578 



UNITED STATES MINERAL RESOURCES 



grains. The impurities in some sands are themselves 

 valuable v^^hen isolated, and their sale can help pay 

 for the cost of beneficiation. Among these are clay 

 and zircon. 



Silica sand and sandstone are common rocks of 

 v^orldwide distribution. Inevitably, any large coun- 

 try that has a varied and representative geologic 

 terrane such as the United States has a large share 

 of these, and the chance that other nations will 

 have deposits distinctly superior to those in the 

 United States is negligibly small. Because of the 

 large resources of silica-rich sand and the opportuni- 

 ties for substitution and for beneficiation of low- 

 grade deposits, the United States should be self- 

 sufficient in silica sand for an indefinite period of 

 time, notwithstanding the fact that a little silica 

 sand is imported occasionally from foreign coun- 

 tries. Importation is the result of very low sea 

 freight rates as compared to the overland freight 

 rates from domestic sources, and it does not reflect 

 adversely on either the quantity or quality of do- 

 mestic sands. 



EXPLOITATION 



The annual production of silica sand increased 

 rapidly between the depression year 1932 and 1944, 

 fluctuated around 17 million tons until 1961, and 

 then shot upward. In 1970, the last year for which 

 complete figures are available, production was about 

 30 million tons, of which one-third was glass sand 

 and one-third was molding sand (fig. 65). The cur- 

 rent rate of increase is about 1 million tons per year. 



GEOLOGIC ENVIRONMENT 



High-silica sands are the products of especially 

 long continued or often-repeated cycles of weather- 

 ing, abrasion, and winnowing. The hardness and 

 chemical inertness of quartz work in its favor to 

 form a concentrated quartz sand deposit after al- 

 most all other minerals have been reduced to powder 

 and wafted away. The most extensive and purest 

 silica sands were formed in shallow seas. Less ex- 

 tensive and generally less pure deposits were formed 

 as dunes along the shorelines of lakes and the sea, 

 as dunes of desert regions, and as beds and flood 

 plains of streams. 



Silica-sand deposits have formed in all geologic 

 periods. Some of the oldest deposits, those of Pre- 

 cambrian age, are very large and very pure but are 

 generally too distant from markets or are too well 

 cemented to compete at present with silica sand 

 from other sources. Many lower Paleozoic deposits 

 are uniquely widespread and pure. For example, the 



Figure 65. — Production of silica sand in the United States, 

 1930-70. Source of data: U.S. Bureau of Mines. 



St. Peter Sandstone and related deposits which ex- 

 tend from Wisconsin and Michigan to Oklahoma, an 

 area of more than 300,000 square miles, are com- 

 posed principally of nearly pure quartz. The Eureka 

 Quartzite and equivalent deposits which extend from 

 Idaho to southern California, an area of more than 

 100,000 square miles, are likewise composed of 

 nearly pure quartz. Analyses of the Eureka Quartz- 

 ite indicate that the silica content generally ranges 

 from 95 to 99 percent. The principal commercial 

 deposits of the United States are in locally dece- 

 mented parts of lower Paleozoic marine sandstones. 

 The process of decementation by which hard sand- 

 stone is converted to weakly coherent sand is not 

 well understood, but it is probably the result of local 

 near-surface weathering. 



Although many silica sands deposited since early 

 Paleozoic time are widespread and some are very 

 pure, none of them rival those of the early Paleo- 

 zoic aee in extent and purity. Upper Paleozoic sand- 



