Garnet Abrasives

Garnet Abrasives

The mention of Garnet as a Gem is one of the very common misunderstandings about this mineral. The Garnet as a semiprecious stone does not belong in the story of Garnet, the industrial tool, and yet it shares with the diamond the double, virtue of being a thing of beauty and utility.

Prior to about 1878 the Garnet was regarded as a curiosity and minor gem and has to this day retained much of that mystic. As a “Gem” the total output of known Garnet deposits probably is less than several hundred pounds per year. Therefore, this portion of the paper can be laid to rest with the showing of a picture of a nice Gem Stone and its evolution into a cut stone for jewelry.

By far the greatest use of the Garnet is as an abrasive. Garnet is the name of a group of iron aluminum silicate minerals having similar physical properties and crystal form. Almandite, the most important type of Garnet industrially, ordinarily has a hardness of 7.5 to 8.5, a specific gravity of 4, a melting point of 1315°C, a color of various shades of red, and a chemical composition of Fe3Al2 (SiO4)3; it crystalizes in the cubic system. While there are several operating properties mining Garnet in the United States, namely Idaho, and more recently in Maine, the longest history belongs to the “Garnet” of the Adirondack Mountains in Upstate New York. These deposits occur in dense consolidated material. The product being mined is an almondite-pyrope Garnet found in an altered gabbro or diarite. The rocks, are of metamorphased igneous intrusions of the precambrian age. Generally the contact is composed of hornblende and feldspar with some hypersthene and clinohypersthene, augite, biotite and small amounts of pyrite and apatite. Magnetite and ilmentie are also present in minor amounts which contribute to Garnet waste separation problems.

While the bulk of Garnet mined is in the United States, there are other countries with some activity also. Garnet production in India is reported to be about 4,000 tons per year. Estimated production in the USSR is about 500 tons. Production in the rest of the world is about 100 tons, of which Australian output is estimated to be 80 tons. Other countries that produce small amounts of Garnet are Sri Lanka, Argentina, Malagasy Republic, the Republic of South Africa and Tanzania. Additionally samples of Garnet have been received from China which indicate their interest in using Garnet as a technical abrasive.

Garnet mining in Idaho is in an alluvial deposit. The chemical composition is enough different from the Adirondack almondite Garnet to cause substantial differences in the technical results achieved when that material is used either for coated abrasives or abrasive powder. There can be little doubt that tremendous reserves of Garnet can be located if needed. Because the industry is small, reserves are not accurately known and have not been evaluated. Table I shows reserve estimates as of 1975. Not being expertly versed in Garnet Mining of other areas, this paper restricts further comments to that of the Adirondack Almondite Garnet as developed by the Barton Mines Corporation.

The Barton effort in Garnet and its use as an industrial abrasive extends from the year 1876 when Henry Hudson Barton opened a business serving the then woodworkers industry. In his search for a better abrasive, by happenstance his path crossed someone with a Garnet Stone. In short, by 1878 Barton had leased enough property to begin the hand picking of Garnet Crystals. This type of mining continued for a number of years, along with many other Garnet enterprises in the same vicinity By 1928 Barton Mines was the sole survivor in the area and has continued in that position.

In conclusion, the production of a natural abrasive from a total United States source is now well over 100 years old. With the ore reserves available, and a new plant; Barton Mines will be able to produce and deliver a steady source of ’’grains” and “powders” for years to come.

powder grades

garnet - abrasive and gems