaskite kaolin deposits of North Carolina: Am. 

Ceramic Soc. Bull., vol. 19, p. 98, 1940.

61 Sterrett, D. B.. Mica deposits of the United States: U. S. Geol. Survey 

Bull. 740, pp. 28-30, 70-71, 281-282, 307-308, 1923.

M Pegau, A. A., The pegmatites of the Amelia, Goochland, and Ridgeway 

areas, Va.: Am. Jour. Sci., 5th ser., vol. 17, pp. 543-547, 1929; Pegmatite deposits of Virginia: Virginia Geol. Survey Bull. 33, 1932.

52 Brown, C. B., Outline of the geology and mineral resources of Goochland County, Va.: Virginia Geol. Survey Bull. 48, 1937.

"Brown, W. R., Mica deposits of Virginia: Virginia Geol. Survey Bull, 

(in preparation).

w Sterrett, D. B., Mica deposits of western North Carolina: U. S. Geol. 

Survey Bull. 315, pp. 400-422, 1907; Mica deposits of North Carolina: U. S. 

Geol. Survey Bull. 340, pp. 593-638, 1910.

M Maurice, C. S., The pegmatites of the Spruce Pine district, N. C.: Econ. 

Geology, vol. 35, pp. 49-78, 158-187, 1940.

"Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, 1942.

58 Olson, J. C., Economic geology of the Spruce Pine pegmatite district, 

N. C.: North Carolina Dept. Cons, and Devel., Div. Min. Resources, Bull. 

43, 1944.

62 Galpin, S. L., Feldspar and mica deposits of Georgia: Georgia Geol. 

Survey Bull. 30, 1915.

80 Furcron, A. S., and Teague, K. H., Mica-bearing pegmatites of Georgia: 

Georgia Geol. Survey Bull. 48, 1943.

81 Clark. G. H., Mica deposits of Alabama: Alabama Geol. Survey Bull. 24, 

1921.

the pegmatite districts is available in the maps and 

reports of Stose,62 Stose, Jonas and others,63 Keith,84 

La Forge and Phalen,65 Stose and Smith,66 Keith and 

Sterrett,67 and others. In addition the literature contains many papers and reports of less general scope 

or less immediate application. They are referred to in 

this report in specific connections only.

PEGMATITES

The pegmatites of the Southeastern States consist 

chiefly of plagioclase and quartz, with subordinate 

perthite, museovite, and biotite and accessory garnet, 

apatite, beryl, columbium-tantalum minerals, uranium minerals, sulfides, and other species. They are 

granodioritic rather than truly granitic in composition. Muscovite is present in some deposits as disseminated flakes, foils, and tiny books, but in others it 

occurs as very large books, some of which are 2 feet 

or more in diameter and weigh several hundred 

pounds or more. All variations between these extremes are known, and many pegmatites contain book 

museovite in a wide variety of sizes and forms. In 

general the distribution of 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 of mica 

that it contains.

Both concordant and discordant pegmatites are 

common. Dikes, sills, discoidal lenses, pods, tongues, 

and cigar-shaped bodies are most widespread, and 

archlike or troughlike bodies and more irregular 

masses are locally abundant. The bodies range from 

lenses and stringers less than an inch thick to dikes 

and sill-like masses several hundred feet long and 200 

feet or more thick. The emplacement of these pegmatites appears to have been controlled by primary 

layering and other planar structures in the igneous 

rocks, by bedding, foliation, and schistosity in metamorphic rocks, and to a considerable degree by fractures in both rock types. Numerous pegmatite 

masses, especially those that are highly irregular, 

probably owe their form to combinations of these features. The orientation of many pegmatites—and of 

minor structures within them—can be correlated

42 Stose, G. W., Geologic map of Alabama, Alabama Geol. Survey and U. S. 

Geol. Survey, 1926.

** Stose, G. W., Jonas, A. I., and others, Geologic map of Virginia, Virginia 

Geol. Survey and U. S. Geol. Survey, 1928.

w Keith, Arthur, U. S. Geol. Survey Geol. Atlas, Cranberry folio (no. 90), 

1903; Asheville folio (no. 116). 1904; Mount Mitchell folio (no. 124), 1905; 

Cowee folio (unpublished manuscript), 1906; Nantahala folio (no. 143), 

1907; Pisgah folio (no. 147). 1907; Roan Mountain folio (no. 151), 1907; 

Lincolnton folio (unpublished manuscript), 1911.

m La Forge, Laurence, and Phalen, W. C., U. S. Geol. Survey Geol. Atlas, 

Ellijay folio (no. 187), 1913.

w Stose, G. W., and Smith, R. W., Geologic map of Georgia, Georgia Div. 

Mines, Mining and Geol. and U. S. Geol. Survey, 1939.

w Keith, Arthur, and Sterrett, D. B., U. S. Geol. Survey Geol. Atlas, Gaffney-Kings Mountain folio (no. 222), 1931.
COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

With the orientation of major and minor structures 

in the adjacent country rock.

A general systematic arrangement of mineralogic 

and lithologic units in many pegmatites has long been 

recognized, and references to "shoots," "ribs," 

"pipes," "barrels," "columns," "veins," "zones," 

"pods," "streaks," and "layers" are common among 

miners and in geologic literature. Recent detailed examination and mapping of pegmatites in many parts 

of the United States have fully confirmed these earlier 

observations and have added abundant detail and new 

information as well. As suggested by members of 

the United States Geological Survey,68 the units of 

pegmatites can be divided into three groups: zones, 

fracture fillings, and replacement bodies.

Zones, where fully developed, are successive shells 

concentric about an innermost zone or core. In a general way they reflect the shape of the pegmatite body. 

Where incomplete or discontinuous they commonly 

form curving lenses, layers, pods, and hoodlike shells. 

Fracture fillings are bodies that fill fractures in previously consolidated pegmatite; generally they are distinctly tabular. Replacement bodies are units formed 

by replacement of preexisting pegmatite, with or 

without obvious structural control, and are not to be 

confused with the relatively simple pegmatite formed 

by replacement of country rock. They generally occur 

along boundaries between zones, between zones and 

fracture fillings, between pegmatite and wall rock, or 

along any other structural element within the pegmatite. Both fracture fillings and replacement bodies 

form structural patterns that are superimposed on 

the concentric or quasi concentric patterns of the 

earlier-formed zones; hence a division of pegmatite 

units into two general age groups is easily made. 

Such a broad division is perhaps in part compatible 

with the findings and conclusions of Schaller,69 

Landes,70 Hess,71 and many other students of pegmatite geology, although markedly differing interpretations of these age groups have been advanced from 

time to time.

The following classification of pegmatite zones has 

been proposed: 72

1. Border, or outermost, zones.

2. Wall zones.

3. Intermediate zones.

4. Cores, or innermost zones.

The idealized diagram in figure 16 shows the distribution of zones and later units in a typical mica-bearing pegmatite body.

88 Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal structure of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 13-16, 

1949.

69 Schaller, W. T., The genesis of lithium pegmatites: Am. Jour. Sci., 6th 

ser., vol. 10, pp. 269-279, 1925.

70 Landes, K. K., The paragenesis of the granite pegmatites of central 

Maine: Am. Mineralogist, vol. 10, pp. 355-411, 1925.

71 Hess, F. L., The natural history of the pegmatites: Eng. and Min. Jour., 

vol. 120, pp. 289-298, 1925.

72 Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal structure of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 20-24, 

1949.

40 Feet

FIGURE 16.—Idealized plans of a pegmatite body, showing typical distribution of pegmatite units. A, Zones, comprising a core of massive quartz surrounded successively by an inner intermediate zone of coarse, blocky 

perthite, a middle intermediate zone of perthite-plagioclase-quartz-muscovite pegmatite, an outer intermediate zone of perthite-plagioclasequartz pegmatite, a wall zone of plagioclase-quartz-muscovite pegmatite, 

and a border zone of fine-grained plagioclase-quartz-muscovite pegmatite; 

B, Same zones, with fracture fillings and replacement bodies of albitemuscovite pegmatite (solid black) formed in them or at their expense.

Most border zones are fine-grained selvages that 

are not more than a few inches thick. Others, particularly those that have formed wholly or in part 

through replacement of country rock, are much 

thicker, more irregular, and not sharply defined. Wall 

zones, which are next inside the border zones, are 

characteristically coarser and much thicker. Although they are actually the zones second from the 

margins of pegmatite bodies, the designation "wall 

zone" has been retained in recognition of a terminology firmly established among pegmatite miners 

and mine operators. Most border zones are of little 

significance in the mining of pegmatites; hence in the 

industry they have not been distinguished from ad-
SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 33

joining zones. As border zones rarely are economic 

or mappable units, the term "wall zone," though 

strictly a misnomer, is nevertheless useful and 

widely understood.

The core, or innermost zone, generally occurs at or 

near the center of the pegmatite body. Any zone between the core and the wall zone is an intermediate 

zone. There is no theoretical limit to the number of 

intermediate zones in a single pegmatite body, but 

few contain more than three such units. The innermost zone of a pegmatite cannot always be recognized 

as such, for not every core can be seen or even predicted with reasonable assurance. Thus a zone identified as a core at one level may prove to be an intermediate zone when the top or edge of the true lenslike 

core is exposed by subsequent mining.73 The cores, 

and possibly other zones as well, are not exposed in 

many pegmatites that have been only slightly cut by 

erosion.

Numerous pegmatite bodies contain one or more 

podlike concentrations of minerals. These pods generally can be recognized as segments of discontinuous 

cores where they are clearly distributed midway between the walls of the pegmatites, but elsewhere their 

distribution is not so easily interpreted. In most deposits, however, they can be considered as parts of 

discontinuous zones.

The zones in most pegmatite bodies coarsen progressively inward from the walls. Moreover, many 

zones are transected by offshoots or apophyses from 

other zones that lie nearer the center of the pegmatite, 

but the reverse relation has not been observed. On the 

basis of widespread observation it seems clear that 

zones in the mica-bearing pegmatites of the Southeastern States were developed successively from the 

walls inward. The internal structure of many pegmatites in New England has been described in detail 

by Cameron and others.74 Pegmatites in western 

North Carolina have been reported on by Olson 75 and 

those in northern New Mexico by Jahns.76 In addition, other reports on mica deposits in Idaho, South

78 Bannerman, H. M., and Cameron, E. N., The New England mica industry: Am. Inst. Min. Met. Eng. Tech. Pub. 2024, 1946 ; Jahns, R. H., Strategic 

pegmatite minerals from the Southwestern and Southeastern States during 

World War II: Am. Inst. Min. Met. Eng. Tech. Pub. (in press, 1949) ; Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal structure of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 21-24, 1949.

74 Cameron, E. N., Larrabee, D. M., McNair, A. H., Page, J. J., Shainin, 

V. E., and Stewart, G. W., Structural and economic characteristics of New 

England mica deposits: Econ. Geology, vol. 40, pp. 369-393, 1945.

75 Olson, J. C., Economic geology of the Spruce Pine pegmatite district, 

N. C.: North Carolina Dept. Cons, and Devel., Div. Min. Resources, Bull. 

43, 1944 ; Mica deposits of the Franklin-Sylva district, N. C.: North Carolina Dept. Cons, and Devel., Div. Min. Resources, Bull. 49, 1946.

7* Jahns, R. H., Mica deposits of the Petaca district, Rio Arriba County, 

N. Mex.: New Mexico School of Mines, State Bur. Mines and Min. Resources, 

Bull. 25, 1946; Strategic pegmatite minerals from the Southwestern and 

Southeastern States during World War II: Am. Inst. Min. Met. Eng. Tech. 

Pub. (in press, 1949).

Dakota, Colorado, Wyoming, and the Southeastern 

States are currently in preparation by members of 

the United States Geological Survey.

MICA DEPOSITS

Some deposits of book muscovite in the Southeastern States occur in pegmatites that are not clearly 

zoned, but most are in well-defined units that in general are quite distinct from adjacent barren units, and 

their distribution is clearly governed by zonal structures. Entire zones in some pegmatites are sufficiently 

rich in mica to be mined, but most deposits of commercial interest are confined to certain portions of 

zones and hence occur as shoots not unlike the shoots 

of ore minerals in metalliferous deposits. The position 

and distribution of such shoots commonly can be correlated with the over-all shape of the containing zone 

or with rolls, bends, bulges, constrictions, protuber- 

"ances, or other irregularities in the zone. Little salable 

material is associated with fracture fillings or re- 

.placement bodies, particularly in the pegmatites 

yielding the greatest production. Such late-stage mica 

is common only in a few pegmatites in the Amelia district of Virginia and in some larger, feldspar-rich, 

pegmatites in other districts.

The classification of zones provides a ready means 

of naming many mica deposits. Such designations as 

"wall-zone deposit," "intermediate-zone deposit," 

"footwall deposit," "pod deposit," or "core-margin 

deposit" require a minimum of explanation. If two or 

more intermediate zones are present, they can be 

distinguished by means of letters, numbers, or such 

position modifiers as "outer," "inner," "lower," and 

the like.

Wall-zone deposits yield most of the sheet muscovite produced in the Southeastern States. Substantial 

quantities also are obtained from disseminated deposits in poorly zoned pegmatites and from coremargin or other intermediate-zone concentrations. 

Detailed field studies have demonstrated that the mica 

within a given zone is rather constant in color, clearness, type and distribution of structural defects, and 

other physical properties, whereas the books from 

different zones within the same pegmatite commonly 

differ very strikingly. Green "A" mica, for example, 

is especially abundant along the edges of quartz cores 

in many pegmatites, whereas the mica in the earlierformed wall zones of the same pegmatites is cinnamon 

brown, brown, or brownish olive and is relatively free 

from reeves. Strongly reeved mica is most abundant 

in the pegmatites and pegmatite zones that are rich 

in potash feldspar, and it is later than the flat mica 

in those pegmatites that contain both kinds of books. 

Systematic variations in other properties are discussed in the following section of this report.
34 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TESTS OF SHEET MUSCOVITE FROM THE SOUTH. 

EASTERN STATES

GENERAL STATEMENT

Electrical tests of sheet mica from numerous specific deposits in the southeastern United States were 

first made in 1940 and 1941, when material collected 

from 124 mines by the Federal Geological Survey was 

studied by the National Bureau of Standards.77 As 

previously pointed out, several of these samples were 

subsequently treated in greater detail by two large 

fabricators of electrical equipment. More extensive 

investigations, later made by the War Production 

Board in Asheville, N. C., involved the testing of several thousand pieces of block mica supplied by the 

Colonial Mica Corporation. This material represented 

most of the Southeastern mines operated in 1942 and 

1943. It was graded and classified visually and by 

means of the Q-meter and spark-coil test set, but the 

results of this work have not yet been published.

The studies described in this report were made 

under the joint auspices of the Geological Survey, the 

State of North Carolina, and the Tennessee Valley 

Authority during the period July 1945-April 1946. A 

total of 2,502 lots containing 237,764 pieces of mica 

was examined petrographically and tested electrically. This material, obtained from at least 850 deposits in the states of Alabama, Georgia, North Carolina, South Carolina, and Virginia, was broadly classified as follows:

839 lots: Material previously collected, labeled, and

stored by the Geological Survey. 

474 lots: Special material collected by the Geological 

Survey, including large lots of mica from the 

Southeastern States and some mica from New England, South Dakota, and New Mexico. 

493 lots: Material collected by the Geological Survey 

and the State of North Carolina during the period 

of the mica-testing program.

243 lots: Stored material loaned by the Colonial Mica 

Corporation and previously graded by visual 

means.

41 lots: New material prepared and graded by visual 

means and loaned by the Colonial Mica Corporation. 

78 lots: Heavily stained electric mica, prepared and

loaned by H. A. Knight of High Point, N. C. 

334 lots: Miscellaneous material furnished or loaned 

by mine operators, manufacturers, and various organizations.

The lots furnished by the Geological Survey comprised raw mica obtained from mines, mine dumps 

and waste piles, and rifting shops, as well as some 

prepared stock from shops and storage sheds. Most

n Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942.

of this mica was specifically identified as to source, 

and in many instances its position within a given 

mine and with respect to the walls, crest, or keel Of 

the containing pegmatite was known.

