Гранат

Garnets are nesosilicates having the general formula X3Y2(SiO4)3, the chemical composition is SiO2 Al2O3 Fe2O3 CaO MgO TlO2. Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs Scale of about 6.5 to 7.5. The harder species, like almandine, are often used for abrasive purposes. Pure crystals of garnet are used as gemstones. The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. 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 are normally used for sand blasting. The pieces between 60 mesh and 200 mesh are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh 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. 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.