Garnet

A₃B₂(SiO₄)₃

Garnet is a common mineral of metamorphic rocks such as gneiss and schist of all description from basic to acid, crystalline limestone and pegmatites.

The History Says

It is said that Noah used a garnet on the bow of his ark to help navigate at night, and garnets have long been carried by people who are traveling, because they believe it will protect them from accidents. Legend has it that garnets protect their owners from nightmares, and garnets were even used as bullets because the shooters thought the red color would increase the intensity of the wound.

The Present Scenario

Garnet is found in Africa, Brazil, Canada, India, Madagascar, Sri Lanka, and the USA (Arizona, Colorado, Georgia, Idaho, New Hampshire, New Mexico, North Carolina, Pennsylvania, Utah, and Virginia).

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GARNET comprises a group of orthosilicta minerals of divalent and trivalent elements having the general chemical formula 3RO´. R´2O3. 3SiO2. The divalent elements present are calcium, magnesium, ferrous iron, or manganese and the trivalent elements may be chromium and ferric iron. Six minerals are commonly regarded as belonging to the garnet family. These are:-

• Grossularite 3CaO.Al₂O₃.3SiO₂
• Pyrope 3MgO. Al₂O₃. 3SiO₂
• Almandine 3FeO. Al₂O₃. 3SiO₂
• Spessartite 3MnO. Al₂O₃. 3SiO₂
• Andradite 3CaO.Fe₂O₃. 3SiO₂
• Uvarovite 3CaO. Cr₂O₃. 3SiO₂

They all crystallize in cubic system with rhombdodecahedral or trapezohedral forms or a combination of both; the hardness varies between 6.5 to 7.5. Massive and crystalline garnets are tough and break with difficulty. A garnet tends to break into small pieces with sharp angular and uneven faces which is regarded as a characteristic feature of a good abrasive. Garnet, therefore, is valued as a natural abrasive. Garnet is a common mineral of metamorphic rocks such as gneiss and schist of all description from basic to acid, crystalline limestone and pegmatites. Almandine is of wide occurence, found in mica schists and metamorphic rocks containing alumina and iron.

"Garnet" is the name given to a group of chemically and physically similar minerals. A very small number of garnets are pure and flawless enough to be cut as gemstones. The majority of garnet mining is for massive garnet that is crushed and used to make abrasives. Garnet is a silica mineral; in other words, garnet's complex chemical formula includes the silicate molecule (SiO₄). The different varieties of garnet have different metal ions, such as iron, aluminum, magnesium and chromium. Some varieties also have calcium. Garnets all crystallize in the isometric (meaning equality in dimension. For example, a cube, octahedron, or dodecahedron.) crystal system. Garnets all are quite hard, ranging between 6 and 7.5 on the Mohs' hardness scale. They also lack cleavage, so when they break, they fracture into sharp, irregular pieces. The combination of the hardness and fracture make garnet a valuable abrasive material.

Note : Garnet is the birthstone for the month of January.

Name

The name garnet has been used since ancient times. It was derived from the Latin word granatium which means a pomegranate because small, red garnet crystals were thought to resemble pomegranate seeds. The original name given this mineral group was granat. In time the "r" and "a" were transposed giving us garnet. The name was officially proposed to mineralogists by the German theologian and philosopher, Albertus Magnus.

Sources

In the United States, only a few companies in three states (Idaho, New York, and Montana) produce garnet for industrial use. There are many significant garnet-producing countries. Noteworthy among them are Australia, China, and India, all of which export significant amounts of garnet. Russia and Turkey also produce large amounts of industrial garnet, but they are not yet exporting much of this material.

Uses

Garnet is ground to a variety of sizes to be used as an abrasive. Garnet sandpaper was the original application of this mineral. It is also used to make a number of similar products, including sanding belts, discs, and strips. Today, the vast majority of garnet is used as an abrasive blasting material, for water filtration, in a process called water jet cutting, and to make abrasive powders.

Substitutes and Alternative Sources

A number of natural and synthetic materials could be used in place of garnet for abrasive purposes. The natural materials include the minerals staurolite, quartz, diamond and corundum. The synthetic materials include fused aluminum oxide and silicon carbide.

Specification and Industry

Only iron garnet, almandine and to a lesser extent, glosularite are used in the manufacture of abrasive like coated garnet paper and cloth and discs glued with sodium silicate. The common bonding materials used for the manufacture of discs are magnesium oxide and magnesium chloride. Garnet is crushed, sieved and graded into various sizes and thus coated abrasives of various fineness are prepared. Garnet abrasives are used mostly for wood polishing. Finer grains of micron sizes are used as tumbling chemicals, grits for optical glass polishing and also ceramics and glasses.

In modern practice, broken garnet pieces are preheated to a temperature varying from 700° to 1000°C before crushing to various grain sizes. It is reported that the toughness, fracture and colour of the garnet is improved on heating and quenching. The degree of heat treatment is judged by the change in colour of the garnet. Heat treatment imparts to garnet the colour of ruby red at low temperature to silvery dark ruby clour at high temperature. Heat treatment also provides better adhesive property to the grains.