32 



UNITED STATES MINERAL RESOURCES 



Hampshire, and in North CaroUna. Those in North 

 CaroHna apparently are very large. Nonlamellar 

 garnet also is common in metamorphic rocks and 

 in placer deposits in the Eastern States, particularly 

 in the Adirondak Mountains of New York State. 

 Many of these deposits have been little prospected, 

 and the resources are not well known, but it is 

 certain that each of a reasonably large number of 

 deposits contains from half a million to many mil- 

 lions of tons of garnet. Overseas resources of garnet 

 are unknown, inasmuch as garnet is little used out- 

 side the United States. 



SILICA SAND 



Silica sand, both crushed and uncrushed, is used 

 in large quantities ; one-half million tons is used in 

 this country annually for grinding stone and glass, 

 and more than a million tons for sandblast purposes. 

 It is, or could be, produced in each of the 50 States 

 by nearly any of the more than 5,000 producers of 

 sand and gravel currently operating. Resources are 

 large. (See chapter "Silica.") 



TRIPOLI 



Tripoli is iine-grained silica which remains as a 

 bedded residuum of the weathering of siliceous lime- 

 stone. It is roughly similar in characteristics and 

 uses to rottenstone, mined in Pennsylvania from 

 weathered siliceous shale, and to diatomite, bedded 

 deposits of the siliceous remains of marine micro- 

 organisms. A combined total of about 40,000 tons 

 of these materials is consumed each year as abra- 

 sives, mostly in scouring soaps and powders and in 

 metal- and lacquer-polishing pastes. Large deposits 

 of tripoli are known in Illinois, Missouri, Oklahoma, 

 Tennessee, Georgia, Alabama, Arkansas, California, 

 Texas, and Nevada. Deposits of diatomite are known 

 in several Western States. (See chapter "Diato- 

 mite.") 



THE ARTIFICIAL ABRASIVES 



After the commercial introduction of artificial 

 abrasives about 1900, the industry became almost 

 immediately dominated by these products. The domi- 

 nance is increasing even today. Aluminum oxide in 

 particular found its principal application, among a 

 host of other applications, in the rough grinding of 

 ferrous metals, an operation formerly accomplished 

 slowly and expensively by metalworking machinery. 

 Nearly all subsequent research on artificial abra- 

 sives has been directed to the discovery of an abra- 

 sive even more efficient than aluminum oxide for 

 grinding ferrous metals. None has been found, al- 

 though large families of carbides, borides, nitrides, 

 silicides, oxides, and even a few artificial silicates 



have been tried. A few abrasives useful for other 

 purposes have been discovered, and research con- 

 tinues. 



Artificial diamond, as discussed above, was first 

 produced in 1955. It is manufactured from a mix- 

 ture of graphite and a catalyst of iron, manganese, 

 chromium, cobalt, nickel, or other metal under con- 

 ditions of high temperature and pressure, typically 

 3,000°C., 70,000 bars of pressure, and several hours 

 of time. Although available for less than 20 years, 

 artificial diamond already infringes seriously on the 

 market for natural industrial diamond, all of which 

 must be imported. This trend is expected to accel- 

 erate in the years ahead. Artificial diamond itself 

 may one day be challenged as an abrasive because 

 artificial cubic boron nitride, invented in 1957 and 

 manufactured in much the same way as diamond, 

 has been demonstrated in grinding tests to abrade 

 diamond as rapidly as does diamond. It also has the 

 advantage of remaining stable at high temperatures, 

 whereas diamond tends to revert to graphite at the 

 temperatures often reached at the grinding surface, 

 resulting in rapid ablation of the diamond in the 

 abrasive wheel. Diamond also is difficult to incor- 

 porate in ceramic-bonded wheels because of high- 

 temperature effects. 



Artificial aluminum oxide, chemically but ^ot 

 physically identical with corundum, is manufac- 

 tured by fusing or sintering bauxite or intermediate 

 products in an electric furnace. Annual production 

 is more than 200,000 tons and requires about 

 320,000 tons of bauxite, or about 2 percent of the 

 annual consumption of bauxite in the United States. 

 Ten to fifteen percent goes into nonabrasive appli- 

 cations, mostly refractory products. New tech- 

 niques over the years have lowered the required 

 alumina content of the furnace charge, previously 

 60 percent, to less than 50 percent, and further 

 technological changes are expected soon to allow 

 alumina contents of only 35 percent, making avail- 

 able vast resources of syenitic and other aluminous 

 rock for this purpose. 



Commercial production of silicon carbide is ac- 

 complished by the reaction of a mixture of silica 

 sand and coke, preferably petroleum coke, in an 

 electric furnace. The principal abrasive use of sili- 

 con carbide is in the wire sawing of stone, but the 

 abrasive successfully substitutes in many operations 

 to which hard natural abrasives also are put, in- 

 cluding the skiving of rubber and leather, and it is 

 the leading abrasive in the dressing of nonferrous 

 metals. Nearly 200,000 tons of silicon carbide is 

 produced each year in the United States, about half 

 of which is used as an abrasive. Refractory appli- 