All raw mica was cobbed free of quartz, feldspar, 

and other waste when necessary and was then rifted 

and trimmed before testing. Trimming was done by 

means of small, hand-operated, sickle-bladed knives, 

with formation of beveled edges on the finished material. Most sheets were full-trimmed, whether or not 

they were marred by reeves, ribs, clay stains, or 

mineral stains, and many of them could be classified 

visually as little better than scrap grade. Some samples consisted of such poor material that they were 

rifted but not trimmed, thereby yielding pieces that 

were loosely classed as "washer stock." Others, however, contained clear sheets of apparently excellent 

quality. During preparation of the mica considerable 

quantities of very thin laminae or "skimmings" were 

produced. These were later stacked to thicknesses of 

30 mils (0.030 inch), and the stacks were tested for 

power factor and conducting impurities.

Mines and prospects from which test specimens 

were obtained are:

Alabama:

Clay County:

M and G mine. 

Eandolph County:

Arnott mine. 

Tallapoosa County:

Berry mine.

Collum (McCray) mine.

Collum "Quartz-Blowout" deposit.

Kidd mine.

McCray. See Collum mine.

Mica Hill mine. 

Georgia:

Elbert County:

Cooley mine.

Crawford-Daniel mine.

Gaines, C. U., prospect.

Gaines, M. L., mine.

New Bethel M. E. Church prospect.

Skelton, J. M., prospect.

Turner prospect. 

Hall County:

Merck (Old Hope) mine.

Old Hope. See Merck mine. 

Hart County:

Alien, Lon. See Wood mine.

Bailey mine.

Carter mine.

Garner mine.

Gully. See Wood mine.

Harper-Pierman mine.

Horsehead mine.

Jones, Ruth, mine.

Scrap mine.

Waterhole mine.

Wood (Gully, Lon Alien) mine.
35

Georgia—Continued 

Lamar County:

Coggins prospect.

Vaughn, Early, mine. 

Monroe County:

Battles mine. 

Babun County:

Kell mine.

Norton mine.

Tunnel mine. 

Troup County:

Hogg mine.

Smith Store prospect. 

Upson County:

Adams mine.

Barron (Bennie Barren, Walker Wakefield) mine.

Barren, Bennie. See Barron mine.

Boyt mine.

Brown (Parrish) mine.

Carter mine.

Corley mine.

Gibson, B. S., prospects.

Herron mine.

Johnson mine.

Marshall. See Stevens Eock mine.

Mauldin mine.

Mauldin Eoad prospect.

McKinney. See Stevens Eock mine.

Mitchell Creek mine.

Parrish. See Brown mine.

Stevens Eock (Marshall, Sullivan, McKinney) mine.

Sullivan. See Stevens Eock mine.

Wakefield, Walker. See Barron mine. 

North Carolina:

Alexander County:

Dagenhart mine.

Gwaltney prospect.

Patterson mine. 

Ashe County:

Buck Mountain mine.

Hodson prospect. 

Avery County:

"A" mine (on Brushy Creek).

"A" mine (on Little Elk Ridge).

Aldrich mine.

Alfred mine.

Benfield mine.

Benfield, Byard, mine.

Big Meadow. See Moulton mine.

Bluff mine.

Branch mine.

Brushy Creek East mine.

Buchanan, Aaron, mine.

Buchanan, Franklin, (Lee Cook) mine.

Buck Hill Eock mine.

Bug Eock (Westphalen) mine.

Burleson, Charley, mine.

Carpenter, Tom, mine.

Charlies Ridge mine.

Comina, Jose. See Vance, Freel, mine.

Cook, Lee. See Buchanan, Franklin, mine.

Corn mine.

Cow Camp South mine.

Doublehead mine. '

Dugger mine.

Eli Eock mine.

North Carolina—Continued 

Avery County—Continued

Elk (Milton English) mine.

Emmons Knob mine.

English, Milton. See Elk mine.

Ewing mine.

Four Foot Square mine.

Franklin mine.

Green, Euben, mine.

Gusher Knob mine.

Hoppey mine.

Houston Eock mine.

Johnson mine (on Plumtree Creek).

Johnson, I. M., mine.

Justice mine.

Lincoln Eock mine.

Lincoln Eock No. 2 mine.

Madam Bank mine.

Meadow mine.

Mill Eace mine.

Moulton (Big Meadow) mine.

Old Eock mine.

Ollis, Jake, mine.

Pancake mine.

Plumtree (Race) mine.

Powdermill Eough mine.

Pyatte, Zeb, mine.

Eace. See Plumtree mine.

Eed mine.

Eed prospect.

Slippery Elm mine.

Taylor, Fate, mine.

Vance, Freel, (Jose Comina) mine.

Westphalen. See Bug Eock mine.

White Eock mine.

Wiseman, Honey Waits, mine.

Wolf Eidge mine. 

Buncombe County:

Abernathy Watershed. See New Balsam Gap mine.

Burco mine.

Corner Eock mine. 

' Mitzes mine.

New Balsam Gap (Abernathy Watershed) mine.

Smith, Beede. See Swannanoa mine.

Swannanoa (Beede Smith) mine.

Tipton mine. 

Burke County:

Brittan, Floyd, mine.

Hudson prospect.

Eeed mine.

Young, Noah, mine. 

Caldwell County:

Butler mine.

Daniels, Cling, mine. 

Caswell County:

Old Milton. See Yarboro No. 1 mine.

Slaughter, J,, G., prospect.

Yarboro No. 1 (Old Milton) mine. 

Catawba County:

Abernathy Long Cut (Hickory) mine.

Abernathy Water mine.

Drum mine.

Hickory. See Abernathy Long Cut mine.

Moose mine.

Sigmon mine.

Tallant prospect.
COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued 

Cleveland County:

Anthony prospects.

Archie mine.

Big Hill. See Bonnett Split mine.

Blanton prospect.

Blanton, C. Robert, mine.

Blanton, Cliff, mine.

Blanton, Coleman, mine.

Bonnett Split (Big Hill) mine.

Bowen mine.

Bridges, Pleaz, mine.

Bumgarner mine.

Cabaniss prospect.

Cabaniss, Alma, mine.

Cabaniss, Tom, mine.

Campbell mine.

Carpenter mine.

Chrysolite mine.

Cooke mine.

Cornwall, Frank, (Old J. S. Blanton) mine.

Cornwell, Charles, prospect.

Davis, Walter, mine.

Elliot, L. R., mine.

Fortenberry, W. H., prospect.

Foster, J. L., prospect.

Gantt, B. T., prospect.

Gantt, M. H., mine.

Gettys No. 1 mine.

Gettys No. 2 mine.

Glover, Eli, mine.

Gold, Mary, mine.

Green, J. F., mine.

Griggs mine.

Harris mine.

Herndon mine.

Homestead. See Mauney, S. S., mine.

Hoyle, A. F., mine.

Humphries, Joe E., mine.

Humphries, W. H., prospect.

Hunt mine.

Indian Graveyard mine.

Indiantown (Mull) mine.

Jimmy mine.

Jones mine.

King, Marvin, prospects.

Lail prospect.

Lattimore mine.

Ledford mine.

Martin mine.

Martin, J. T., prospect.

Mauney, Bailey, mine.

Mauney, M. M. See Mauney, S. S., mine.

Mauney, S. S., (M. M. Mauney, Homestead) mine.

McGinnis, F. G., mine.

McSwain, G. B., mine.

Metcalf mine.

Mill Race mine.

Moose mine.

Mull. See Indiantown mine.

Niagara mine.

Norman, Archie, (W. H. Thompson) mines.

Norman-Thompson (W. H. Thompson) mine.

Old Carroll. See Patterson, Bun, mine.

Old J. S. Blanton. See Cornwall, Frank, mine.

Patterson, Bun, (Old Carroll) mine.

Peeler No. 1 mine.

North Carolina—Continued

Cleveland County—Continued

Powell (Sugar Barrel) mine.

Putnam mines.

Randall mine.

Rice mine.

Seism prospect.

Spangler, D. H., prospects.

Spangler, Ruben. See Spangler, T. N., prospects.

Spangler, T. N., (Ruben Spangler) prospects.

Stroud, T. C., prospects.

Sugar Barrel. See Powell mine.

Thompson, W. H. See Norman, Archie, mines.

Thompson, W. H. See Norman-Thompson mine.

Warlick, Clyde, mine.

Weathers mine.

Webb mine.

Williamson mine.

Wright prospect. 

Gaston County:

Beam, Claude, prospect.

Big Bess mine.

Huskins mine.

Old Neale. See Self, E. R., mine.

Self, E. R., (Old Neale) mine. 

Haywood County:

Arrowood. See Little East Fork mine.

Big Ridge mine.

Champion Fibre Co. prospect.

Gray. See Putman mine.

Little East Fork (Arrowood) mine.

Medford, Frank, prospect.

Putman (Gray) mine.

Shiney mine.

Spruce Ridge mine.

Stringfield mine.

Wilson, W. T., prospect. 

Jackson County:

Abbs Creek mine.

Ashe, Bob, mine.

Bald Ridge mine.

Bearwallow Fork. See Piney Mountain Creek mine.

Betts Gap mine.

Bettys Creek No.. 1 mine.

Big East Fork mine.

Big Flint (Grassy Ridge, Glassey Rock) mine,

Big Terrapin mine.

Blackjack mine.

Blanton prospect.

Brown's Coward Mountain. See Wood mine.

Bryson mine.

Bryson, C. D., mine.

Bryson, C. V., mine.

Bryson, Diller, mine.

Buchanan, B. C., prospects.

Buchanan, Henry, prospects.

Buchanan, Marsden, prospect.

Buchanan, Pole, mine.

Buchanan, Ramsey, mine.

Buchanan's Old. See Gradin mine.

Buckeye Gap mine.

Buck Knob mine.

Bumgarner mine.

Buzzard Roost mine.

"C" (Puncheon Camp) mine.

Cabe mine.

Cedar Cliff mine.
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 37

North Carolina—Continued

Jackson County—Continued

Chastine Creek. See Woods mine.

Choga (Chogey) mine.

Chogey. See Choga mine.

Clark, Hardy, prospect.

Clouse mine.

Collins mine.

Cox (Cox and Davies) mine.

Cox and Davies. See Cox mine.

Cox Murray Mountain. See Engle Cope mine.

Dave Ridge mine.

Davis prospect.

Dead Timber Ridge mine.

Deets, Early, mine.

Dietz, Jeanie, mine.

Dillard, W. G., prospect.

Double Gap mine.

East Fork prospect.

East Laport mine.

Engle Cope (Cox Murray Mountain) mine.

Far Top Field prospect.

Ferguson, Judge, mine.

Flukens. See Long, John, No. 1 mine.

Frady mine.

Frady Creek. See Woods mine.

Glassey Rock. See Big Flint mine.

Gradin (L. C. Presley, Buchanan's Old) mine.

Grassy Ridge. See Big Flint mine.

Gregory mine.

Higdon mine.

Hooper, Aaron, mine.

Island Ford mine.

Jasper mine.

Kolb mine.

Long, John, No. 1 (Flukens) mine.

Long, John, No. 2 mine.

Long Branch mine.

McCall, Lin, mine.

McKay. See Rocky Branch mine.

Mills, Luce, prospect.

Moody. See Toy mine.

Moss mine.

Murray Cove. See Murray Mountain mine.

Murray Mountain (Murray Cove) mine.

New Wolff (New Wolff No. 1, Wolff, Old Wolff) mine.

New Wolff No. 1. See New Wolff mine.

Nichols, Doc, mine.

Nicholson, Andy, mine.

Old Wolff. See New Wolff mine.

Painter mine.

Parker, Jerome, mine.

Parker Knob mine.

Pine Ridge mine.

Piney Mountain Creek (Bearwallow Fork) mine.

Pinhook Creek mine.

Pinhook Gap mine.

Potato Cove (Tater Cove) mine.

Presley, Bud, mine.

Presley, L. C. See Gradin mine.

Presley, Mack, mine.

Price, J. B., mine.

Prince, J. S., (Ben Queen) prospect.

Puncheon Camp. See "C" mine.

Queen, Ben. See Prince, J. S., prospect.

Queen, Oscar, mine.

Raco mine.

North Carolina—Continued

Jackson County—Continued

Radeker, J., mine.

Reed, Sally, mine.

Rhoda mine.

Rice mine.

Richland Balsam prospect.

Ridge mine.

Roaringhole (Sheep Knob) mine.

Rock mine.

Rocky Branch (McKay) mine.

Rogers mine.

Sheep Knob. See Roaringhole mine.

Sheep Mountain mine.

Shell Ridge mine.

Spence mine.

Stephens, Duke. See Hooper, L. E., mine.

Stillwell mine.

Sugarloaf Creek mine.

Sugarloaf Mountain prospects.

Tater Cove. See Potato Cove mine.

Tennessee Creek mine.

Tilley mine.

Toy (Tustin, Moody) mine.

Tustin. See Toy mine.

Tustin mine.

Upper Sugar Creek Gap prospect.

Wayehutta Clay (Weary Hut, Worry Hut) mine.

Weary Hut. See Wayehutta Clay mine.

Wetmore prospect.

Wilkes mine.

Williams, Bud, mine.

Wilson prospect (in Cashiers Valley).

Wilson, Cleve, mine.

Wilson, Shirley, mine.

Wolff. See New Wolff mine.

Wood (Brown's Coward Mountain) mine.

Woods (Chastine Creek, Frady Creek) mine.

Woodward, Tyler, mine.

Worry Hut. See Wayehutta Clay mine. 

Lincoln County:

Baxter, Jack, (Tom Baxter) mine.

Baxter, Jack, prospect.

Baxter, Tom. See Baxter, Jack, mine.

Beam prospect.

Bess mine.

Biggerstaff (Deadman) mine.

Deadman. See Biggerstaff mine.

Eaker, Doris, mine.

Foster No. 1 (W. A. Thompson No. 1) mine.

Foster No. 2 (W. A. Thompson No. 2) mine.

Foster, W. L. C., prospect.

Foster, W. T., (W. A. Thompson) mine group.

Hallman mine.

Houser, Plato, mine.

Houser, Plato, No. 2 mine.

King (Norman and Cecil) mine.

Leatherman, Pink, mine.

Norman and Cecil. See King mine.

Thompson, W. A. See Foster, W. T., mine group.

Thompson, W. A., No. 1. See Foster No. 1 mine.

Thompson, W. A., No. 2. See Foster No. 2 mine. 

Macon County:

"A" mine.

Allman Cove mine.

Ammons (Henry) mine.

Anderson mine.
38 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued 

Macon County—Continued

Angel, George, (Cabe, Moore) mine.

Angel, Mel, mine.

Annie Laurie (Wade Moody) mine.

Arnold, Fred, prospect.

Arrowwood mine.

Bailey mine.

Baird (Smith) mines.

Barnard. See Burke, John, mine.

Bearpen mine.

Beasley No. 1 (Bradley Butt) mine.

Beasley No. 2 mine.

Berry. See Deerlick Knob mine.

Berry mine.

Boyd Knob. See Shepherd Knob mine.

Bradley Butt. See Beasley No. 1 mine.

Bradley-Campbell (Campbell, Higdon) mine. 

• Brawley mine.

Bryson. See Burr Knob mine.

Bryson prospect.

Bryson, Terrell, (Old Bryson) mine.

Buoy No. 1 mine.

Buoy No. 2 mine.

Burke, John, (Barnard) mine.

Burleson, John, mine.

Burningtown. See Poll Miller mine.

Burr Knob (Bryson) mine.

Buttermilk mine.

Cabe. See Angel, George, mine.

Camp Branch prospect.

Campbell. See Bradley-Campbell mine.

Chalk Hill mine.

Corbin Knob mine.

Corbin Knob prospects.

Crawf ord, Lester, prospect.

Cunningham mine.

Dalton mine.

Deerlick Knob (Berry) mine.

Dills mine.

Dobson mine.

Downs, Charlie, prospect.

