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is a common mineral of metamorphic rocks such as gneiss and schist of
all description from basic to acid, crystalline limestone and
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).
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:-
3MgO. Al2O3. 3SiO2
3FeO. Al2O3. 3SiO2
3MnO. Al2O3. 3SiO2
3CaO. Cr2O3. 3SiO2
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.
Garnet is the birthstone for the month of January.
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.
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.
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.
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