80 NEW YORK STATE MUSEUM 



Use. The utility of garnet as an abrasive depends upon the 

 combination of several qualities, particularly hardness, toughness, 

 cleavage or fracture, and the purity of the mineral. The varieties 

 of garnet recognized by the mineralogist, of which there are a nimiber 

 that bear specific names, differ among themselves in respect to these 

 qualities; this is true also to an important degree for the same kind 

 of garnet in samples from separate occurrences. Almandite is the 

 variety mostly used for abrasive work and ordinarily has a hardness 

 on the mineralogists scale of 7 to 7. 5. It is thus harder than quartz, 

 but not so hard as corundum. From tests of the Adirondack alman- 

 dite in the crystal form it would appear that the hardness may even 

 exceed the degree indicated, as it has been found in certain instances 

 to range from 7.5 to 8. Toughness has a direct bearing upon the 

 service that may be expected from an abrasive, especially when 

 used under pressure. A material may be hard but too brittle or 

 too crumbly to stand up under heavy duty. The crystallized alman- 

 dite has great strength and lasts well when used on abrasive 

 machines. 



Much of the Adirondack garnet has an imperfect cleavage, evident 

 on crushing the crystal by the appearance of particles with one or 

 more plane surfaces, but rarely with complete boundaries of that 

 kind. Such parting, as it is called to distinguish it from cleavage 

 proper, is an advantage if not so well developed that the mineral 

 breaks down too easily; the planes supply sharp chisel-like cutting 

 edges that make the garnet very effective on moderately soft 

 materials. 



The impurities that are commonly associated with garnet are 

 mica, chlorite and hornblende. They reduce the strength of the 

 garnet and also its cutting powers. Color is not a criterion of 

 quality, but abrasive manufacturers express a preference for the 

 true garnet red which shows up better on abrasive papers than the 

 paler sorts. It is important that the crystals as they occur in the 

 rock be large enough to stand the effects of the crushing and sepa- 

 rating operations and still yield the necessary assortment of sizes. 

 With small crystals the result of milling is to produce an excess of 

 the fine sizes. A garnet rock with very small crystals would hardly 

 afford a basis for commercial production, no matter how rich it 

 might run in the mineral. 