Duvall, Grady. See Raby mine.

Elmore mine.

Elmore Branch prospect.

Evans, A. J., prospect.

Fox mine.

Gibson mine.

Gibson prospect.

Gum Gap mine.

Gurney mine.

Hall prospect.

Hall-Burra mine.

Harris mine.

Henry. See Ammons mine.

Henry, Jacob W. See Henry, Jake, mine.

Henry, Jake, (Jacob W. Henry, Rocky Face) mine.

Higdon. See Bradley-Campbell mine.

Higdon prospect.

Holbrook. See Kasson mine.

lotla. See lotla-Bradley mine.

lotla-Bradley (lotla, lotla Bridge) mine.

lotla Bridge. See lotla-Bradley mine.

Jack Knob mine.

Kasson (Holbrook) mine.

Kelly, Lassie, (Pine Knob) mine.

Kinsland prospect.

North Carolina—Continued 

Macon County—Continued 

Kiser mine.

Leatherman, S. C., prospect. 

Ledford Cove mine. 

Lenoir prospect. 

Lin Cove prospect. 

Little Spring mine. 

Littlefield mine. 

Locust Tree prospect. 

Lower Mack mine. 

Lucas, Doc, mine. 

Lyle, Doc, mine. 

Lyle Cut mine. 

Lyle Knob mine. 

Malonee mine.

Mashburn, Norman. See Roaring Fork mine. 

Mason, Lee, mine. 

May mine. 

McCrary mine. 

Mill Knob mine. 

Miller (Sanders) mine. 

Moody mine.

Moody, Wade. See Annie Laurie mine. 

Moore. See Angel, George, mine. 

Moore, A. J., mine. 

Moore, Jim, mine. 

Morgan, Denver, prospect. 

Moss Knob mine. 

Mount Hope Church prospects. 

Mud Hole mine. 

Mud Hole prospect. 

Old Bryson. See Bryson, Terrell, mine. 

Passamore. See Turkey Knob mine. 

Pendergrass prospect. 

Pine Knob. See Kelly, Lassie, mine. 

Poll Miller (Burningtown) mine. 

Poplar Cove prospect. 

Quizzenberry mine. 

Raby (Grady Duvall) mine. 

Ray, Lissie, prospects. 

Ray, Tom, prospect. 

Ray Cove prospect. 

Rickman mine.

Roaring Fork (Norman Mashburn, Smith) mine. 

Rocky Face. See Henry, Jake, mine. 

Rocky Face mine. 

Roper, Bud, mine. 

Rough Fork mine. 

Russell prospect. 

Sanders. See Miller mine. 

Sanders, Doc, mine. 

Shepherd Knob (Boyd Knob) mine. 

Shotgun mine. 

Siphon Hole mine. 

Slagle mine. 

Slagle-Drake mine. 

Smith. See Baird mines. 

Smith. See Roaring Fork mine. 

Stamey mine. 

Swell prospect. 

Taylor, Fred, prospect. 

Thorne Mountain Northeast mine. 

Thorne Mountain Southwest mine. 

Turkey Knob (Passamore) mine. 

Turkey Nest mine. 

Upper Raby mine.
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 39

North Carolina—Continued 

Macon County—Continued

Verdell mine.

Welch mine.

Wildes, Jud, prospect.

Winding Stair mine.

Winecoff mine.

Zachary, Hal, mine.

Zoom mine. 

McDowell County:

Thunder Knob mine. 

Mitchell County:

A. S. O. mine.

Abernathy mine.

Adams mine.

Adams prospect.

Autrey, Al, mine.

Bardon (Bordon) mine.

Bear Creek prospect.

Bearden mine.

Birch mine.

Birdeye. See Phillips mine.

Bloodworth mine.

Bordon. See Bardon mine.

Branch. See Young, Zack, mine.

Buchanan, Adam, mine.

Buchanan, Erby. See Buchanan, J. K., mine.

Buchanan, J. K., (Milt Wilson, Erby Buchanan) mine.

Buchanan, Jim, mine.

Buchanan, Jim, prospect.

Buckeye mine.

Bunker Hill mine.

Burleson, W. C., (Joe Stevenson) mine.

Byrd. See Byrd, Jeff, mine.

Byrd, Jeff, (Byrd) mine.

Carolina China Clay Co. mine.

Carolina Mineral Co. No. 3 mine.

Carolina Mineral Co. No. 6 mine.

Carolina Mineral Co. No. 12 mine.

Carolina Mineral Co. No. 12 (White) prospect.

Carolina Mineral Co. No. 29 mine.

Carters Eidge prospect.

Case mine.

Chalk Mountain mine.

Chalk Mountain prospect.

Chalk Mountain Northeast prospect.

Chestnut Flat mine.

Chestnut Flat prospect.

Chestnut Flats mine.

Clarissa mine.

Cloudland (Pizzel) mine.

Cook. See Sparks, Eube, mine.

Cook mine.

Cook prospect.

Cox prospect.

Cox, Ben, mine.

Cox, Wood, mine.

Cox, Wood, prospect.

Crabtree No. 1 (Wildcat) mine.

Crabtree Northwest prospect. 

7 Crabtree Summit prospect.

Dake. See Deake mine.

Davis mine.

Deake (Dake) mine.

Deer Park No. 1 mine.

Deer Park No. 2 mine.

Deer Park No. 3 mine.

North Carolina—Continued

Mitchell County—Continued 

Deer Park No, 5 mine. 

Devils Looking Glass mine. 

Dog Pond mine. 

Drawbar mine. 

Duck Branch mine. 

Eagles Nest mine. 

English Knob mine.

Estatoe Northeast (Mossy Bock) prospect. 

Estatoe Southeast prospect. 

Field mine. 

Field prospect. 

Flat Rock mine. 

Flukens Hill mine. 

Glenn mine. 

Gopher mine. 

Gouge, Slim, prospect. 

Gouge Falls prospect. 

Graveyard prospect. 

Gudger mine. 

Guy mine.

Hall and Boone mine. 

Harlan mine. 

Harlan prospect. 

Haw Flat mine. 

Hawk mine. 

Hawkins mine. 

Hootowl mine. 

Hootowl prospect. 

Hoppus prospect. 

Hoppus Northeast mine. 

Horton Rock mine. 

Howell, George, (Waterhole) mine. 

Howell, Jeff, mine. 

Jack Eock mine. 

Jase mine. 

Jase prospect. 

Jeff. See Jeff Cut mine. 

Jeff Cut (Jeff) mine. 

Jimmy Cut mine. 

Johnson prospect. 

Johnson, Ernest, mine. 

Johnson, Theo, mine. 

Landers mine. 

Liberty Hill prospect. 

Lick Eidge mine. 

Little Hawk Eidge mine. 

Long Cut mine. 

Lower Sugar Tree Cove mine. 

Lower Sugar Tree Cove prospect. 

Miller mine (north of Penland). 

Miller mine (west of Spruce Pine). 

Mossy Eock. See Estatoe Northeast prospect. 

Mountain Top mine.

Murphy Eock mine (on Crabtree Creek). 

Murphy Eock mine (on Eebels Creek). 

McBee, Ed, mine. 

McChone prospect. 

McKinney mine. 

McKinney prospect. 

New. See Wisemari'No. 2 mine. 

Pannell mine. 

Pegram mine.

Penland Southeast prospect. 

Pepper Pot (Perrin) mine. 

Perrin. See Pepper Pot mine.
40 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued

Mitchell County—Continued

Phillips (Birdeye) mine.

Pine Mountain mine.

Pizzel. See Cloudland mine.

Potato Hill mine.

Poteat mine.

Poteat prospect.

Poteat Southwest prospect.

Poteat West prospect.

Putman mine.

Putman, Marsh, mine.

Queen mine.

Randall mine.

Raven Cliff mine.

Renfro mine.

Richards prospect.

Ridgecrest prospect.

Self mine.

Silvers Ridge mine.

Silvers Ridge prospect.

Sinkhole mine.

Smith, Calhoun, mine.

Smith, Lissie, mine.

Soft Ridge mine.

Sparks, Ken, mine.

Sparks, Rube, (Cook) mine.

Stevenson, Joe. See Burleson, W. C., mine.

Summer mine.

Twiggs mine.

Upper Crabtree prospect.

Vern Muse prospect.

Waterhole. See Howell, George, mine.

White. See Carolina Mineral Co. No. 12 prospect.

Wildcat. See Crabtree No. 1 mine.

Willis prospect.

Wilson, Milt. See Buchanan, J. K., mine.

Wiseman No. 2 (New) mine.

Wiseman, W. W., mine.

Wolfden prospect.

Wolf den North prospect.

Young prospect.

Young, George, mine.

Young, S. S., mine.

Young, Zack, (Branch) mine. 

Rockingham County:

Evans, Rosa, mine.

Holland mine.

Knight mine.

Smith, Ben. See Smith, Short Tom, mine.

Smith, Ernest, mine.

Smith, Short Tom, (Ben Smith) mine. 

Rutherford County:

Dycus mine.

Isinglass Hill mine.

Kay mine.

Maurice mine.

Stroud, Lax, mine. 

Stokes County:

Brown mine.

Hawkins (Joe Hawkins) mine.

Hawkins, J. C. See Ruby King mine.

Hawkins, Joe. See Hawkins mine.

Hole (Jack Hole) mine.

Jack Hole. See Hole mine.

Ruby King (J. C. Hawkins) mine.

Shelton, G. R., mine.

North Carolina—Continued 

Stokes County—Continued

Spencer mine.

Steele mine. 

Transylvania County:

Bee Tree No. 1 (Bee Tree Fork) mine.

Bee Tree Fork. See Bee Tree No. 1 mine.

Farlow Gap. See Furlow Gap mine.

Furlough Gap. See Furlow Gap mine.

Furlow Gap (Farlow Gap, Furlough Gap) mine.

Reid mine.

Wagon Road Gap prospect. 

Wilkes County:

Creek. See Shell, Zolly, mine.

Ferguson mine.

Hall mine.

Proffitt, W. A., mines.

Shell, Zolly, (Creek) mine. 

Yancey County:

Aley Creek. See Ayles Creek mine.

Alford mine.

Alien, Henry, mine.

Allis Creek. See Ayles Creek mine.

Anglin mine.

Autrey (Linzie Autrey) mine.

Autrey, J. W., mine.

Autrey, Linzie. See Autrey mine.

Ayles Creek (Allis Creek, Aley Creek) mine.

Bailey Mountain mine group.

Balsam mine.

Barger (Spruce Pine Mica Co. No. 21) mine.

Barger prospect.

Bee Ridge mine.

Bennett. See Red mine..

Bittner and Beech mine.

Black Brothers. See McKinney, Chet, mine.

Black Dixie mine.

Blake (Boomer Tom Young) mine.

Bland. See Old Gibbs mine.

Blevins, D. 0.,-Wes Thomas mine.

Boomer Tom Young. See Blake mine.

Boone, Nelson, mine.

Boone, W. K., prospect.

Branch mine.

Brown Creek mine.

Buckeye mine (on Crabtree Creek).

Buckeye mine (west of Boonford).

Carolina Mineral Co. No. 20 mine.

Carson Rock mine.

Cattail (Cattail Creek, Ison) mine.

Cattail Creek. See Cattail mine.

Celia. See Cilley mine group.

Chestnut Branch mine.

Cilley (Silly, Celia) mine group.

Commissary Ridge mine.

Cora mine.

Creson mine.

Eagle Bluff mine.

Edge mine (near Fawn Mountain).

Edge, John, mine.

Edwards mine.

Edwards, J. W., mine.

Flukens Ridge mine (head of Blue Rock Branch).

Flukens Ridge mine group (at Newdale).

Georges Fork mine.

Gibbs mine.

Gibbs, Elmyra, mine.
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 41

North Carolina—Continued

Yancey County—Continued

Gibbs Clay. See Old Gibbs mine.

Gibbs Spar mine.

Gimbel prospect.

Googrock mine.

Gouge. See Shehan, John, mine.

Gouge,, Fannie, (Spruce Pine Mica Co. No. 10) mine.

Gouge, Jim, mine.

Green Mountain mine.

Griffin. See Griffith, Mills, mine.

Griffith, Mills, (Griffin) mine.

Hall, Andy» mine.

Hall, Cleveland, mine.

Harding (Charley Young) mine.

Harp and Blevins prospect.

Heaton, Sport. See McKinney, W. A., mine.

Hector mine.

Hensley-Laurel (Charley Young) mine.

Higgins mine.

Hilliard mine.

Irby Cut mine.

Ison. See Cattail mine.

James mine.

Laurel Branch mine.

Laws mine.

Ledford mine.

Letterman, M. P., mine.

Little Zeph mine.

Locust Rough (B. T. Snopp) mines.

Lower Crabtree prospect.

Mathis, Wyman, prospect.

McDowell, Gaston, mine.

McKinney, Chet, (Black Brothers) mine.

McKinney, W. A., (Sport Heaton) mine.

McPeters, Flem, mine.

Middle Ridge mine.

Old Gibbs (Washout, Bland, Gibbs Clay) mine.

Old Owlie mine.

Poll Hill mine.

Presley. See Presnell mine.

Presley, S. W., mine.

Presnell (Presley) mine.

Ray (Wray) mine.

Red (Bennett) mine.

Riddle, Jim, mine.

Robinson, Charles, mine.

Robinson, Corb, mine.

Rock mine.

Sally Knob mine.

Shakerig mine.

Shehan, John, (Gouge) mine.

Silly. See Cilley mine group.

Silvers mine.

Sleepy Hollow mine.

Snake Den mine.

Snopp, B. T. See Locust Rough mines.

Spruce Pine Mica Co. No. 10. See Gouge, Fannie, mine.

Spruce Pine Mica Co. No. 21. See Barger mine.

Tantrough mine.

Tolley Bend. See Tolley Bent mine.

Tolley Bent (Tolley Bend) mine.

Washout. See Old Gibbs mine.

Westall mine (on Colberts Ridge).

North Carolina—Continued

Yancey County—Continued

Willis Shanty mine.

Wray. See Ray mine.

Young mine (near Harding mine).

Young, Charley. See Harding mine.

Young, Charley. See Hensley-Laurel mine.

Young, Josh, mine.

Young, Wilt, mine.

Young, Zeph, mine group.

Young, Zeph and Alien, mines. 

South Carolina:

Anderson County:

Burgess (L. E. Hunter) mine.

Gaillard mine.

Hunter, L. E. See Burgess mine.

Martin, Ben, mine. 

Cherokee County:

Blanton, Troy, mine. 

Pickens County:

Bolding mine.

Fowler, Will, prospect. 

Spartanburg County:

Cowpens mine. 

Virginia:

Amelia County:

Berry mine.

Bland. See Champion mine.

Champion (Jefferson No. 4, Bland) mine.

Dobbin prospect.

Jefferson No. 4. See Champion mine.

Jefferson No. 6 mine.

Ligon mines.

Maria (Old Pinchbeck, Smith) mine.

Mays mine.

McCraw No. 1 (Old Pinchbeck No. 2) mine.

McCraw No. 2 (Old Pinchbeck No. 3) mine.

McCraw No. 3 (Old Pinchbeck No. 1) mine.

Morefield mine.

Old Pinchbeck. See Maria mine.

Old Pinchbeck No. 1. See McCraw No. 3 mine.

Old Pinchbeck No. 2. See McCraw No. 1 mine.

Old Pinchbeck No. 3. See McCraw No. 2 mine.

Rutherford mines.

Rutherford No. 2 mine.

Smith. See Maria mine.

Vaughn mine.

Wingo mine. 

Bedford County:

Young mine. 

Charlotte County:

Crews No. 1 prospect.

Crews No. 2 prospect.

Howell prospect. 

Goochland County:

Amber Queen mine.

Monteiro (Monteiro Tract) mine.

Monteiro Tract. See Monteiro mine. 

Hanover County:

Saunders No. 2 mine. 

Henry County:

Coleman No. 2 mine.

DeShazo mine.

Eanes No. 2 mine.
42 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

Virginia—Continued

Henry County—Continued

Garrett mine.

