THE MINING AND QUARRY INDUSTRY I9I4 31 



employment of garnet as an abrasive is peculiarly an American 

 development, and although firmly established in certain manufactur- 

 ing lines here, as shown by the steady demand in spite of the very 

 active competition of other abrasives natural and artificial, appar- 

 ently it has made little progress in foreign countries. 



The Adirondack deposits have been repeatedly described in the 

 reports of the New York State Museum and elsewhere so that 

 there is no need to discuss their features at length in this place. 

 Briefly, they consist of various gneisses and massive rocks which 

 carry disseminated crystals and aggregates of garnet of the variety 

 known as almandite. In some places, as instanced by the Rogers 

 mine on Gore mountain, the garnet is found in crystals that range 

 up to several feet across and weigh a ton or more, but elsewhere 

 the mineral is mostly in individuals that measure an inch or less in 

 diameter. The larger individuals seldom show any outward 

 crystal development but have rounded or lenticular outlines and 

 are traversed by innumerable fractures or parting planes. When 

 the crystals are broken into, the particles of garnet are picked out 

 by hand with the aid of cobbing tools, such garnet being known as 

 shell and pocket garnet. This method of hand work, with the aid 

 of blasting to break up the rock, is practised on Gore mountain 

 where the conditions are exceptionally favorable for its use on 

 account of the relative richness of the rock and the large size of 

 the crystals. In the other deposits the rock is quarried with no 

 attempt at sorting and is then crushed in mills to a size which 

 releases the garnet from the matrix. The mineral is removed 

 from the crushed product by treatment in jigs or by pneumatic 

 separators. The mechanical separation yields a clean concentrate, 

 although there is a small margin between the specific gravity of 

 almandite and that of the hornblende which is an important con- 

 stituent of the rock. 



The utility of garnet for abrasive purposes depends upon its 

 hardness, toughness and cleavage or fracture. Almandite, which 

 is the variety employed, is usually assigned a hardness of 7 to 7.5 

 by mineralogists; that is, it exceeds quartz in that respect but is 

 not quite so hard as topaz. It appears that Adirondack almandite 

 of the crystal variety possesses superior hardness, and is rated as 

 7.5 or 8. Toughness and strength are important qualities, especially 

 when the abrasive is used under considerable pressure. A material 

 may be very hard and yet brittle or crumbly under pressure. The 

 well-crystallized almandite, free from impurities, has great strength 

 and gives good service on polishing machines. The presence of 



