30 



UNITED STATES MINERAL RESOURCES 



commonly taper from the middle toward each end 

 and exhibit a pronounced transverse parting, and 

 (2) large polycrystalline blocks. Corundum crystals 

 normally are translucent or opaque white, gray, 

 blue, red, brown, black, or green, but some are 

 transparent red, pink, blue, or bluish-green, or ex- 

 hibit asterism or other pleasing chatoyancy, and 

 these are valued as gemstones known to the jewelry 

 trade as ruby and sapphire. 



Abrasive corundum is used nearly exclusively as 

 crushed, carefully sized grit and powder for finish- 

 ing optical parts and for lapping and polishing 

 metals. The transverse parting characteristic of 

 much corundum presents to the work a sharp, chisel- 

 like edge which scrapes rather than scratches the 

 work and produces a smooth surface. When the abra- 

 sive wears, a flake spalls off so as to expose a new 

 sharp chisel edge. Probably it is this characteristic 

 that encourages a continuing small demand for 

 corundum despite the fact that garnet, diamond, 

 artificial aluminum oxide, and other abrasives can 

 successfully substitute for all the uses to which 

 corundum is now put. 



About 2,000 tons of corundum is imported each 

 year, and about one-tenth of this is reexported as 

 prepared grinding powder. For many years the 

 source of supply was Rhodesia, but political consid- 

 erations in mid-1968 cut off this supply, encourag- 

 ing many users of corundum to convert immedi- 

 ately — and probably permanently — to artificial 

 abrasives. South Africa has become the new source. 

 It is perhaps a measure of the relative unimportance 

 of corundum in our economy that the long-term 

 Rhodesian source was cut off only a few months 

 before the U.S. Government removed corundum 

 from the list of strategic materials and offered for 

 sale the last 2,000 tons of corundum in the strategic 

 stockpile. 



The United States has no reserves of corundum, 

 and the resources are poorly known. Not much more 

 than 10,000 tons was produced between 1870 and 

 1906; none has been produced since that time ex- 

 cept for small test lots during the World Wars. 

 Probably the one area in the United States con- 

 taining the largest amount of potential corundum 

 ore is that area embracing Clay, Macon, and Jack- 

 son Counties, N.C., and Towns and Rabun Counties, 

 Ga., although corundum also is known in all of the 

 Appalachian States, from Maine to Alabama, and 

 in a few places in California, Montana, and Idaho. 



Deposits of corundum in the contact zone between 

 peridotitic intrusive rocks and enclosing highly 

 aluminous metamorphic rocks, especially those in 

 Georgia and North Carolina, were the principal 



source of corundum in the United States. The gene- 

 sis of these deposits has been debated by geologists 

 for many years. Corundum also is a constituent of 

 placer deposits and of more than 20 kinds of igne- 

 ous rocks, including granite pegmatite, nepheline 

 syenite, anorthosite, and gabbro. In some gneisses 

 and schists, corundum makes up from 5 to 60 per- 

 cent of the material in zones that range from a few 

 inches to several tens of feet in width and that may 

 extend several miles, although no such concentra- 

 tion currently represents an economic deposit. Pan- 

 ning would seem to be the method of choice for 

 prospecting, and although perhaps not applicable 

 to deposits in the United States, prospectors should 

 be aware that peridotites rich in chromite generally 

 contain no corundum, whereas those without chro- 

 mite may. 



EMERY 



Emery is a gray to black granular material con- 

 sisting of corundum and magnetite with admixed 

 spinel, hematite, garnet, and other minerals. The 

 proportion of some constituents ranges widely; 

 some specimens are nearly pure corundum, others 

 nearly pure magnetite, and still others nearly pure 

 spinel. 



Synthetic abrasives have reduced the formerly 

 multiple uses of emery to two. About 3 percent cur- 

 rently is made into coated abrasive sheets; the rest 

 is used in nonskid pavements and stair treads. The 

 true emery wheel has now become an antique curi- 

 osity. Only custom and economics encourage the 

 continued use of emery because manufactured alu- 

 minum oxide grain probably would be superior for 

 the same applications. Even granular magnetite, 

 hematite, spinel, or garnet may substitute ade- 

 quately, as indeed they probably have, judging 

 from reported variations in the mineral content of 

 emery ore used in the past. 



Emery deposits are known in New York, Massa- 

 chusetts, Virginia, North Carolina, and Pennsyl- 

 vania. Those in New York, a group of deposits 

 about 31/2 ini southeast of Peekskill and concen- 

 trated within an area of about 1 square mile, prob- 

 ably represent the largest single source in the United 

 States. The U.S. Bureau of Mines estimated reserves 

 of these deposits in 1955 at 60,000 tons, but since 

 then the entire U.S. production of about 150,000 

 tons has been from these deposits and production 

 continues from a single mine at an annual rate of 

 5-10 thousand tons. The extent of surface openings 

 indicates that, with full development and the accom- 

 modation of several grades of ore, these deposits 

 may well be capable of producing more than 10,000 