Greer and Merriman mines.

Jones No. 1 mine.

Jones No. 2 mine.

Taylor, Nettie, mine.

Morrison, C. R., (Oak Level) mine.

Oak Level. See Morrison, C. R., mine.

Price mine.

Ridgeway mine. 

Pittsylvania County:

Broomfield prospect.

Roach. See Sycamore mine.

Sycamore (Roach) mine. 

Powhatan County:

Dolphin, Clinton, mine.

Herbb No. 1 mine.

Herbb No. 2 mine.

Miller. See White Peak No. 1 mine.

Purcell. See White Peak No. 1 mine.

White Peak No. 1 (Purcell, Miller) mine.

COLOR TESTS

METHODS OP TESTING

All lots of mica were classified according to color, 

so that systematic comparisons could be made between this property and the results of electrical and 

petrographic tests. Three general methods of classification were considered. Simple inspection, long and 

successfully used by experienced men in the industry, 

is rapid but is subject to human errors. Determination of color by means of a chromaticity-difference 

colorimeter 78 and photometer is an accurate and 

readily reproducible method and was used by Judd 79 

in establishing a color standard for ruby mica. It is 

based upon the derivation of indices for lightness and 

hue and gives results in fundamental terms. Unfortunately, however, this method requires time and 

equipment not available to the writers when the Asheville tests were made. This was true, also, of several 

other methods that were suggested. Direct visual 

comparison of the mica with color standards was the 

scheme finally adopted, as it yields rapid and reproducible results that are accurate within the limits required in the investigations.

The difficulties of judging color by ordinary inspection are clearly summarized by Judd,80 who notes that 

the distinction between ruby and nonruby micas 

"often depends upon color differences so small as to be 

scarcely perceptible." He goes on to state that

a given borderline mica will exhibit a progression of colors 

from nearly colorless through yellowish gray to dark brown,

78 Judd, D. B.. Specification of uniform color tolerances for textiles: Textile Research, vol. 9, p. 258, 1939.

TM Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. 

Research, vol. 36, pp. 246-256. 1945.

80 Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. 

Research, vol. 36, pp. 245-246. 1945.

inclining more toward red as the thickness of the specimen 

viewed is increased; so the inspector must take into account 

the thickness of the specimen in order to arrive at a judgment 

characteristic of the mica independent of its thickness. This 

he does by viewing the mica in several ways; single specimens 

of various thicknesses viewed against the sky, a handful of 

specimens viewed against the sky, single specimens of various 

thicknesses placed on a white surface and viewed by light that 

passes through each specimen twice, once to* illuminate the 

white surface, once to reach the eye of the observer, and 

finally directly in the stock box or barrel in an opaque layer by 

light reflected from the nearly parallel faces of the various 

specimens in random position at various depths in the pile. 

His judgment is complicated by the fact that mica from a 

single mine to be classified exhibits more or less variation in 

color from specimen to specimen and from spot to spot within 

the same specimen.

It is true that an experienced inspector learns to apply 

mental evaluations and comparisons in achieving remarkably good color indemnifications for block mica, 

but his results cannot be as accurate nor as consistent 

as those based upon direct comparison with material 

standards. Such standards may themselves change 

through normal deterioration or through careless use, 

but they can be checked from time to time against 

more absolute standards.

All samples used for color classification in the present tests comprised pieces of mica at least 1^4 inches 

long and half an inch wide. These were cleaved to a 

uniform thickness of 15 mils (0.015 inch) and were 

then correlated with elements of the filter-type color 

chart prepared by Ridgway.81 Direct comparisons 

were made by means of a white sheet of cardboard, 

each test being placed upon the sheet beside a rectangular hole slightly smaller than one of the colored 

elements of the chart. The cardboard was moved until 

one of the standard colors, as seen through the hole, 

could be matched with the mica under test. All comparisons were made under illumination from two 

General Electric 17-inch 3,500° white fluorescent 

lamp tubes placed 15 inches above the working surface. No difficulty was experienced in satisfactorily 

matching the test pieces with the standard colors of 

the chart, although great care was necessary in the 

observation of samples heavily stained by clay or 

other minerals.

RESULTS

More than 75 different colors were identified among 

the micas tested. No samples were found to match 

pure spectrum colors or intermediate hues; instead, 

the identifications reflect variations in lightness, or 

tone, and variations in brightness and dullness. In 

general the colors range from buff and drab through 

shades of brown and green to pale yellowish green.82

"Ridgway, Robert, Color standards and color nomenclature, 43 pp., 63 

color pis., Washington, D. C., 1912.

w Jahns, R. H., Color characteristics of sheet muscovite in the Southeastern 

States (abstract): Geol. Soc. America Bull., vol. 66, p. 1170, 1945.
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 43

They are readily grouped into seven main categories, 

as follows:

General terms of the trad

I I 

I I

Ruby

Rum

Green

Main color categories

Pinkish buff and drab 

Cinnamon brown 

Brown

Brownish olive 

Yellowish olive 

Yellowish green 

Green

None of the colored groups is sharply bounded, and 

all gradations between groups adjacent on the list are 

known. Each is easily subdivided on the basis of light 

tints and dark shades, as well as in terms of relative 

brightness or dullness. The darkest shades are browns 

and brownish olives; the lightest tints, those of yellowish green, buff, and drab. The results of the color 

tests are included in table 10.

In general the buff, drab, and cinnamon-brown 

micas are the "ruby" micas of the trade, and the 

brown and brownish-olive varieties are the so-called 

"rum" micas. The trade terms are inexact and are 

not everywhere consistently used, so that only a generalized correlation between them and the more accurate color designations has been attempted. In 

many instances the trade terms are misnomers. As 

pointed out by Judd,83 most specimens that are near 

the ruby-nonruby border line actually are weak 

yellow, very pale brown, or pale brown according to 

more appropriate designations.84

It has been indicated in previous sections of this 

report that few mica books are colored uniformly 

throughout. Color zoning, with gridiron or chessboard patterns, concentric color bands parallel to 

crystal outlines, stripes parallel to pressure-figure directions, and faint, irregular mottling are widespread. Further, nearly all books contain pale rims 

that tend to be more greenish than the interior. Ordinarily, however, it is possible to assign a single 

general color to a given sheet of mica, and it is only 

in exceptional instances that two or more distinct 

colors merit individual note in this connection. (See 

table 10.)

Although variations within individual books are 

common, the color of muscovite in the Southeastern 

deposits is fairly constant within a single shoot or 

even within a single zone in the pegmatite body in 

which it occurs. Micas of more than one general color

** Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. 

Research, vol. 35, p. 248, 1945.

M Judd. D. B., and Kelly, K. L., Method of designating colors: Nat. Bur. 

Standards Jour. Research, voL 23, p. 355, 1939.

are known from many pegmatites, but they typically 

occur in separate concentrations within those pegmatites. Where green and brown muscovite occur in the 

same pegmatite body, the brown is near the walls. 

Those pegmatites with green book muscovite near 

their walls contain no brown books and generally contain no biotite other than that possibly derived 

through reaction with wall-rock material. Biotite is 

characteristically associated with buff, drab, and 

brown muscovite.

Most of the book muscovite in the Piedmont province is brown and pinkish, in marked contrast to that 

in the Blue Ridge province. Green mica in the Piedmont province is common only in the RidgewaySandy Ridge district and in numerous outlying areas 

in Virginia and North Carolina. Pinkish and brown 

micas in the Blue Ridge province are common only in 

the Franklin-Sylva district, in outlying parts of the 

Spruce Pine district, and in areas farther north. The 

outer parts of many color-zoned books in the Alabama 

deposits are green.

Broad and systematic color variations in the mica 

of some areas appear to be related to nearby masses 

of intrusive rock that probably are genetically related to the pegmatites. Such variations are particularly clear in the Spruce Pine, Franklin-Sylva, Hartwell, and Ridgeway-Sandy Ridge districts. The 

pegmatites in and near the intrusive masses generally 

contain muscovite of dominantly green color, whereas 

those farther from such masses are more likely to 

contain buff, drab, or brown muscovite.

PETRO GRAPHIC EXAMINATION 

STAINING AND INCLUSIONS

Nearly all lots of mica were examined megascopically for staining and inclusions and then were further studied under the petrographic microscope. Many 

of the test pieces contain mineral stains in the form 

of intergrown hematite, magnetite, and biotite, either 

singly or in combination, but only 286 lots—13.3 percent of the total—consist of such stained pieces to 

the extent of 25 percent or more (table 9). The 

amount of stain varies greatly from one lot to another 

and even from one piece of mica to another. The bulk 

of material in 96 lots—4.5 percent of the total—is 

marred by moderate to large quantities of such impurities, but numerous other samples are entirely 

clear, and still others contain biotite or iron oxides in 

such minor quantities or in such tiny specks that 

they are virtually clear.

The proportion of material with mineral stain and 

the type and distribution of stain in each lot are indicated in columns 6 and 7 of table 10. It should be 

pointed out that these data apply only to the mica that 

was tested and hence may or may not represent the
44 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

average of all material obtainable from any given 

mine.

TABLE 9.—Occurrence of stain in 2,148 test lots of muscovite from the 

Southeastern States

Type of stain

Hematite and /or magnetite and /or biotite interMineral stain in 25 percent or more of test pieces.. 

Moderate to large quantities of mineral stain in

Vegetable stain and /or green mottling and /or 

brown mottling present in one or more test pieces.

Clay stain and /or iron stain and /or manganese

Number of 

lots

550

286 

96

192 

514

Proportion of 

total lots 

(percent)

25.6

13.3 

4.5

8.9 

23.9

Much of the included hematite and magnetite occurs as specks (less than 0.5 millimeter or 0.02 inch 

in diameter), small spots (0.5 to 1.5 millimeters or 

0.02 to 0.06 inch in diameter), or large spots (more 

than 1.5 millimeters or 0.06 inch in diameter). Nearly 

all the spots and specks of magnetite have rather 

smooth and regular edges, but a substantial proportion of the hematite occurs in simple or complex dendritic forms. The designations "dendrite specks" and 

"dendrite spots" are appropriate for these irregular 

particles. Numerous large inclusions are most easily 

referred to as "plates," "laths," or "blotches." 

"Blotches" are irregular masses or groups of masses 

whose diameters generally are greater than 3 millimeters. The arrangement and distribution of stain 

and inclusions are readily described by means of such 

terms as "rows," "belts," "triangular lattices," "rectangular lattices," "clusters," "sprays," "clouds," 

"scattered," "widespread," "local," and "isolated."

Biotite that is intimately intergrown with muscovite as very thin films without good crystal form 

is considered a type of mineral stain in this report. 

Individual films are platy, wispy, or shredlike and 

commonly cause the host muscovite to appear greenish or brownish and distinctly curdy. The biotite 

staining is concentrated near the centers of some 

muscovite books, and locally, where it forms triangular latticelike patterns, it closely resembles some of 

the lighter-colored types of hematite intergrowths. 

In contrast to the "mineral-stain" type of biotite are 

larger, euhedral inclusions of brown or black biotite, 

some of which are an eighth of an inch or more in 

thickness. These seem best classed with inclusions 

of such minerals as apatite, garnet, pyrite and other 

sulfide minerals, quartz, tourmaline, zircon, and 

zoisite. When removed, inclusions of this type leave 

holes or distinct depressions in the host muscovite. 

Their occurrence and distribution in the tested material are summarized in table 10, column headed 

"Remarks."

Green and brown mottling, vegetable stain of the

inorganic type, and several other forms of pale, curdy 

stain mar some or all test pieces of 192 lots, or nearly 

9 percent of all lots examined (table 9). Much of this 

thin, finely divided foreign material occurs in specks, 

spots, lines, rows, clumps, clusters, or more irregular 

aggregates. "Bursts" and "cigarette burns," large 

spots with dark, opaque centers and pale, irregular 

margins, are abundant in some of the lots (table 10).

Clay stains are widespread, and nearly a fourth of 

the test lots include some pieces of mica marred by 

this defect (table 9). Secondary iron oxide stains 

are almost as abundant, but manganese oxide stains 

are common in only a few lots. None of the pieces of 

Southeastern mica is appreciably air-stained.

Hematite is the most abundant staining mineral in 

the entire mass of tested mica, but inclusions of this 

mineral are rare in the buff or brown micas. Nor is 

magnetite abundant in such material. In contrast, 

shredlike intergrowths of biotite are common in muscovite of nearly all colors. Ordinarily the biotite in 

green muscovite is green, whereas that in brown and 

buff muscovite is deep brown. The coarse, euhedral 

type of biotite generally is intergrown with reddish 

muscovite, rarely with green. It is very rare in material that is marred by hematite or magnetite stains. 

Garnet appears to be a characteristic associate of 

green muscovite; apatite, of the buff and brown 

varieties.

In ge\ieral stained mica is most abundant in the 

outer parts of the containing pegmatite body and also 

is concentrated along the margins of wall-rock septa 

and inclusions. Pegmatites with core-margin or other 

centrally distributed concentrations of stained mica 

generally contain few clear green books, and those 

with wall-zone concentrations of clear green mica 

rarely contain many stained books. Stained mica is 

abundant in some pegmatites with wall-zone concentrations of buff or brown books but is rare in those 

with centrally located concentrations of such books.

PINHOUES AND HAIR CRACKS

Pinholes are tiny flaws that generally extend 

through only a few laminae of the host mica book. 

Many are extremely difficult to detect without the aid 

of a microscope or suitable spark-test apparatus, and 

a substantial proportion of condenser failures may 

well be attributable to the presence of such imperfections in visually qualified mica. Hair cracks (or hair 

lines), the extremely thin, fine cracks that cause failure of laminae during filming, are somewhat more 

easily recognized by careful inspection, although 

many of them do not become apparent until the mica 

is split into very thin sheets.

In the present investigation no attempt was made 

to determine the proportion of pinholed sheets in each 

lot of mica by means of petrographic examination
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 45

alone, as such quantitative data were much more 

readily obtained during the electrical testing. On the 

other hand, it was possible to demonstrate under 

the microscope that most pinholes are formed by 

the "popping out" of tiny inclusions of zircon, apatite, 

garnet, and other "stony" minerals. Not only are 

such inclusions similar in size and shape to the pinholes occurring in the same lot of mica, but the general proportion of these inclusions in block mica can 

be correlated with the proportion of pinholed films 

obtained from the blocks. Moreover, it has been long 

recognized by experienced mica inspectors that the 

proportion of pinholes in a given lot of mica increases 

during its preparation and especially during filming, 

presumably as a result of separation of the stony inclusions from the mica laminae. These imperfections 

are much more abundant in the brown and buff, or 

ruby, micas than in the green ones, probably because 

of the greater numbers of zircon and apatite inclusions in the brown and buff varieties.

Some hair cracks are irregular, with trends that 

do not form a recognizable pattern within the host 

book of mica. Most, however, are straight and systematically disposed. Many of these appear to be 

ruling developed on a very small scale, whereas 

others are parallel to rays of the percussion figure. 

According to some mica buyers, hair cracks commonly develop after the rifting and trimming of sheet 

mica from certain deposits, particularly if the books 

are not thoroughly dried before preparation. If such 

block mica is filmed promptly, however, relatively 

few hair cracks are said to form. This may well be 

true, but it also is known that hair cracks are much 

more readily recognizable in mica films than in book 

or block material. In general these tiny imperfections 

are more abundant in green micas than in the brown 

and buff, or ruby, varieties. They are especially common in the yellowish-olive books from several mines 

in the Spruce Pine district.

ELECTRICAL TESTS 

APPARATUS AND METHODS OF TESTING

The Bell Telephone Laboratories' resonance-circuit vacuum-tube-voltmeter portable set D-167113 

was used to determine the Q value of all samples of 

block mica, and the spark-coil test set D-l 67407 was 

used to establish the presence of pinholes, cracks, and 

conducting impurities. The circuits and operation of 

these instruments have been described in a foregoing 

section of this report. Before the testing began, the 

Q-meter was overhauled, cleaned, and checked for 

satisfactory tube characteristics, battery voltages, 

and calibrations. The meter was again checked by 

the supervisor of the electrical work before actual 

test runs were begun, and it was rechecked after each 

2-hour period to insure continued accuracy. It was

operated throughout each 8-hour work day for the 

duration of the program.

