|Infobox on Garnet
|Example of Garnet
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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 garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus ("grain"), possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds similar in shape, size, and colour to some garnet crystals.
Six common species of garnet are recognized by their chemical composition. They are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: 1. pyrope-almandine-spessarite and 2. uvarovite-grossular-andradite.
"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.
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.
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow and orange.
Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Alluvial garnet grains which are rounder are more suitable for such blasting treatments. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. For water jet cutting, garnet extracted from hard rock is suitable since it is more angular in form, therefore more efficient in cutting.
Garnet paper is favoured by cabinetmakers for finishing bare wood. Garnet sand is also used for water filtration media.
As an abrasive garnet can be broadly divided in two categories, blasting grade and water jet grade. The garnet, as it is mined and collected, is crushed to finer grains; all pieces which are larger than 60 mesh (250 micrometers) are normally used for sand blasting. The pieces between 60 mesh (250 micrometers) and 200 mesh (74 micrometers) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 micrometers) are used for glass polishing and lapping. Regardless of the application, the larger grain sizes are used for faster work and the smaller ones are used for finer finishes.
There are different kinds of abrasive garnets which can be divided based on their origin. The largest source of abrasive garnet today is garnet rich beach sand which is quite abundant on Indian and Australian coasts and the main producers today are seen to be Australia and India.
This material is particularly popular due to its consistent supplies, huge quantities and clean material. The common problems with this material are the presence of ilmenite and chloride compounds. Since the material is being naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at the Tuticorin beach is 80 mesh, and ranges from 56 mesh to 100 mesh size.
River garnet is particularly abundant in Australia. The river sand garnet occurs as a placer deposit.
Rock garnet is perhaps the garnet type used for the longest period of time. This type of garnet is produced in America, China and western India. These crystals are crushed in mills and then purified by wind blowing, magnetic separation, sieving and, if required, washing. Being freshly crushed, this garnet has the sharpest edges and therefore performs far better than other kinds of garnet. Both the river and the beach garnet suffer from the tumbling effect of hundreds of thousands of years which rounds off the edges.
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.
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 aluminium oxide and silicon carbide.
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 colour at high temperature. Heat treatment also provides better adhesive property to the grains.