UNITED STATES MINERAL RESOURCES 



FELDSPAR 



By Frank G. Lesure 



CONTENTS 



Page 



Abstract of conclusions 217 



Introduction 217 



Use and economic importance 217 



Exploitation 219 



Geologic occurrence 220 



Resources 221 



Identified and hypothetical resources 221 



Speculative resources 221 



Problems for research 221 



Selected references 222 



FIGURE 



25. 



Graph showing crude feldspar sold or used by 

 producers in the United States, 1900-70, com- 

 pared with imports of nepheline syenite, 

 1937-70 



217 



ABSTRACT OF CONCLUSIONS 



Feldspar is the general name for an important group of 

 aluminum silicate minerals used primarily in the glass and 

 ceramic industries. Although consumption in the United 

 States has more than doubled since 1940, domestic reserves 

 and resources are considered adequate for anticipated de- 

 mand. Transportation costs are a major factor in the de- 

 livered price of feldspar and contribute to the use of sub- 

 stitutes where available. Research problems involve iden- 

 tifying potential sources nearer to consumers. Demand for 

 feldspar in the United States by the year 2000 is expected 

 to be three to five times present production. This demand 

 is in large part based on the increasing use of container 

 glass and will depend partly on the recycling of bottle glass 

 and the continued increase in the use of throwaway bottles. 



INTRODUCTION 



Feldspar is the general name for a group of an- 

 hydrous aluminum silicate minerals that contain 

 varying amounts of potassium, sodium, and calcium. 

 The feldspars are important rock-forming minerals 



and constitute nearly 60 percent of many igneous 

 rocks. The principal potassium feldspars, orthoclase 

 and microcline, have the same chemical composition 

 (KAlSiaOs), but diiTerent crystal forms. The sodium- 

 calcium feldspars, called plagioclase, form a com- 

 plete series of minerals ranging from pure 

 NaAlSisOs (albite) to pure CaALSiaOs (anorthite). 

 The plagioclase minerals between albite and anorth- 

 ite in order of decreasing sodium content and in- 

 creasing calcium content are oligoclase, andesine, 

 labradorite, and bytownite. Most natural orthoclase 

 and microcline contain 10-25 percent NaAlSiaOs, 

 and most plagioclase contains 5-15 percent KAlSi.sOs. 

 Intergrovirths of orthoclase or microcline with albite 

 are called perthite and they are a common constitu- 

 ent of pegmatites. 



Producers and consumers know feldspar as a bene- 

 liciated mixture composed of feldspar minerals with 

 ideally less than 5 percent but sometimes as much as 

 20 percent of quartz, and minor quantities of other 

 minerals. Perthitic microcline (potash spar), albite 

 (soda spar), and oligoclase (soda-lime spar) are the 

 principal types of feldspar mined in the United 

 States. 



USE AND ECONOMIC IMPORTANCE 



From 1955 to 1970, about 55 percent of the feld- 

 spar sold in the United States was used in glass, 31 

 percent in pottery, 4 percent in enamel, and 10 per- 

 cent in soaps, abrasives, mineral fillers, welding-rod 

 coatings, and other miscellaneous uses. Feldspar is 

 a source of alumina, soda, and potash for glass. The 

 alumina improves workability, retards devitrifica- 

 tion, and increases the chemical stability of the fin- 

 ished product. Iron oxide, a coloring agent, is the 

 most common undesirable impurity in glass-grade 

 feldspars. The maximum acceptable iron-oxide con- 

 tent of feldspar for flint glass is 0.1 percent, but as 

 much as 0.5 percent is acceptable for green or amber 

 glass. Minerals too refractory to melt at tempera- 



U.S. GEOL. SURVEY PROF. PAPER 820 



217 



