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Minerals of Garnet Group
The garnets comprise a group of isomorphous minerals with the general formula X3Y2(SiO4), where X may be Ca, Mg, Mn, or Fe+2 and Y may be Al, Cr3+.
The common garnets are:
- Pyrope: Al2Mg3O12Si3: Red to Purple with a hardness 7-7.5 on Moh’s scale.
- Almandine: Fe++3Al2(SiO4)3: Red brown in colour, and has a hardness 7-7.5 on Moh’s scale.
- Spessartite: Mn++3Al2(SiO4)3: Orange to red to brown, and has a hardness equal to 6.5 – 7.5 Moh’s scale.
- Grossularite: Ca3Al2(SiO4)3: Green, yellow, red, pink, clear in colour. It has a hardness of 6.5 to 7.5 Moh’s scale.
- Uvarovite: Ca3Cr2(SiO4)3: The green coloured Uvarovite has a hardness on Moh’s scale 6.5 – 7.
- Andradite: Ca3Fe2(SiO4)3: It is green, yellow, black in colour. Its hardness is 6.5-7 on Moh’s scale.
Physical Properties of Garnet Group of Minerals:
|Chemical Classification||X3Y2(SiO4)3, where X may be Ca, Mg, Mn, or Fe+2 and Y may be Al, Cr3+|
|Color||Typically red, but can be orange, green, yellow, purple, black, or brown. Blue garnets are extremely rare.|
|Streak||All garnets have a Colorless streak.|
|Diaphaneity||Transparent to translucent|
|Mohs Hardness||6.5 to 7.5|
|Specific Gravity||3.5 to 4.3|
|Diagnostic Properties||Hardness, specific gravity, isometric crystal form, lack of cleavage|
|Chemical Composition||General formula: X3Y2(SiO4)3|
|Uses||Waterjet cutting granules, abrasive blasting granules, filtration granules, abrasive grits and powders, gemstones|
Uses of Garnets:
- Garnet has been used as an ornamental and gem material.
- In recent years Garnets are being used in industries.
- Garnet can be used as an abrasive material. As garnet has a high hardness, that is equal to 6.5-7, therefore, the sediments of garnets have sharp edges, and thus are used as abrasive sediment in water jet cutters.
- Small granular sediments of garnet are applied on the paper to make sandpaper, which is used in wood workshops.
- The sandpapers of garnet are used to clean the rust of metals and to smoothen the surfaces.
- Garnet granules are also used in abrasive blasting (“sand blasting”). In these processes, a pressure pump propels a stream of abrasive granules (also known as “media”) mixed in fluid are sprayed against a surface with very high velocity and pressure. This stream of fluid containing the abrasive material erodes the surface very quickly. Abrasive blasting is done for smoothening, cutting, and designing a surface.
- Surfaces of very hard materials can also be cleaned and polished by means of garnet abrasives without damaging the surface.
- Very small granules of garnet are used as a filter. Filling the liquid medium with garnet granules forms a semipermeable filter, which allows the liquid to passer through it, but does not allow the impurities. Garnet is a very useful filter to filter the water and other liquid very easily and cheaply.
- Generally, garnets are formed within the crust and are found on the earth’s surface. But some of the garnets are extruded deep from the mantle along with the volcanic eruptions. These eruptions also extrude the interior rock pieces, which are known as “xenoliths”. Most of the surface diamond is found in these rocks (xenolith). Xenoliths of Kimberlite (rock) are a major source of the diamond mineral.
How Garnets are formed:
- Garnets in Igneous Rocks: Garnet often occurs as an accessory mineral in igneous rocks such as granite. Many people are familiar with almandine garnet because it is sometimes seen as dark red crystals in the igneous rocks used as granite countertops. Spessartine is an orange garnet found as crystals in granite pegmatites. Pyrope is a red garnet that is brought to Earth’s surface in pieces of peridotite that were torn from the mantle during deep-source volcanic eruptions. Garnet is also found in basaltic lava flows.
- Garnets in Sedimentary Rocks: Garnets are relatively durable minerals. They are often found concentrated in the soils and sediments that form when garnet-bearing rocks are weathered and eroded. These alluvial garnets are often the target of mining operations because they are easy to mine and remove from the sediment/soil by mechanical processing.
Garnet vs Ruby:
Not all garnets red, but those which are red, always cause confusion in differentiating them from ruby. But still we can differentiate ruby from garnet in the following ways:
- Red garnets can have hints of orange or pink while rubies are vivid red with occasional purple or blue secondary hues.
- By positioning both the gemstones in light, you can observe two rainbows with no yellow or green bands in ruby stone, while the rainbow of full colour can be seen in a garnet stone.
- We can differentiate both on the basis of their hardness on Moh’s scale. Rubies rank 9, right after diamonds. Garnets rank 6.5 to 7.5, a much softer stone in comparison.
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Writer at Geo Studies Organization; https://geostudies.org