Diffusion

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. It is a key mineral in interpreting the genesis of many igneous and metamorphic rocks via geothermobarometry. Diffusion of elements is relatively slow in garnet compared to rates in many other minerals, and garnets are also relatively resistant to alteration. Hence, individual garnets commonly preserve compositional zonations that are used to interpret the temperature-time histories of the rocks in which they grew. Garnet grains that lack compositional zonation commonly are interpreted as having been homogenized by diffusion, and the inferred homogenization also has implications for the history of the host rock.

Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. Garnet sand is also used for water filtration media. 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.

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 has been mined in western Rajasthan for the past 200 years, but mainly for the gemstone grade stones. Abrasive garnet was mainly mined as a secondary product while mining for gem garnets and was used as lapping and polishing media for the glass industries.

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