UNITED STATES MINERAL RESOURCES 



MICA 



By Frank G. Lesure 



CONTENTS 



Page 



Abstract of conclusions 415 



Introduction 415 



Types and uses of mica 415 



Sheet mica 416 



Scrap mica 416 



Exploitation 416 



Mining and preparation 417 



Prices 418 



Geologic features of mica deposits 418 



Pegmatites 418 



Phlogopite deposits 419 



Scrap- and flake-mica deposits 419 



Resources 420 



Identified and hypothetical resources 420 



Speculative resources 421 



Prospecting techniques 421 



Problems for research 422 



Selected references 422 



FIGURES 



50. 



51. 



Graph showing production of sheet mica in the 

 United States compared with imports of 

 sheet mica, 1900-70 



Graph showing production of scrap mica in the 

 United States, 1900-70, compared with that 

 in the rest of the world, 1925-70 



TABLE 



417 



418 



84. 



Sheet mica on hand in all U. S. Government in- 

 ventories, as reported by General Services 

 Administration, March 31, 1972 



ABSTRACT OF CONCLUSIONS 



Sheet mica has been an important strategic mineral used 

 in electrical appliances, vacuum tubes, capacitors, and vari- 

 ous other electrical and nonelectrical products. The United 



States has not been self-sufficient in sheet-mica production 

 since before the turn of the 20th century and has im- 

 ported supplies to meet half to all of its needs, mostly from 

 India, Brazil, and Malagasy Republic. Sheet-mica uses are 

 declining with advancing technology and introduction of 

 substitutes. Although the United States has undiscovered 

 and paramarginal resources of sheet mica, the high cost of 

 the necessary hand labor involved in mining and prepara- 

 tion of sheet mica deters exploration, development, or fur- 

 ther mining. 



Average United States scrap-mica production is increas- 

 ing, but reserves and resources are adequate for foreseeable 

 demand. The United States is self sufficient in flake-mica 

 production and is a net exporter of ground mica. 



INTRODUCTION 



Mica is the general group name for several com- 

 plex hydrous aluminum silicate minerals. The prin- 

 cipal mica minerals are muscovite (white mica), 

 biotite (black mica), and phlogopite (amber mica). 

 All have a perfect basal cleavage and form crystals 

 that can be split into thin sheets having various 

 degrees of transparency, toughness, flexibility, and 

 elasticity. The micas are common minerals in ig- 

 neous, metamorphic, and some sedimentary rocks. 

 Muscovite is the most important commercial mica 

 in the United States. Phlogopite is generally inferior 

 to muscovite except in uses that require resistance 

 to high temperature: muscovite breaks down at 

 temperatures of more than 500 °C, whereas phlogo- 

 pite is relatively stable up to 1,000°C. Biotite is 

 inferior to muscovite and phlogopite and has little 

 commercial value. 



TYPES AND USES OF MICA 



Two types of mica are sold: sheet mica, which 

 must be relatively flat, free from most defects, and 

 large enough to be cut into pieces at least 1 square 

 inch; and scrap mica, which is all mica that does 

 not meet sheet-mica specifications and that is gen- 

 erally ground to a powder. Small sheets of un- 

 trimmed mica of poorer quality that can be punched 

 or trimmed into disks at least 1 inch in diameter 



U.S. GEOL. SURVEY PROF. PAPER 820 



415 



