THE MINING AND QUARRY INDUSTRY I9II 3t 



issues of this report. The mineral is fairly widespread as a con- 

 stituent of the metamorphosed igneous and sedimentary rocks, but 

 only in a few places is it found in sufficient abundance and with 

 the requisite characters to be worked for commercial abrasive 

 purposes. 



The value of abrasive garnet depends, of course, primarily upon 

 its hardness. This is a variable character and on the usual min- 

 eral scale garnet is classed as having a hardness of from 6.5 to 7.5. 

 The limits as given are only approximate, as it is difficult and even 

 impossible to estimate hardness with precision. Chemical compo- 

 sition is undoubtedly a factor in determining the hardness of the 

 common kinds of garnet found in the metamorphosed rocks, like 

 gneisses, schists and crystalline limestones. The iron-alumina 

 variety (almandite) is generally harder than the lime-alumina 

 (grossularite), or the lime-iron variety (andradite). Well-crys- 

 tallized garnet is tougher and probably also harder than the granu- 

 lar or massive garnet of similar chemical composition. The prop- 

 erty of toughness or tenacity is very important in an abrasive 

 which has to withstand considerable pressure as when used as pol- 

 ishing machines. Another factor which has a bearing upon the 

 value of abrasive garnet is the size of the product which can be 

 secured in the ordinary practice of mining and separation. If the 

 crystals are small or have been badly shattered by compression 

 after crystalHzation the product may be too fine to yield the neces- 

 sary assortment of commercial sizes. It is an advantage, however, 

 that the garnet should possess an imperfect cleavage or parting, 

 so that on crushing the grains show one or more smooth surfaces. 

 These surfaces permit firm attachment to the cloth or paper and 

 also provide a sharp cutting edge. Color is no criterion of quality 

 in ordinary garnet, but abrasive users seem to prefer the darker 

 shades of red which approach the distinctive garnet color. 



The local industry has very little competition from other mines 

 in this country. Mines have been worked at dififerent times in 

 New Hampshire, Massachusetts, Pennsylvania and North Carolina 

 but have not proved permanent factors in the trade. The impor- 

 tation of Spanish garnet, first noted in 1907, has assumed some 

 importance as a substitute for the finer sizes of the Adirondack 

 mineral. This garnet is said to be obtained by concentration of 

 alluvial sands and can be produced cheaper than the domestic 

 garnet, but comes only in the finer sizes. It pays no import duty. 

 The importations in 191 1 were 693 short tons, with an invoice 