Three women, selected on the basis of their knowledge and experience in the rifting, trimming, and 

visual inspection of sheet mica, were contracted to 

prepare the samples and operate the test sets. Each 

was trained to adjust, calibrate, and manipulate the 

electrical equipment. All work was done in a nonair-conditioned ground-floor room, 25 feet wide and 

40 feet long. The relative humidity varied from 45 

percent to 98 percent, and the temperature from 68° 

to 97° F. The spark-test machine was used in a 

darkened room, so that the operator could readily 

distinguish any conducting material or structural 

flaws in the samples.

The entire program was organized on a continuous 

production-line basis. The mica books were rifted by 

one operator, placed in labeled boxes, trimmed by a 

second operator to sizes recommended by the American Society for Testing Materials, and then tested in 

the Q-meter by the third operator. Job assignments 

were rotated each day, in order to avoid monotony 

and increase the efficiency of the work. In only a few 

days the women acquired sufficient technique and 

skill to test adequately 15 to 25 mica specimens per 

minute on the Q-meter. The best operator was able 

to test 20 specimens per minute for periods of 15 

minutes or more. Even so, the time and precision required for the Q-meter work made it the slowest part 

of the procedure, and some balancing of operations 

was necessary. This was done by assigning either 

of the workers engaged in preparing the mica to operation of the spark-test set for appropriate periods.

In the Q-value or power-factor test, the pieces of 

mica were designated as E-l, E-2, or E-3, and the 

results of all 30-mil stack tests were recorded. After 

each lot of mica was tested, it was placed in paper 

envelopes according to its electrical classification. 

Washer material was kept in separate bags. The lots 

were then counted, labeled, and checked, and the 

numbered envelopes were boxed in groups of 50 and 

taken to the spark-test bench. In the spark testing 

a record was made of all mica with conducting impurities, hair cracks, and pinholes, and the general 

proportion of pieces with conducting material or 

other defects was noted for each lot (table 10). The 

samples also were inspected visually, and a record 

was made of color, transparency, and any stain or 

large cracks that were present.

GENERAL TESTS

The results of the general electrical tests are listed 

by sample lot and by mine in table 10, where they are 

correlated with descriptions of color, transparency, 

and other properties of the mica. Of the 2,502 lots 

tested with the Q-meter 2,441, or 97.6 percent, con862911°—BO—4
46 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

tained only mica that lay within the 95-100 range 

on the dial and hence qualified as E-l material. All 

the mica in 16 lots was classed as E-2 in quality, and 

all of that in two other lots as E-3. Of the remaining 

samples 21 contained E-l and E-2 material, 19 contained E-l, E-2, and E-3, and 3 consisted of E-2 and 

E-3 pieces only* The results of electrical tests of the 

samples that include material of other than E-l quality are listed separately in tables 11 and 14.

In terms of individual test pieces, 229,781 of the 

total 237,764 specimens—more than 96.6 percent— 

qualified as E-l; 4,747 specimens were classed as E-2 

and 3,236 as E-3. Most of the E-2 and E-3 mica was 

slightly to heavily stained with magnetite or hematite, and approximately half of it was so badly claystained, iron-stained, rippled, reeved, cracked, or 

otherwise blemished that it could be classed only as 

washer stock. Some electrically inferior sheets, on 

the other hand, were perfectly clear and otherwise 

of excellent appearance.

Of the 61 lots containing mica of E-2 or E-3 quality, 29 were obtained from mines in the Spruce Pine 

district, N. C.; 5 from the Shelby-Hickory district, 

N. C.; 11 from the Franklin-Sylva district, N. C. and 

Ga.; 2 from Charlotte County, Va.; 4 from Troup 

County, Ga.; and 1 from Tallapoosa County, Ala. 

Nine lots were not specifically identified as to source. 

This distribution by no means reflects the general 

quality characteristics of sheet mica in one area or 

another, as no attempt was made to obtain the original samples in any systematic manner with respect 

to such factors as the relative production of mica and 

the relative quantities of mineral-stained mica in each 

area.

There is little correlation between power factor and 

color of the test pieces, or between power factor 

and amount of shredlike biotite inclusions, mottling, 

vegetable stain, large biotite inclusions, or brown 

bursts in the test pieces. The spark tests verified the 

presence of cracks and holes in much of the trimmed 

sheet material, and a. few of the hematite and magnetite inclusions in some pieces reacted as conducting 

substances (table 10). Pinholes and hair cracks were 

found to be much more common than they appeared 

to be on the basis of careful visual examination and 

were present in some of the test pieces that ordinarily 

would have qualified as No. 1.

The testing program included the study of 550 lots 

that contained a total of more than 39,000 pieces of 

very slightly to densely mineral-stained mica. Nearly 

half of these pieces were typical No. 3 or electric 

stock, with scattered to dense specks, spots, blotches, 

laths, and lattices of iron oxides. Most of the mica 

was found to be E-l, and less than a tenth of it could 

be classed as E-3. A 15,987-piece sample of half- and 

three-quarter-trimmed No. 3 material from the 

Knight mine, Rockingham County, N. C., tested 61 

percent E-l, 23 percent E-2, and 16 percent E-3. Only 

3 percent of the mica contained conducting impurities. Most of the specimens were heavily stained 

with brown to black hematite, and the entire lot was 

a representative sample of the lowest-quality sheet 

mica from the mine. There appears to be a partial 

correlation between power factor and the amount of 

hematite-magnetite stain in the mica, in that most of 

the E-2 and E-3 pieces were heavily stained, but 

evidently there are numerous exceptions. Some pieces 

that qualified as E-l, for example, were more densely 

stained than many with a higher power factor.
GENERAL TEST DATA FOR MICA SPECIMENS
48 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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74 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TABLE General dat 10. for mica arranged specimens, bylot tes and mineContinued prospect or — — 

Electrical tests

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TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 75

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76 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

Continued mine andspecimens, arranged byGeneral for mica prospect data TABLE 10.lot test or — — 

Electrical tests

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94 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TABLE General 10. data for mica specimens, arranged bytest lo mineContinued and prospect or — — 

Electrical tests

f i|||l *3^£

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Sparse hematite and shresspots and raremagnetite spots.

-----dO-. --------

•tTS'S 

|1 £

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hematite.

.-.-dO.- —— ......

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brown.

---.dO- ——-..

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OHOH O" OH o^
Young, Zeph,

mine group.

Do—.- —

Alien, mines

(in Zeph 

Young group).

mine.

Do.........

... ..do.. .......

---..do.--..---.

..---do.. . ------

__---do.-.- — ..

--——dO———- —— -

Yancey, N. C...

-—— dO-_ —— ——

—— -dO—— —— ——

... -do— ... —

-. ---dO-_ ------

5, 6......

7 _

7----....

—do———

Yellowish to

brownish olive.

---.-do.----....

— -.do— — —

some pale

brownish olive.

....-do.........

Trace 

Trace

20 

<5

<5

0

Rare magnetite 

specks and spots.

-—do—.. — —

Scattered magnetite and hematite 

spots.

Rare dendrite 

specks and spots 

of hematite.

.....do...........

Locally abundant vegetable stain and 

green biotite 

shreds; garnet inclusions.

.--.-do.----.

Sparse green 

shreds of 

biotite.

Clay-stained; 

curdy brown 

stain at centers of some 

books, 

-.---do.......

iron-, and 

manganesestained.

81

12

137

40

100

100

100

100

100

100

0

0

0

0

0

0

E

SPECIMENS FROM SOURCES OUTSIDE THE SOUTHEASTERN STATES

Do.—————

mica (localities 

unknown) .

mica (localities 

unknown) .

-—do—;.—

-----dO-. -----

----dO--------

N. Mex.

———— dO—— —— ——

-L-do— — —

-i-.do-..- — -

---do—— -----

1, 2, 3, 4,

5. 

4, 5, 6 _ .

3. ... — --

4 —

5—-—- _

3, 4— — .

green and yellowish to 

brownish olive.

green. 

— .-do- — ——

green to yellowish olive. '

-——dO- —— —— .

<5

Trace

30

0

100

0

S ^

drite specks and 

triangular lattices.

tite and hematite specks, 

.-..do- — ......

blotches of hematite with heavy 

triangular lattices.

specka and spots.

green mottling.

some green 

mottling.

2

1

1

1

1

1

35

17

9

4

20

211

f;n

100

100

100

100

0

100

100

0

0

0

0

85

0

0

0

0

0

0

15

0

0
96 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TABLE 11.—Results of electrical tests of mica samples, from specific mines or deposits, that include material of E-% or E-S quality

Name of mine 

or prospect

Black Dixie mine (in 

Bailey Mountain 

group) .

Boone, Nelson, mine..._ 

Buchanan, Henry, prospects . 

Buchanan, Jim, prospects . 

Campbell mine. _ _______

Do _

Carolina Mineral Co. 

No. 6 mine.

Chalk Mountain prospect.

Cox, Wood, mine__-____ 

Crab tree No. 1 (Wildcat) 

mine. 

Crabtree Summit prospect. 

Crews No. 1 prospect.. ._ 

Emmons Knob mine_ _ _ 

Estatoe Northeast 

(Mossy Rock) prospect. 

Furlow Gap (Farlow 

Gap) mine.

Hogg mine (wall-zone 

mica).

Hoppus Northeast mine.. 

Irby Cut mine ___ __..

Jimmy mine (in Archie 

Norman group) . 

Kell mine _ ... __ _ .

Locust Rough (B. T. 

Snopp) mines.

New Wolff (Wolff, Old 

Wolff) mine.

Ollis, Jake, mine __ _

Renfro mine __ _______

Rickman mine __ -_-___

Do ___________

Smith, Calhoun, mine 

Smith, Ernest, mine _____

Tunnel mine____.___ _

Wiseman, W. W., mine. 

Young, George, mine ____

District

—— -_dO—— -- —— _-___

Frariklin-Sylva__ 

Shelby-Hickory. 

Spruce Pine __ _____

Spruce Pine __ _____

Spruce Pine _ _ _ ___. 

.....do _ -_— __ .

..—do.——————— 

.....do _ ..._....__

.....do _ ._.-...-._ 

——do————__—

— _.do_— __.—__.

Outlying Virginia- — 

Spruce Pine__ _ _ _

Franklin-Sylva. — 

Spruce Pine_ ________

_---dO-__ __________

Outlying Georgia—.. 

Spruce Pine. _____

—..do... ____ ____ 

Outlying North Carolina Piedmont. 

Franklin-Sylva ___

Franklin-Sylva _ ... 

Ridge way-Sandy 

Ridge.

-_-_dO-_ _--_-_-____

_——— do———————— —— __

Spruce Pine __ __.._

———do—— —— — ..

.....do.. .._.____.__

Franklin-SylvaOutlying Georgia.... 

_-_--do__-_ — -__-._

Spruce Pine ___ ____ 

Ridge way-Sandy 

Ridge.

Franklin-Sylva- ----- 

Outlying Virginia- _„-

Shelby-Hickory- . _ 

Spruce Pine __ ----- 

—— -do.. ___ ------

County and

State

Mitchell, N. C- _ ..

Jackson, N. C-__--_ 

Cleveland, N. C._- .

Yancey, N. C __ ---

Mitchell, N. C ______ 

Cleveland, N. C __ _

_ ..do -

Mitchell, N. C .-_-_. 

. _ _do __ . ________

.__„ do _ _._ _______

Mitchell, N. C ___ 

__ .do _ _ ______

__ _do._ _ . _______

Charlotte, Va _.____.

Avery, N. C— _____ 

Mitchell, N. C——— _

Transylvania, N. C__

Yancey, N. C— .___ 

Mitchell, N. C.-----

Mitchell, N. C--._-

... ..do... ____._.___

Yancey, N. C_— ... 

Rutherford, N. C_-_

Jackson, N. C_____- 

Cleveland, N. C____.

Rookingham, N. C-_

Mitchell, N. C ___ _

Avery, N. C _ - __

Mitchell, N. C__ _ _

Mitchell, N. C ___

Macon, N. C_--_.-_ 

Troup, Ga ____ ...

Mitchell, N. C— .„ 

Rockingham, N. C._

Mitchell, N. C——— _

Rabun, Ga_----_--_

Cleveland, N. C __... 

MitcheU, N. C ___

—— - -dO——- —— ... _.-

Type of 

material

Washer _

-_.--dO——

_.__ _do—

------dO---

— - -do... 

__— do...

--—— do——

——— do— 

6— _ ——

Washer... 

—— .do—

_ _ .do—

.....do... 

3-7 ______

Washer _ 

———do—

5, 6_ __ _

5-7————

5__------

Washer .__

5-6. — --

Washer ___ 

-----do...

5-7.- __

4-6-__-_.

Washer _ 

----_do___ 

fi, 7— -__

S-7---.--

Washer _

————dO——

3-7 ______

Washer . . 

5, 6 __ -.

Washer - _ 

5-7 _ -_.

5-7- ———

3-7— —

5,6 _ --.

6 _ -__-.

Washer __ 

.---.do— 

6

Washer . _

.--.-do— 

6, 7— —

3,4———-

6,7——..

4-6 ___.._

5-7 ___ .

5-6- — --

Washer . . 

Washer . .

Proportion 

of 

material 

with 

mineral 

stain 

(to nearest 

5 percent)

20 

0 

0

100 

35 

50 

35

25

20 

100 

100

100 

0

65 

100 

5

100

100 

5 

60

100

65 

100 

100

100 

5 

0 

100

Trace 

Trace

100 

90

0 

100

100 

65 

10

85 

70 

10 

95 

50 

0 

100 

100 

30 

95

100 

60 

100 

75 

15 

60 

100 

<5

Number 

of 

pieces 

tested

100 

21 

12

8 

81 

7 

17

9

89 

7 

18

16 

11

842 

8 

63

6

185 

48 

5

184

20 

3 

410

248 

8 

7 

94

1,112

7

30

15,987

7 

85

12 

849 

113

1,269 

150 

15 

200 

15 

15 

83

11 

175

3

174 

1,216 

128 

114 

15 

28 

30

Percent E-2

100 

8

42

100 

6 

100 

29

100

60 

100 

0

44 

55

0 

100

19

67

3 

2 

100

35

35 

100

41

2 

100 

100 

13

4 

100

50 

23

100

7

0 

6 

0

2 

6 

47 

4 

100 

100 

14 

100 

18 

6

0 

9 

20 

4 

4 

100 

4 

37

Percent E-3

0 

0 

0

0

0 

0 

18

0

0 

0 

100

0 

0

2 

0 

0

33

11 

0 

0

3

0 

0

9

0 

0 

0 

0

0 

0

0 

16

0

5

25 

0 

3

14 

3 

0 

3 

0 

0 

0 

0 

82 

0

100 

2 

3 

1 

1 

0 

0 

0

Proportion of tested 

defects (to nearest 5 percent)

Material 

with 

conducting 

substances

0 

0 

0

0 

0 

0 

0

0

0 

0 

0

0 

0

0 

0 

0

100

65 

0 

0

0

0 

0 

100

45 

0 

0 

100

0 

0

0 

5

0 

0

0 

5 

5

80 

10 

0 

100 

0 

0 

65 

0 

0 

5

100 

75 

40 

50 

10 

0 

100 

0

Material

with 

cracks

0

0

0

0 

0

0

<5

0

0 

10

10

5

<5 

0

Trace 

0

<5

0

0

0 

0 

0 

0

, o

0

Material 

with 

pinholes

25 

0 

0

0 

0 

0 

0

10

0 

0 

0

Trace 

0

0 

0 

0

0

0 

0

0

0

0 

0 

<5

0 

0 

0 

5

<5 

50

0 

15

0 

0

100 

<5 

0

<5 

Trace 

0 

<5 

0 

0 

0 

0 

0 

<5

0 

0 

5 

25 

15 

0 

0 

20

SAMPLES FROM SOURCES OUTSIDE THE SOUTHEASTERN STATES

New Hampshire mica 

(localities unknown) .

