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Hutton Garnet Beaches

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Mineral Info

Garnet: The Mineral

Garnet is the general name given to a group of complex silicate minerals with widely varying chemical composition, but all with isometric crystal structure. Garnet has relatively high specific gravity and is considered a heavy mineral. It is non-toxic, chemically inert, and has no crystalline silica. Garnets are classified into three main groups, with extensive substitutions possible within each:

Aluminum Garnet: Almandine (Fe, Al garnet), grossular (Al, Ca garnet), pyrope (Mg, Al garnet), and spessartine (Mn, Al garnet)
Chromium Garnet: Uvarovite (Cr, Ca garnet)
Iron Garnet: Andradite (Fe, Ca garnet)

The mineral garnet ranges widely in colour, from almost black to purple, red and pink, as well as varieties of yellow, orange and green. Colour alone is not always indicative of the type of garnet.

Garnet occurs worldwide in many rock types, principally gneisses and schists; other sources include contact metamorphic rocks, metamorphosed crystalline limestones, pegmatites, and serpentinites. Occurrences of hard rock and placer garnet are numerous; however, relatively few commercially viable garnet deposits have been discovered. Placer garnet is recovered from heavy-mineral sand and gravel deposits in many parts of the world.

The most common variety within the garnet family is almandine. Its color, from red, reddish orange, to reddish purple depends on small changes in its chemical composition. It is hard (6.5-7.5), heavy (4.1 sg), without cleavage, and insoluble in acids. The grains survive erosion and are concentrated by wave and wind action, as is the case at the Hutton garnet beaches.

Almandine garnet is an effective abrasive largely due to its hardness and lack of cleavage. It is hard enough to scratch and cut most common metals and glass, and is the primary garnet type commonly used in industry, although some andradite is used in filtration. Almandine is currently produced at several hard rock and placer mines in the USA, with much imported from Southeast Asia. India, Australia, and China are important garnet producers, with lesser amounts mined in the Ukraine and in South Africa.

Hutton Garnet

Hutton garnet is mainly pale pink to orange almandine (Fe3Al2Si3O12), and similar to commercially available garnet around the world. Garnet grains typically contain few inclusions and are unfractured -- important for abrasive waterjet cutting as breakage resistance is increased. As the Hutton garnet beaches are geologically 'young', the garnet grains are mostly sub-angular: Photomicrographs illustrate grain shapes ideal for nearly all waterjet applications.

The Hutton sand is mineralogically quite homogeneous. The deposits have a natural particle size distribution well suited to water-abrasive jet cutting. The garnet grains are generally 0.30mm to 0.15mm in diameter -- the sizing required for waterjet products. Lesser amounts of coarser material may be used for sand blasting applications.

Processing of the Hutton sand has shown a commercial product is easily produced. Waterjet testing indicates the garnet is very suitable for use in commercial applications.

Uses of Garnet

The main markets for garnet sand are abrasive waterjet cutting, abrasive blast media and water filtration.

Abrasive waterjet cutting

Perhaps the most exciting market for garnet is in the waterjet cutting industry. Patented in the early 1980's, this emerging computer-based technology uses ultra-high pressure water forced through very small nozzles to cut a wide range of materials. If fine garnet is introduced into the flow, precision cuts become possible in a wide range of materials, from titanium metal to glass.

In comparison to lasers, which operate at high temperatures and are limited to cutting 1.3 -- 2 cm thick material, abrasive jets are more versatile and more cost-effective. Commonly used on 10 cm thick metal sheets, waterjets can cut up to 25 cm steel and 60 cm glass at slow speeds, or surface etch for decorative purposes. Any complex two-dimensional shape may be machined with high precision and excellent quality finish. Some waterjet machines cut with a tolerance of +/- 0.005 cm. For these reasons, the aerospace industry makes extensive use of this technology.

Total demand for this sector is difficult to estimate, and published figures range substantially. Waterjets utilize garnet in the 0.30 mm to 0.15 mm size range. The natural size distribution of the Hutton sand is naturally suited to a #80 mesh product most commonly in use for waterjet applications.

The waterjet business sector will remain an important and growing opportunity, not just in North America, but worldwide in the years to come. The Hutton project is well located geographically to benefit from future growth, particularly in North America and Europe. More comprehensive information on abrasive jet waterjet machining can be found at the Waterjet Web Reference.

Abrasive blasting

In abrasive blasting, a grain of material is held in a pressurized steel pot until released through a system of hoses and a nozzle to impact a surface. The grain, often traveling at velocities approaching the speed of sound, cleans the surface through a combination of "cutting" through layers of dirt and paint, and through a mechanical action of disrupting the surface through shock impact. Nearly any material of the proper size to pass through the pneumatic system with sufficient mass to carry energy can be used as a blasting abrasive.

In abrasive blasting, nearly any sizing shown to be effective can find a market niche. Garnet sizes used for blast cleaning range from very coarse material (about 2.0 mm) used in some specialized recycling applications to fine grained (0.15 mm) material used to clean aluminum and thin steel. Most blasting is done with material in the 0.8 x 0.5 mm and a slightly finer 0.6 x 0.2 mm size range. Published prices for garnet blasting products are slightly lower than those for waterjet-grade garnet.

Water filtration

The third major market for garnet grains is in single or multi-media, high-density sand filters. The use of garnet as a high-density layer under the sand and anthracite layers can increase the efficiency of the filtering process in some areas.