ABRASIVES 



31 



tons per foot of depth to depths of many hundreds 

 of feet. Overseas supplies have come from Greece 

 and Turkey at an annual rate of from a few hundred 

 tons to about 2,000 tons. Reserves in these countries 

 aggregate 5-10 million tons and the resources prob- 

 ably amount to many millions of tons. Large depos- 

 its of emery are known in the Soviet Union, but 

 this emery does not enter world trade. 



Emery deposits in Greece and Turkey occur as 

 pods and lenses in metamorphosed limestone; those 

 in the United States are in the margins of mafic 

 intrusive rocks which are in contact with alumi- 

 nous country rock. All probably result from contact- 

 metamorphic processes, although geologic contro- 

 versy very similar to that involving corundum 

 deposits also has accompanied the study of deposits 

 of emery. 



GARNET 



Garnet is the term applied to about 15 varieties 

 of a complex silicate mineral which crystallizes in 

 the cubic system, generally as rhombic dodecahedra. 

 This form is nearly spherical and gave rise to the 

 name garnet, derived from the Latin "granatus," 

 meaning grainlike or seedlike. Common commer- 

 cial garnet is almost entirely the variety almandite, 

 a dark-red to dark-brownish-red iron aluminum 

 silicate. 



Garnet currently has three major uses as an abra- 

 sive. (1) About 45 percent is used as a sandblast 

 medium to clean an impart a textured surface to 

 both ferrous and nonferrous metals in the shipbuild- 

 ing, aircraft, and automotive industries, and to a 

 much lesser extent for such other manufactures as 

 home appliances, stone, and artworks. The tough 

 round alluvial garnet crystals impart a pleasing 

 work-hardened matte surface to these materials. 

 (2) About 35 percent is used as crushed grit and 

 powder to grind and polish lenses and other optical 

 parts, to surface semiconductor materials, and to 

 accomplish the intermediate grinding in the finish- 

 ing of plate glass. The recent advent of the Pilking- 

 ton, or "float," process of plate-glass manufacture, 

 whereby a ribbon of hot glass is drawn across a bed 

 of molten tin resulting in a finished surface of plate- 

 glass quality, will probably reduce substantially the 

 future demand for crushed and sized garnet grit for 

 glass-grinding purposes, currently about 4,000 tons 

 per year. (3) About 20 percent is crushed and made 

 into abrasive sheets. In this form it is used to dress 

 rubber, plastics, leather, soft metals, and wood. 



The use of garnet as an abrasive is almost en- 

 tirely American ; more than 95 percent of the garnet 

 produced in the world is produced in the United 



States, and more than 80 percent is consumed here. 

 Yet there are only two producing mines of conse- 

 quence, and total annual production is only about 

 20,000 tons. Spain, Portugal, India, and Canada 

 have produced small quantities very occasionally. 



The garnet from New York State has an unusual 

 structure, a closely spaced lamellar parting ordi- 

 narily not developed in garnet, that results in 

 chisel-edged granules when the garnet is crushed, 

 which scrape the work surface rather than scratch 

 it. This property makes the garnet similar to corun- 

 dum, for which it commonly substitutes. This gar- 

 net is unmatched as a tool for surfacing furniture 

 woods or soft metals, or for skiving leather and 

 rubber, and its use for grinding glass results in 

 fewer scratches and a lower rejection rate than 

 does use of other abrasives. Yet, garnet has no ap- 

 plication in which it cannot be replaced by other 

 appropriately prepared and adapted abrasives, rang- 

 ing from sand, glass beads, and steel grit for sand- 

 blasting, to diamond powder for finishing optical 

 parts. A continuing, even moderately increasing, 

 use of garnet is probable, subject nearly entirely 

 to changes in economics and technology. 



Garnet, as a source of commercial abrasives, oc- 

 curs almost exclusively in metamorphic rocks and 

 in placer deposits derived from them. Garnet-rich 

 zones in gneisses and schists may contain 4^60 per- 

 cent garnet, may be several hundreds of feet wide, 

 and may extend for many miles. Many placers con- 

 tain as much as 20 percent garnet, and very few 

 beach, dune, glacial, and stream deposits through- 

 out the world do not contain from a fraction of a 

 percent to a few percent of garnet. The garnet easily 

 could be saved by any of several simple mechanical 

 or electrical methods of extraction, but it almost 

 never is; large quantities of garnet are lost every 

 day in the production of sand and gravel from these 

 deposits for other purposes. Garnet deposits have 

 been reported in more than half the States — all the 

 States west of the Great Plains — and the Appala- 

 chian States from Maine to Georgia. The alluvial 

 sandblast garnet recovered from Idaho comes from 

 a single placer deposit estimated to contain 500,000 

 tons of recoverable garnet. There are unmined 

 placers in the Western States, and perhaps some 

 undiscovered placers. The garnet with the lamellar 

 parting currently being mined from a coarsely 

 crystalline gneiss at North Creek, N.Y., is in a de- 

 posit which if fully developed could produce more 

 than 7,000 tons of garnet per foot of depth to a 

 depth perhaps in excess of 1,000 feet. Several other 

 deposits of garnet with lamellar partings are re- 

 ported elsewhere in northern New York, in New 