-—— — - ——— New Hampshire _ __ 4 ________ 100 20 85 15 5 0 5
TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 97

TESTS OF VISUALLY QUALIFIED MICA

Through the courtesy of W. J. Alexander, Southern 

District Manager, 354 lots of prepared and qualified 

mica were loaned by the Colonial Mica Corporation 

for electrical testing. This material had been processed and qualified by visual methods in Asheville, 

N. C. The test data for 243 lots taken from the production line of the Colonial Mica Corporation shops 

in 1945 are shown in table 12. Twelve of these had 

been qualified by careful inspection as No. 1; 19 as 

No. 2; 208 as No. 2 inferior; 2 as No. 3; and 2 as No. 1 

and No. 2.

The results of electrical testing were in marked 

contrast to this grouping. All pieces of mica in all 

the lots registered E-l on the Q-meter dial, and nearly 

all were in the uppermost part of the E-l range. On 

the other hand, the spark testing revealed pinholed 

material in 64 percent of the lots, hair cracks in 

nearly 7 percent of the lots, and conducting inclusions 

in one lot. Pinholes and cracks were present in 12.6 

percent of the test pieces, yet none of these flaws was 

readily observable with the naked eye. This high 

proportion of blemished sheets evidently is unusual, 

as only 3.35 percent of the 237,790 pieces tested during the entire program contained pinholes or hair 

cracks.

Another series of mica samples had been prepared, 

qualified, taken from the production line, and placed 

in storage by the Colonial Mica Corporation in 1943. 

These samples were retested by the electrical methods, and the results of the tests are compared with 

those of the earlier grading in table 13. Thirty-nine 

lots of green mica yielded only 10 specimens of E-2

material and 1 of E-3, and 72 lots of buff and brown 

mica yielded only 68 specimens of E-2 and 13 of E-3. 

Yet 92 of the lots had been graded as No. 2; 5 as No. 1 

and No. 2; and 14 as No. 2, No. 2 inferior, and No. 3. 

Most of the down grading apparently had been done 

on the basis of waviness and inclusions, but the spark 

testing showed that pinholes were present in 33 percent of the lots, hair cracks in 8 percent, and conducting impurities in more than 6 percent of the lots. The 

proportion of pieces in each lot so marred varied 

from 2 percent to 100 percent. The results of electrical tests of the sample lots that contain material of 

other than E-l quality are listed separately in 

table 14.

The results of these comparisons fully support the 

conclusions of Coutlee, Townsend, and others who 

have pointed out that much mica ordinarily classed 

as No. 2 or No. 2 inferior by visual appraisal actually 

has excellent dielectric properties, as determined by 

means of the Q-meter. Moreover, the spark-coil test 

set provides a means for identification of conducting 

regions and tiny structural flaws that may well be 

more effective than even the most critical visual scrutiny. In its present form the Q-meter permits the 

testing of 15 to 20 pieces of mica per minute by a 

reasonably competent operator, the average rate depending in part upon the type of material under test 

and in part upon the period of time between rest 

pauses for the operator. The spark-coil test set permits much more rapid manipulations. Adaptation of 

the Q-meter to semi-automatic or even fully automatic 

operation appears entirely feasible and would appreciably lessen the cost of electrical testing.
98 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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TABLE i_J.—Test data for mica, taken from the production line of the Colonial Mica Corporation in 19451 previously qualified by visual means—

Name of mine 

or prospect

Do ..........

Do——— _ ..

Do— — ——

Do— — ——

Do. _ . _ -

Do.. — _ ._

DoDo—————

Do——._. ...

Do __

Do——————

Do. —— _. ...

Do... ______ .

Do. _ ... ...

Do—— — —

Do.—— _ ..

Do——————-

Do— _ ....

Do. _ ......

Do....... _

Miller Douglas — _ 

Do——— _ ..

Do..___..._.

Do—— ______

Do. _ . .....

Do. __ ......

DoDo——————

Do...... ....

Do—— .... __ 

DoDo. _ ._.—

Do— ————

Do. _ ... _

Do. _

Do—— ... ...

Do— ————

Do. _ ......

Do. _ ______

Do— _..._ __

Do.__ ___ ._

Do ____ ....

Do... .... ...

Do———— ....

Do— .... ...

DoDo——————

Do. _ . _ ..

Do. _ . .....

Do—————— .

Do..... _ „

Do————— ..

Do........ ..

DoDo——— _ ..

Do. _ . _ ..

Do ____ . ...

Do————— „

DoDo——— _ ..

Do... .......

Do... _ ....

Do. _ ......

Do____..__ __

Do—— .... __

District

Alabama. _..._.._ 

——— do— _______ .

..... do... ... .... .

— __ do.... .._____

——— do— —————

. _ .do.. — ......

— ——dO———— .....

————— dO——————— ———

——— -dO—— ...... . .

..... do— _._... „

—— .do———.. __

-----do __ ---___.

...__ do—— _. _____

———do—— ———— . __ do—— ——— .

———do— __ . _

..... do..... .... „

—— .do _ —— ....

—— -dO—— —— —————

——— -dO.-.——— ...

Shelby-Hickory . . .

- —— -dO......——..

. __ do———— _ ..

. ___ dO- — -- __ ..

—— .do.... _ ....

.....do — ... _ „

___ do.... _ ....

.. _ do- ___ ....

HartweU..— . _ . 

.....do _ ________

... -do... ....... . 

—— .do.. _ ______

— _._do..._ _ .„-

- ___ dO-- __ -_..

--..do _ ........

.. ...do.. ______ ___

.. _ do. ___ .....

— ..do _ . _ ....

— _ do— __ . _

. __ do ____._..._.

.....do ___

— —do _ ...... __

. —— -dO. — . __ ———

—— ..dO—— —— __ —

.——do _

——do.. ———— —

.... -do_—__ ———

— __.do_. __ __.__

-—do ___

..-.do... __ ....

- __ .dO——.._ ——— _

———do—— _______

—— .do——— ______

_ — .do..... ..... .

. _ .do... .... ..__

— ...do—— __ ...

—— -do———— ....

———do——— ......

— __ dO—— --..._.

——— do—— ————

T ... .do. ——— ....

-- —— dO—— ...... ._

... ..do——— ______

..... do————— _

County and State

Randolph, Ala... ...

——— do— —— . _____

——— do— ———— —

__ .do.. — — .-_..-

—— .do— — -----

— —do——— — ——— —— do— — ——— .

———do—— —— — —— .do. — — —— -

---. do.- — . — —

..—do-—————-

—— .do— —— ..... .

--.do———. _ ——

——— do. —— ——— ..

———do———— _____ —

— _.do_—— ——— —

.--..do—— _______ —

---dO—— —— — __ --

— -do— ——— ——— — ..do— — — ——

Caldwell, N. C.__...

._.__ do— -_.____._

... ..do—— ———— ..

--.do— — _ — -

-———dO——— ————— ——-

. — .do. — _ -...--

. _ .do— — ————

... -dO—— -_.- —— .--

Hart, Ga _ . _ .... 

— —do— __________

——— .dO--- —— ——— . —

——— do———— ———

— -.do— ——— — - 

---..do— — _______

-----dO——— ------

- — .do————— ——

——— do.'—— — _ —

——— do— ...... ....

——— do—— ————— ——— do— — —— —

.. — do——— ——— —

-.— do— --..-. -.

——— do... ——— — __

_.__ .do— ——— _____

—— .do- —— ______ .

---.dO—— ______ -- —

--.do-.- — _ .--.

———do— — ——.

.-..do— — ._____.

— -do— ——— — .

—— -dO.-.--.. —— -

———do—— — — — . . do

— -do——— ————

____ .do— _________ .

——— do__. _____ __...

——— .dO.----. ——

---dO—— ———————— -

—— do—— ————

— -do— -----

— -do——— ———— _ ——— do—— —————

-—do——.————

. _ .do———— .....

———— dO——— ——— —— —

Type of 

material

5,6 

5 

6

5>_!6 

5>_. 6

5)1! 6 

5)_, 6 

5)_, 6 

5)_, 6

5j|!6 

5)3,6 

5)_,6 

5)3,6 

5)_,6 

6

5)3,6 

5 

5,6 

5,6 

6 

6 

5, 5)_,6 

5, 5)_,6

5, 5)_,6 

5, 5)_,6

5, 5)_, 6 

5, 5)_,6 

5, 5)_,6 

5, 5)_, 6

4, 5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4, 5,6 

4,5,6 

4,5,6 

4,5,6 

4,5, 6 

4,5,6 

4,5,6 

4, 5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6 

4,5,6

Color

——— do—— — — —

——— do— — — ——

—— -do— ... __ — -

. _ .do... ... -----

— ..do— .. ------- 

-...do— .-.-. — - 

-—do—— — — —

— ..do— ..... -----

———do———— — —

.....do——— —— — -

..—do— ... — ——

—— do—— —— —

_____ do—— — — —

. — ..do— --------

-— do——— — -----

_ __ .dO—— -. ------

——— do. — —— ———

... _.do——— — ———

.....do—— _

——— do— ——— ——

-.... do—— ----- -

———do— ——— — -

.....do— ——— _-___

—— _do___ --.-. — -

..... do—— —— ——

———do—— ——— —— 

__... do— —— . —— _ ———do————— —— -

_——do— — — ———

——— do— — ______ . 

... -do. — ... — —

—— .do— ... -- — 

———do——— - — —

.....do ___ _______

.....do _____ _____

— __do ____ . — ._

.——do _ . __ ———

_ ..do _____ .....

.....do _____ .....

.....do .............

.....do .............

__...do _____ .....

. __ do.. ___ .....

.. _ do _____ — ...

.. _ do ____ . .....

.....do _____ .....

.....do— — _. ......

-...do—..— — ._ 

———do——— _____

——— do— ——— — -

..... do. — ... ... —

.....do—— —— ———

..... do——— ....——

—— .do——— — ———

.—.do——— — ——— 

__ .do— ___ ... .. .

———do——— ————

do _ -_. .

.....do—.——- _ 

__ .do—— —— ———

__._.do———— _. — .

..—do— --------

Proportion 

of material 

with mineral 

stain 

(to nearest 

5 percent)

0 

10 

25 

50 

20 

20 

0 

20 

20 

20 

20 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Listed 

quality, 

determined 

by visual 

inspection

2 inf... _ .

2 inf—— „

3— ________

3.... ... .

2 inf ___ ..

2inf— — .

2 inf ___ ._

2 inf ______

2 inf... ... .

2 inf__ — ..

2 inf _______

2 inf.. _ „

2 inf... ____

2 inf..———

2 inf.......

2 inf ..._.__

2 _._._____.

2— ........

2 ........__

2 _ _. _____

1 __________

2 inf ____...

2 inf.._ ....

2 inf _______

2 _..._._.__

2 _..._.____

1 __________

1 _ _______

2 ______ .

1 ______ .

2 inf ___ .

2 inf.......

2 inf.. —— .

2 inf—— _.-

2 inf.......

2 inf ..__.__

2 inf __._.__

2 inf————

2 inf .....__

2 inf _ ,-.-

2 inf _______

2 inf.......

2 inf————

2 inf.......

2inf—— —

2 inf ____ .

2inf___ ... _

2 inf————

2 inf ___ ..

2 inf.. .. ...

2 inf ___ ..

2 inf... .___

2inf———— .

2inf____ ___

2 inf.. _ ...

2 inf.. _____

2 inf _______

2 inf _______

2 inf..... „

2 inf. .__ —

2inf— ___ _

2 inf————

2 inf. ______

2 inf _______

2 inf _______

2 inf... _ .

2 inf .____..

2 inf _______

Electrical tests

Total 

number 

of pieces 

tested

367 

317 

169 

133 

412 

439 

300 

425 

400 

425 

400 

425 

410 

405 

400 

310 

235 

300 

300 

300 

265 

385 

188 

193 

145 

128 

133 

142 

303 

365 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200 

200

Percent 

E-1

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100

Percent 

E-2

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Percent 

E-3

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

30-mil 

stack 

(number 

of stacks 

in parentheses)

E-1 (!)___ 

E-1 (!).__ 

E-1 (!)___ 

E-1 (!)___ 

E-l (1)_- 

E-1 (!)___ 

E-1 (1)___ 

E-1 (!)_._ 

E-1 (!)___ 

E-1 (1) __ 

E-1 (I)..

E-1 (!)___ 

E-1 (!)_._ 

E-1 (1)- — 

E-1 (1)..

E-1 (I)..

E-l (!)-__ 

E-1 (1)..

E-1 (1)..

E-1 (!)._

E-1 (!)_.

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 

E-1 

E-1 

E-1 

E-1 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 

E-1 

E-1 

E-1 

E-1 

E-1 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 ( 

E-1 (

'.)"'

I)..

.)--

'.)'.".

.)'.'.

I)..

11::

,)..

.)'.'.

I III

I ..

I ..

1 ..

I ___

I ..

1 ___

'.)'

l)._

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.)--

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TESTS OP SHEET MUSCOVITE PROM THE SOUTHEASTERN STATES 101

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102 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TABLE 13.—Test data for mica (not identified by mine}, taken from the production line of the Colonial Mica Corporation in 1943 and placed in

storage, qualified by visual means in 1943

Lot 

number

1 1 3 PL

1 1 37

1 1 3ft

1139

1140

1141_,— __-_-

1 -I AO

1 1,13

1144 _ . ___

1145 _________

1146 _____ -

i 1 ei

1155

1156 __.__ — _

1158__ _ ____

1159——— — -

1165 _____ .

1214 ___.___-_

191 1

1216 ____ _

1 9O 1

1931

1 939

1234 _ . ___

1 93 PL

1 93ft

1937

19Rn

1261 ____._ — .

1262 ___ . __

1 9ft3

1264 ______

1265— _---

1266 ______

1270 _ . ___

1 971

1 979

1973

197,1

1276_.—._ —— -

1280

•J OftO

1285 _ __-...

1 9ftfi

1 9S7

1288. _____

1 9ftQ

1294— -----

1305 _________

1306 ___ __._

1307 ______

1 3flft

1 3flQ

1310 ___ . _

1311

1319

1313

1314 _________

1^1 1

1316 _________

1317 ______

1 31 ft

1319. ________

1320 ______

1 391

1 399

1323 _ _ _ __

1324 __ _ _ _

1 39 PL

1326 ___ ____

1327 ______

1 39Q

1337

1338 _________

1^4.1

1343 _________

1345 _________

1346 ___.__—

1347 _________

1348 ___ _

1349 ____ ..

1369 _________

1371 _________

1376 ___ . _

1377 ______

1378 _________

1379 _ _ _ _.

1381. __ -___

1385 ____ _.

1386 _________

1387 _________

1388 _ . _____

1389 _________

1390 ___ _ ___

1391——- ——

Type of 

material

6 

6,7 

6 

5, 6J_, 6 

6, 7 

6, 7 

6, 7 

6,7 

6,7 

6, 7 

5, 5^_ 

4 

6 

7 

5 

5*. 

5, 6 

6 

6, 7 

6 

6 

6 

6 

6 

6 

5, 6 

6 

6, 7 

5, 6 

5A, 1 6 

5, 6 

5}_, 6 

6 

5J_, 6 

6 

5, 6 

5, 6 

5}., 6 

6, 7 

6 

6,7 

6 

6 

6 

6,7 

5, 5}_, 6 

6 

5, 6 

5, 5^_

KV^

6H, 6 

6 

6, 7 

6 

5, 5V_, 6 

5}_, 6 

5, 6 

6 

5, 6 

6 

3,4, 5 

5 

5H. 6 

6 

6 

5, 6H. 6 

5}_, 6 

6 

S, 5M, 6 

6, 7 

5, fij_, 6 

6 

6,7 

6 

6 

6 

6 

6 

6 

6 

4, 5 

5H, 6 

6 

6 

5, SH, 6 

6 

6 

6 

6 

5J..6 

6 

6,7 

6 

6,7 

6,7

Proportion 

of material 

with mineral 

stain 

(to nearest 

5 percent)

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

20 

0 

10 

0 

0 

25 

0 

0 

5 

0 

0 

0 

0 

0 

O 

0 

0 

0 

0 

O 

0 

0 

O 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

25 

0 

0 

25 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Listed 

quality, 

determined by,

visual 

inspection

2 _____ . ____

2 ____________

2 ____________

2 ______ - —

2 _____ -. ——

2 ____________

1, 2 ____ —

2— ._ __ _._

2. __- — —— - —

2 ____ —— _ —— _

2 ____ _____

1 ______ .-.

