Garnet

Garnet is a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species 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: pyrope-almandine-spessartine and uvarovite-grossular-andradite.

Chemical composition

Garnets are nesosilicates having the general formula X3Y2(SiO4)3. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)2+ and the Y site by trivalent cations (Al, Fe, Cr)3+ in an octahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed.

Physical properties

The hardness of garnet varies between 6.5 and 7.5 on the Mohs scale. The specific gravity or density varies depending on the chemical composition. The color of garnets can be red, purple, green, yellow, or blue. The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in parts of the United States, Russia, and Turkey. It changes color from blue-green in the daylight to purplish red under incandescent light.

Uses

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 paper is favored by cabinetmakers for finishing bare wood.

See also

• Gemstone
• Mineral
• Silicate minerals
• Abrasive
• Mohs scale of mineral hardness

Garnet Resources
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