1. _____ --.

1. ___ _ __ _

o

2 ______ ——

2- _____ ___

2___ _ —— .____

2.. __ ____ ——

2. __„_____„

2—. __ . _____

2

2. ___________

2. ___________

2__ _________

2 ____________

9

2 _______ ..

2 __ _ _______

2 ____________

2

2 __ __ __

2. _____ ._ .

2 ___________

2. ___________

2 ____________

2 _____ __ .

2 _______ __

1, 2 ____ -_

2. ___________

2 ____________

2 __ __ __ __

2_ ___________

2 ____________

2 ____________

2 ____________

1, 2, 2inf____ 

2 ____________

2_ ___________

2 ____________

2 ____________

2. __ . ___ __

2. ___________

2. ___________

2__ _________

1, 2, 2inf.___ 

1, 2, 2inf____ 

2 ______ ._

1,2 _ ___ _ .

2 ____________

1, 2 ____ ...

2 ____________

2_ ___________

2 ____________

2 ____________

2 ____________

1, 2, 2inf._._ 

2 ____________

2 ___________

2 __ ___ _

2— _ ______

2 __ _ _______

2 _______ _

2 ______ _

2 ____________

2 _______

2 ______ _

2 ____________

1,2—— —— ._

2 ____ _____

2, 2inf _______

1, 2, 2inf____ 

2-_._ _ _ ____

2. _ ________

2 ____________

2____________

2____ ________

2-__ _ _.__.

2— __________

2____________

2_ ___ --_.__

2 ____________

2 ____________

Electrical tests

Total 

number 

of pieces 

tested

204 

115 

109 

100 

108 

104 

101 

110 

102 

195 

100 

100 

100 

' 100 

199 

100 

140 

104 

106 

114 

100 

100 

105 

105 

106 

112 

106 

100 

100 

105 

105 

100 

109 

120 

101 

110 

102 

195 

105 

110 

128 

105 

103 

104 

106 

105 

121 

192 

199 

105 

106 

105 

103 

110 

131 

116 

133 

106 

106 

113 

150 

102 

100 

106 

198 

104 

105 

114 

145 

107 

108 

130 

112 

101 

104 

105 

100 

118 

100 

106 

144 

109 

102 

104 

113 

184 

104 

115 

107 

107 

108 

105 

106 

106 

100

Percent E-l

100 

100 

100 

100 

99 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

97 

100 

99 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 100' 

100 

100 

100 

100 

100 

100 

100 

100 

99 

100 

100 

100 

99 

100 

100 

100 

100 

100 

100 

100 

100 

98 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100

Percent E-2

0 

0 

0 

0 

1 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

1

0

1

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

1

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

2 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Percent 

E-3

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

2 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

1 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

30-mil 

stack 

(number of 

stacks in 

parentheses)

E-l (l)_.-_-

E-l (l)-_- —

E-l (!)---__-

E-l (!)__.___

E-l (1) ______

E-l (!)__„__

E-l(l)-.- —

E-l (!)______

E-l (!)__— _

E-l (1).———

E-l (1)__ ——

E-l (!)______

E-l (!)__--_-

E-l (!)______

E-l (!)______

E-l (!)______

E-l <-)___.__

E-l (1)_.____

E-l (!)_.——

E-l (!)___ —

E-l (!)__„__

E-l (!)______

E-l (!)______

E-l <_)______

E-l (!)__—_

E-l <_)_.„„

E-l (!)_——_

E-l (!)_._„_

E-l (!)____„

E-l (1 ______

E-l (1 ______

E-l (1 ______

E-l (1 ______

E-l (1 __ ____ 

E-l (1 ______

E-l (1)______

E-l (!)___„_

E-l (!)______

E-l (1) ______

E-l (!)__ _ _

E-l (!)___ _

E-l (!)__ __

E-l 1) ______

Proportion of tested defects

Material 

with 

conducting 

substances

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

25 

0 

50 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

5 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

10 

0 

20 

50 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

60 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Material 

with 

cracks

0 

0 

0 

0 

0 

0 

0 

0 

0 

50 

0 

0 

0 

50 

5 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

5 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

5 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

20 

0 

0 

0 

20 

0 

0 

0 

0 

0 

0

Material 

with 

pinholes

0 

SO 

0 

0 

0 

0

so

0 

0 

0 

0 

0 

0 

0 

0 

3 

30 

5 

2 

2 

2 

0 

0 

0 

0 

0 

25 

0 

5 

0 

2 

0 

0 

5 

0 

5 

5 

0 

5 

2 

0 

2 

0 

0 

0 

2 

0 

0 

2 

2 

0 

2 

0 

. 10 

0 

0 

0 

0

so

0 

0 

0 

60 

0 

0 

2 

5 

0 

0 

6 

6 

0 

0 

0 

0 

10 

0 

6 

0 

6 

25 

0 

0 

50 

0 

0 

5 

0 

0 

0 

0 

0 

10 

0 

0
SUMMARY AND CONCLUSIONS 103

TABLE 13.—Test data for mica (not identified by mine), taken from the production line of the Colonial Mica Corporation in 1943 and placed in

storage, qualified by visual means in 1943—Continued

Lot 

number

1392 ______

1393 ______

1400 ______

1404 _________

1405 ____ -.

1406 _________

1407———-.--

1408— — -.-

1409 _ ______

1410 ____ .-

1411 _________

1412—, — --

1413 _________

1414 _ . _ ..

1415 _________

1416 _________

Type of 

material

6,7 

5H, 6 

6,7 

4, 5, 6 

5,6 

5, 6 

5, 6 

4, 5, 6 

5,6 

5,6 

5, 6 

5, 6 

5,6 

5,6 

5, 6 

5, 6

Proportion 

of material 

with mineral 

stain 

(to nearest 

5 percent)

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Listed 

quality, 

determined

>y,

visual 

inspection

2 ______ ..

2 ______ ..

2--. ---------

2, 2 inf., 3 __ 

2

2. ____ — -__.

2 _____ ....-

2

2

2 ____ -----

2

2

2_ — _ . _

2.-- ___ . _

2

2 ______ ...

Electrical tests

Total 

number 

of pieces 

tested

100 

109 

100 

108 

101 

100 

103 

116 

102 

108 

100 

105 

107 

110 

113 

101

Percent 

E-l

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100

Percent 

E-2

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Percent E-3

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

30-mil

stack 

(number of 

stacks in 

parentheses)

&-!(!)-- — .

E-l (1)__ ——

E-l (!)__. ___

E-l (!)_____-

E-l (!)___.__

E-l (1) ______

E-l (I)-.--.

E-l (!)_-— _

E-l (!)__,„_

E-l (!)___.__

Proportion of tested defects

Material 

with 

conducting 

substances

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

Material 

with 

cracks

0 

0

0 

0 

0 

20 

0 

0 

0 

20 

0 

0 

0 

0 

0 

0

Material 

with 

pinholes

0 

0 

0 

0 

0 

0 

0 

20 

0 

0 

0 

0 

0 

5 

0 

0

TABLE 14.—Results of electrical tests of mica samples, taken from the production line of the Colonial Mica Corporation in 1943, that include

material of E-2 or E-S quality

Mine lot No.

1263 _____________

1265 .________—„

1325-.--- _______

1337.. _ ________

1369 _____________

Type of 

material

5,6 

6 5-6 

6,7 

6J_, 6

Proportion of

material with 

mineral stain 

(to nearest 

5 percent)

0 

0 

25 

0 

0

Listed 

quality, 

determined 

by visual 

inspection

2___ _ .... _____

2 _ .

I,2,and2inf___ 

2 ____ _______

2 _____

Number 

of pieces 

tested

105 

109 

145 

112 

109

Percent E-2

1 

1 

1 

0 

2

Percent E-3

2 

0 

0 

1 

0

Proportion of tested defects

Material 

with conducting 

substances

0 

0 

0 

0 

0

Material 

with 

cracks

0 

0 

5 

0 

0

Material 

with 

pinholes

2 

0 

0 

0 

0

SUMMARY AND CONCLUSIONS

The results of the color tests show that Southeastern muscovite ranges from buff and drab through 

shades of brown and green to pale yellowish green. 

Most books are not colored uniformly throughout, 

and both irregular and several types of regular variations are known. Buff and brownish ("ruby") mica 

is most abundant in the Piedmont province and in 

large parts of the Franklin-Sylva, Spruce Pine, and 

Jefferson-Boone districts of the Blue Ridge province, 

but in general the bulk of the sheet material produced 

from the entire Southeast region is yellowish 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 and 

yellowish olive. On a larger scale, brownish books 

generally are nearer the walls of a given pegmatite 

body than green ones, and no brownish books occur 

in those pegmatites with green wall-zone mica. Green 

books are rare in pegmatites with core-margin or 

other interior concentrations of brownish mica.

A substantial proportion of Southeastern muscovite is mineral-stained; that is, it contains specks, 

spots, or broader masses of hematite, magnetite, or 

biotite. Much hematite also occurs as latticelike intergrowths of triangular or rectangular pattern. 

Little mineral stain can be effectively removed by 

careful splitting, owing to the extreme thinness of the 

foreign masses. In marked contrast are thicker and 

larger inclusions of euhedral biotite and such "stony" 

minerals as apatite, garnet, pyrite and other sulfides, 

quartz, tourmaline, zircon, and zoisite. Such inclusions can be removed with little difficulty; when this 

is done, holes or distinct depressions are left in the 

host muscovite. Other types of foreign matter, mainly mottled to curdy pale-greenish or brownish stain, 

are widespread and locally very abundant. Clay 

stains and supergene iron and manganese stains are 

very abundant also, but air-stained mica is rare in 

the Southeastern deposits.

Iron oxide mineral stains are common in brownish 

and yellowish-olive micas but are rare in those that 

are buff or brown. Thin shreds and wisps of biotite 

are common in muscovite of nearly all colors, whereas 

the coarse, euhedral type of biotite occurs almost
104 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

wholly in buff and brown muscovite. Mottling, "vegetable stain," and similar impurities are present in 

nearly all types of muscovite. In general, mineralstained mica is confined to the outer parts of those 

pegmatite bodies that also contain clear greenish 

books and to the inner parts of those that also contain 

clear buff or brown books.

Pinholes appear to be developed in Southeastern 

muscovite by the "popping out" of tiny inclusions of 

zircon, apatite, garnet, and other "stony" minerals 

during splitting of the host books. They are not uncommon in most districts but are abundant in the 

mica from only a few mines. Most occur in brown 

and buff, or ruby, micas, in which zircon and apatite 

inclusions are most abundant. Hair cracks, said to 

develop in part after rifting and trimming of sheet 

mica from certain deposits, are more numerous in 

greenish micas than in the brown and buff varieties. 

They are especially common in the yellowish-olive 

books from several mines in the Spruce Pine district.

The results of the electrical tests are similar in 

many respects to those of several earlier testing programs,85 and the detailed study of 237,764 pieces 

specifically identified with at least 850 deposits gives 

them additional statistical value. The correlation between power factor and physical appearance suggested by Kesler and Olson 86 holds in a general way, 

but evidently there are numerous exceptions, particularly among stained micas. Nearly all these exceptions lie in one direction, 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, in that most test pieces of E-2 and 

E-3 electrical quality were heavily stained. On the 

other hand, many other pieces of heavily stained mica 

tested E-l, and some perfectly clear pieces tested E-2 

and E-3.87 Careful inspection has demonstrated that 

E-l stained mica cannot be distinguished from E-3 

stained mica by ordinary visual means. In general, 

however, 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 iron-poor varieties.

85 Eesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, 

N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942; Horton, F. W., Mica: 

U. S. Bur. Mines Inf. Circ. 6822, pp. 41-46, 54-55, 1935 (revised 1941) ; Hall, 

E. L., Equipment and method for measurement of power factor of mica: 

Inst. Radio Eng. Proc., vol. 32, p. 396, 1944 ; Townsend, J. R., Final report on 

commercialization of mica testing equipment: War Metallurgy Comm., War 

Production Board Rept. W-151, pp. 21-25, 1944; Coutlee, K. G., Judging 

mica quality electrically: Am. Inst. Electrical Eng. Trans., vol. 64, pp. 5-7, 

1945.

88 Eesler, T. L., and Olson, J. C., op. cit., p. 18.

87 Hall, E. L., Equipment and method for measurement of power factor of 

mica: Inst. Radio Eng. Proc., vol. 82, p. 396, 1944; Coutlee, K. G., Saving 

mica by testing: Bell Lab. Rec., vol. 22, p. 513, 1944.

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. Air stain also was found to 

have no effect in the few test pieces with this feature, 

although appreciable increase in power factor due to 

air bubbles in mica has been demonstrated by Hall 88 

at a frequency of 1,000 kilocycles.

Evidently neither the color nor the depth of color in 

clear muscovite has any definite relation to its power 

factor. This is in full agreement with the findings of 

Hall,89 Coutlee,90 and others 91 and is contrary to the 

oft-repeated assertion that green and dark-colored 

varieties have basically inferior electrical characteristics.92 The clear green, yellowish-olive, and brownish-olive micas, both light and dark, were found to lie 

within essentially the same power-factor range as the 

pinkish-buff and light-brown, or ruby, varieties. If, 

however, all greenish muscovite is considered without 

regard to whether or not it is mineral-stained, its general dielectric properties are distinctly inferior to 

those of the other types, simply because damaging 

mineral stain is much more common in such mica. 

This very important point is summarized by the 

American Society for Testing Materials 93 as follows:

Experience has shown that the Q value range of ruby and 

white types of block mica, regardless of source, is 80 to 95 per 

cent E-l, whereas, the Q value range of light green, dark green, 

greenish brown, and rum-colored block mica is 45 to 90 per 

cent E-l. It is important to bear in mind that it is permissible 

for all qualities of mica prescribed in A. S. T. M. Specifications 

D 748, to contain spots and stains, providing the mica meets all 

electrical and physical requirements, whereas, no spots or 

stains (other than air stains) are permitted in the * * * visual 

quality groups per A. S. T. M. Methods D 351.

It should be recognized that power factor alone does 

not determine the applicability of a given piece of 

mica for condenser or other high-grade uses. The 

mica must meet other physical requirements, hence 

must also be tested visually and with the spark-coil 

set. Tightly intergrown plates and wisps of biotite, 

for example, have little effect upon the power factor 

of block muscovite, yet they may well interfere seri88 Olson, J. C., Mica-bearing pegmatites of New Hampshire: U. S. Geol. 

Survey Bull. 941-P, p. 382, 1942.

89 Hall, E. L., Equipment and method for measurement of power factor of 

mica: Inst. Radio Eng. Proc., vol. 82, p. 896, 1944.

80 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 

Eng. Trans., vol. 64, pp. 1, 7, 1945.

91 American Society for Testing Materials, Tentative specifications for 

natural block mica and mica films suitable for use in fixed mica-dielectric 

capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. 

Standards, pt. 3-B, p. 705, 1947.

"Horton, F. W., Mica: U. S. Bur. Mines Inf. Circ. 6822, p. 22, 1935 (revised 1941) ; Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, 

p. 8, 1943; Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards 

Jour. Research, vol. 85, p. 245, 1945.

"American Society for Testing Materials, Tentative specifications for 

natural block mica and mica films suitable for use in fixed mica-dielectric 

capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. 

Standards, p. 3-B, p. 717, 1947.
SUMMARY AND CONCLUSIONS 105

ously with its filming properties or lead to development of cracks and holes in finished films. Pinholes, 

hair cracks, and conducting impurities are particularly objectionable. Reeves, warping, rippling, or 

other structural imperfections may lead to undesirably high proportions of waste during the final stages 

of mica preparation, even though the electrical properties of the mica are perfectly satisfactory for condenser use.94

It was demonstrated more than a decade ago that 

domestic muscovite compares favorably with India 

muscovite, quality for quality, in dielectric strength, 

changes on heating, power factor, flexibility, and 

hardness,95 and it generally has been assumed that 

most consumers' preference for India mica is based 

upon its more careful preparation and better grading. 

Through the efforts of the Colonial Mica Corporation 

during the period of World War II, however, the preparation of domestic mica was standardized and 

greatly improved, and consumers were able to use 

large quantities of such material without further testing. Moreover, the results of the present investigations show that there is no intrinsic difference, quality 

for quality, between clear ruby and clear nonruby 

micas from the Southeastern States. Clear green, 

yellowish-olive, and brownish-olive micas, selected by 

visual means, should prove just as satisfactory for 

condenser use as clear buff and light-brown micas, 

and several manufacturers have indeed found this to 

be the case. In addition, the use of electrical testing 

and adoption of the combined visual-electrical system 

of mica classification should result in upward grading 

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 and lots of apparently 

high-quality material, owing to pinholes or other defects difficult to recognize.

Although possibilities for the use of visually inferior grades of mica in condensers have been pointed 

out, the requisite electrical testing and increasing 

costs of preparation might well exceed the potential 

gains from such a procedure, especially under peacetime conditions. In discussing the mica situation in 

1944, Gwinn and Tucker 96 noted that "by the end of 

the third quarter the supply and stock of strategic 

mica, especially the smaller sizes, reached such a point 

that the War Production Board felt it no longer necessary to purchase either domestic or imported greenand black-stained mica. This curtailment was instituted to eliminate the excessive handling and preparation costs entailed in preparing green- and blackstained material." Although perfectly accurate with 

respect to black-stained mica, this statement—like 

many another in the published record—is not at all 

clear with respect to green material. The use of a 

hyphen after the word "green" implies that mica with 

green stain is intended, whereas the decision of the 

War Production Board applied to all green mica (or, 

more exactly, to all nonruby mica), whether stained 

or clear. There appears to be no good evidence that 

clear green muscovite is any more difficult or expensive to handle and prepare, grade for grade, than 

clear muscovite of any other color.

Some of the economic factors involved in the use of 

electrical testing for the upgrading of visually lowquality mica have been summarized by Watts,97 who 

points out that the cost will be increased by (1) the 

expense of selecting the mica; (2) losses involved in 

the rejection of E-2 or E-3 mica or of mica with conducting impurities; (3) additional rejections in the 

voltage testing of films for large-capacity or pottedtype condensers; (4) difficulty in splitting and punching some types of mica; (5) the additional inspection 

and care required in manufacture; and (6) the 

greater number of rejected capacitors. He adds that 

electrically selected low-quality (visually determined) ruby or green mica can be used to produce 

satisfactory capacitors and that if the industry were 

"to use the lower qualities when sufficient higher 

qualities were obtainable, the anomalous situation of 

greater cost for capacitors made with lower quality 

mica than higher quality mica would result."

It has been established that much mineral-stained 

mica is not as readily split and filmed as the clear 

varieties,98 hence that its preparation is more time 

consuming and involves somewhat greater losses of 

material. No such difference, however, has been demonstrated between clear micas of different colors.

The grading of mica according to the combined 

visual-electrical method proposed by the American 

Society for Testing Materials should result in a substantial conservation of sheet material of condenser 

quality. This saving, which might well amount to as 

much as 60 percent,99 would be particularly important during periods of short supply, especially with 

respect to clear sheet stock. At other times, however, 

competition with foreign sources of supply, with 

ceramic, glass, treated paper, and plastic substitutes,

94 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 

Eng. Trans., vol. 64, p. 7, 1945.

^Horton, F. W., Mica: TJ. S. Bur. Mines Inf. Circ. 6822, p. 54, 1985 (revised 1941).

98 Gwinn, G. R., and Tucker, E. M., Mica, in Minerals Yearbook for 1944, 

p. 1470, U. S. Bur. Mines, 1946.

OT "Watts, F. F., Notes on NRC-587, in Townsend, J. R., Addendum to "War 

Metallurgy Comm. Kept. W-151: War Production Board Kept. W-170, pp. 

1-2, 1944.

88 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 

Eng. Trans., vol. 64, pp. 6-7, 1945; "Watts, F. .F., Notes on NRC-587, in 

Townsend, J. R., Addendum to "War Metallurgy Comm. Rept. "W-151: "War 

Production Board Rept. W-170, p. 8, 1944. 

M Cottttee, K. G., Saving mica by testing: Bell Lab. Rec., vol. 22, p. 518, 1944.
106 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

and even with synthetic mica 10° might well be so great 

that piece-by-piece electrical testing would not be 

economically feasible as a means for bringing visually 

inferior grades of domestic mica into the field of condenser use. As pointed out by Watts,101 though, "the 

use of the instruments by industry need not be confined to selecting the better micas from the lower 

qualities. They are being used to check shipments, 

evaluate mica from untried sources and for selection 

of the highest electrical quality from high-quality 

mica. In the event that the industry adopts A.S.T.M.- 

D748-43T, their use would become mandatory." Moreover, improvements in the design of the Q-meter and 

spark-coil test set are to be expected, and these devices probably will be adapted to continuous semiautomatic or fully automatic operation, either for 

piece-by-piece or for sample-lot testing.

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100 Spence, H. S., Mica as a critical war mineral: Canadian Min. Jour., vol. 

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Hunter, C. E., 1940, Residual alaskite kaolin deposits of North

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folio (no. 124).

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77-81.

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INDEX

Page 

"A" mica — ———————— ___--_— __________—________8-9; pi. 2

Abstract ____________________________________.______1—2

Acknowledgments ———————_—_____—____________:________4

Actinolite inclusions — — — __.—________________________14

Air creep ————————————————————————————————___15; pi. 5 

Air stain ————_.———._———_________________12, 103, 104; pi. 5 

Albite inclusions ——————————____—.—________________,._._14 

Allanite inclusions ————————___—___________________--_14

American Society for Testing Materials, size-grading chart of _____18, 19 

test methods for electrical qualities adopted by—____—________22-24

visual quality classification of ———_——__-_—_—______18, 21-22

visual-electrical method of qualification developed by—________24-26

Apatite inclusions ———————_________________________14, 44, 103

B

Bell Telephone Laboratories, Q-meter developed by————___28-24 ; pi. 9

series of investigations at-___—__—__—_—__________24-25

spark-coil test set developed by—_—_—_—__—_—_________22-23

Beryl inclusions ——————————————_——————_————_—________14

Biotite inclusions ———_____-_________14-15, 43-44, 103-104 ; pi. 8

Block mica, defined ———————_________________—________17, 18

Book mica ————————————————_———————_———__________17

See also Block mica. 

Brittle mica ________________________________________5, 7

Broad-A mica. See Flat-A mica.

Buckling _____.______________________________9-10 ; pi. 3

Bursts —————_———————_—____—_—_—________________12, 44

Chub-A mica ———————————_——___— —— —————— ________9; pi. 3 

Cigarette burns ——————_______________________________12, 44

Cigarette mica ————————_—_———————_—_.._—_—__________16 

Circle mica, defined ———————_———————_—_——————__________18 

Clay stain __—________________________15, 44, 103, 104 ; pis. 2, 8

Cleavage —————————————_—______-_—._—————_____6-7; pi. 1

Cleavage stepping ———————__——___________—______10; pi. 3

Colonial Mica Corporation————————__———_————_—_4, 18, 21, 105

tests on material loaned by—————————__————_—_____34, 97-103

Color—————_——————————_—_________11-12, 42-43, 103, 104; pi. 7

Comparison, Southeastern with India mica—————————_____________105

Composition —————————————————————————————————————____.__5

Conclusions ————————————_————————————————_______.__103-106

Creping ——————————————————————————————————_—_———_9

Cross grains. See Reeves.

Crystallography ———————————_——————————————________5-6 ; pi. 1

Cupping ——————————————————____———____————_————___9; pi. 3

Deposits, distribution of ————______________________—______.-.28-29

location of —————————_--_—-_„-_-—_-____———____________29

occurrence of ————————————————————————————_______._30-33

summary data on ————————————————————————_—_——————30 

Dielectric constant (K) ____________________________16, 22, 25 

Dielectric strength _———_——__________—__________16, 22, 25 

Districts, enumerated ————__—___________——________28-29

location of _————————_________________________________29

summary data on —————_—_———_—_———_—_————___———_30 

Double-A mica ———_—————______.__-__—_____________8; pi. 2

E

Elasticity _____—_______________________________7-8 

Electric mica _•———--————____________________________21-22 

Electrical conductivity ————_—_—_______—____——_—_—_16, 22, 25

Feather mica. See Herringbone mica.

Field and laboratory work———_—___—_———.—_———_—__—_—_—3-4

Fishback (fishbone) mica. See Herringbone mica.

Fishtail mica. See "A" mica.

Flat-A mica———.--________________—______8-9; pis. 2, 4

Flexibility _______________________________________7-8, 16

Fluorite inclusions _————________-__—_-__———_—_—________14

Page

Garnet inclusions——————————————————————————14, 44, 103; pi. 8 

General properties ———————————————————————————————4-6; pi. 1 

Geology _______——————————————————————————————————30-33

Goethite inclusions —————————————————————————————————————14 

Grading and classification————————————————————————17-25; pi. 9 

Gritty mica —————————————————————————————————————————13 

Gummy mica _____——————-—————————————————————————7

Hair cracks (hair lines) ————_———————————————7, 9, 23, 44-45, 104 

Hair mica ————————————————————————————————————10; pi. 4 

Hardness —————————————————————————————————————————7-8 

Hematite inclusions and stains—————————13-14, 43-44, 103, 104; pis. 5-8 

Herringbone mica ———————————————————————————————9; pi. 2 

History of investigations————————————————————————————————3-4 

of mining operations—————_————————————————————————29-30 

Horsetail mica. See Herringbone mica. 

Housetop mica. See "A" mica.

Impurities, primary _———————_——————12-15, 43-44, 103-104 ; pis. 5-8 

secondary ______________________15-16, 44, 103-104 ; pis. 5, 8 

Inclusions ______________________13-15, 43-44, 103-104; pis. 5-8 

Indian standard sizes, chart of————————————————————————————20 

Intergrowths, mineral ——————.——————13-15, 43-44, 103-104 ; pis. 5-8 

Investigations, history of ——————————————————————————————3-4 

Iron stain _____________________-__——__————15, 44, 103, 104

Kyanite inclusions ——-——_--_————_—————————-———-14

Location of deposits——————————————————————————————————28-29 

Locky mica ——————————————————————————————————————————7

M

Magnetite inclusions _———_.„-__.—————————43-44, 103, 104; pis. 5, 8 

Manganese stain ————————————————————————————————15, 44, 103 

Marketing ____————————————_———————————————————————27-28 

Mine-run (run-of-mine) mica. See Book mica. 

Mineral stain __________——_____————————13-15, 43-44, 103 ; pis. 5-8

Mineralogy ——————————————————————————————————————4-5 

Mines and prospects from which test specimens were collected——————34-42 

Mining operations, history of———————————————————————————29-30 

Mottling_____________________________12-13. 44, 103-104; pi. 5

N 

Nailed (nail-locked) mica————_——————————————————— ———— —7

O 

Occurrence of deposits—————————— —————————————————— ———30-33

Pattern mica. See Sheet mica.

Pegmatites, mica-bearing ————————————————————————————31-33 

Percussion figures ———————————————————————————6, 7, 10; pi. 1 

Petrographic examination of test specimens—————————————————43-45 

results correlated with those of electrical tests——————————————47-95 

Pinholes__________________________———-———-23, 44-45, 103 

Plate mica. See Sheet mica. 

Power factor (PF) _————————————_16, 23-24, 25, 26, 45, 104-105

Preparation _——————————————————————————————————26-27 

Prices __________________________———————————————27-28, 30

Production ..._—_—————__—————-—————————————————29-30

Punch mica, defined————————————————————————————————————18 

Pyrite inclusions____———————————————————————14, 44, 103; pi. 8

109
110

INDEX

Page

Q-meter___________________.-_--__.________23-24, 45, 97; pi. 9 

Q value..—________________„___.___..___________16, 104-105

classification based on—————————— ———————————————____—_26

method of testing for____-______-———————____23-24, 25-26, 45; pi. 9

Quality classification by electrical methods— ——— _ — .—————22-24; pi. 9 

by visual methods—.—_____———————_———________18, 20-22 

by visual-electrical method—_ ——————— — ———— ——— ________24-26

Quartz inclusions — ——————————— ————— -——————————_14, 44, 103

Reeves.-.—________________________________________8-9 ; pi. 2

References cited_____________—————_————__._________106-107 

Ribbing—_—____—_—__________———_——————_—______9 ; pi. 3

Ridging ————————————_— ————————— — —— _ ____———.______9

Rippling _____________________————————.——___—_—______9, 10

Ruling—_—_—_—__—_„____———————__.___________10 ; pi. 4 

Rutile inclusions————_————___——————————— —————— __________14

Sandy (sand-pitted) mica—_—____———————————————___________13

Scrap mica, defined—_—_—_—_____—_————_——____________17

Sheet mica, defined__________________________________17-18 

Silver spots.—_—_—_—_____—_______——————______—______12

Size.—_—.___..______________————______________18, 19, 20 

Spark-coil test set____________-_____—_____________22-23, 45 

Spearhead mica. See "A" mica.

Spot-locked (spot-welded) mica————— ———————————————_____13

Staining, primary..__________—————12-15; 43-44, 103-104; pis. 5-8

secondary.._______.____—————.—15-16, 44, 103-104; pis. 5, 8

Stony mica —————_—_—_—_—_—————————————————_________13

Strategic mica ______________-___—_______________17, 21-22

Structural imperfections._______————_———___-_-____8-10 ; pis. 2-4

Substitution of other materials______-___________2-3, 17, 105-106

Tacky mica —...—————_—_—_——————————————_———_.____—7 

Tanglefoot (tangled, tanglesheet) mica—————————————_—_.___._„7

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Testing program ——————————————————————————————————33-103

color tests, discussed———————————————————————————42-43, 103

results correlated with those of electrical tests————————————47-95

electrical tests, apparatus————————————————————————————45

methods ——————————————————————————————————————45

results, discussed ______————_————————45-46, 97, 103, 104-106

tabulated ______________-____—_—__————47-103

mines and prospects represented——————————————————————34-42

petrographie examination, discussed————————————————————43-45

results correlated with those of electrical tests———————————47-95

specimens, preparation of———_—————————————————————————34

source of —————————————————————————————————————34

Thickness ————.———————.————————————————————————————18

Tied mica ———.————————————————————————.——————.——————7

Tourmaline inclusions—————————————————————————14, 44, 103; pi. 8

U

Upgrading of visually inferior grades—————————————————————105-106 

Uses __——_—_—————————————————————.——————.——2,16-17

V-ridge mica. See "A" mica.

Value __—-_——————_—————————————————-.—————-——————30 

properties affecting_________________——————————6-16; pis. 2-8

Vegetable stain, inorganic__—_———————12-13, 44, 103-104; pis. 5-7 

organic__________________——_____————-15-16, 44, 103-104

Visually graded mica, tests on————————————————————————34, 97-103 

upgrading of ——————————————————————————————————105-106

W

Warping__________—__—_————————————————————9-10 ; pi. 3 

Washer (washer-punch) mica, defined———————————————————————18 

Wavy mica —————— ——————————————————————————————— ———9 

Wedging__———_—_—_—_———————————————————————————9 ; pi. 3

Zircon inclusions——————————————————————————————————-44, 103 

Zoisite inclusions___________-______—————————14, 44, 103 ; pi. 8 

Zoning of pegmatites————————————————————————————————32-33

U. S. GOVERNMENT PRINTING OFFICE: 1950—852911