Garnet (mineral)
Garnet is a group of minerals characterized by a common internal structure and varying chemical compositions, making it a diverse and widely occurring mineral. Primarily found in metamorphic and igneous rocks, garnet is recognized for its role as an important component of the Earth’s mantle and can also be found in garnet-rich sands produced by erosion. The mineral displays a range of colors, including red, yellow, brown, and occasionally green, with specific hues influenced by its chemical makeup. Notably, garnet has a hardness of 6.5 to 7.5 on the Mohs scale, making it suitable for various applications.
In terms of usage, garnet is predominantly utilized as an abrasive in industries such as woodworking, metal processing, and waterjet cutting. It also has applications in filtration, ceramics, and even in the electronics sector. Gem-quality garnets, especially the red almandine and the more prized green demantoid, are crafted into jewelry. Significant deposits of industrial garnets are located in the United States, India, China, and Australia, while notable sources of gem-quality garnets can be found in Brazil, Sri Lanka, and Tanzania. Historically, garnet has been valued for its beauty and utility, with uses dating back to ancient civilizations.
Garnet (mineral)
Where Found
Garnet occurs worldwide; it is common in many metamorphic and igneous rocks, especially gneisses and schists, and in garnet-rich sands that develop by erosion of such rocks. Garnet is an important constituent of the Earth’s mantle. Gem-quality garnets are notably found in Brazil, Sri Lanka, Tanzania, and the Ural Mountains. Industrial garnets are mined in the United States, India, China, and Australia.
Primary Uses
Garnet is used primarily as an abrasive and secondarily as a semiprecious gemstone. The color is the principal factor in determining the value of gem-quality garnets.
Technical Definition
Garnet is a family name for a group of minerals that have a common internal structure but vary in their composition and physical properties. The color of garnet is usually red, sometimes yellow or brown, and rarely green. Garnet commonly forms equidimensional crystals that have from twelve to thirty-six faces. The hardness of garnet varies from 6.5 to 7.5 on the Mohs scale. Garnet is brittle and forms subconchoidal fractures when it breaks.
Description, Distribution, and Forms
There are about twenty minerals called garnet. Each has the same general formula, “A”3 “B”2Si3O12, where “A” is calcium, magnesium, iron, manganese, or a combination and “B” is aluminum, iron, vanadium, zirconium, titanium, chromium, or a combination. Most of the formal garnet names are based upon hypothetical pure compositions in which a single element occurs in the A and B sites. Such “pure” garnets are rarely found in nature; most natural garnets are mixtures. The most common mixture is called pyralspite, which is an acronym for a mixture of the pure garnets named pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), and spessarite (Mn3Al2Si3O12). The second most common mixture is calledugrandite, an acronym for a mix of uvarovite (Ca3Cr2Si3O12), grossularite (Ca3Al2Si3O12), and andradite (Ca3Fe2Si3O12).
The color of a garnet is controlled by its chemical composition. Garnets rich in iron are dark red to nearly black. Garnets containing mostly calcium and aluminum are yellow to cinnamon brown. Shades of green result when abundant chromium is present. Garnets grow as isometric crystals that commonly develop as dodecahedron or trapezohedron forms or as a combination of both.
Garnets grow in a variety of geological settings. Pyralspite forms during metamorphism of shale or basalt at moderate temperatures and pressures, whereas ugrandite forms during metamorphism of limestone at moderate temperatures and low pressures. Semipure pyrope occurs in rocks of the lower mantle, and gem-quality almandine can be found in igneous pegmatites.
New York State has the largest known deposit of high-quality abrasive garnet. Bodies of ore can be found 30 to 120 meters wide, more than 30 meters thick, and approximately 1.5 kilometers long. Once the ore is mined and taken to the mill, the garnet is separated from other minerals by a combination of crushing and grinding, screening, tabling, flotation, magnetic separation, watersedimentation, and/or air separation. The maximum grain size of the garnet concentrate is less than one-half of a centimeter. Grains of differing grades are grouped into a variety of sizes depending on the requirements of the specific industrial use.
History
Garnet has been prized as a gemstone for most of history. Some Bronze Age jewelry contained garnet. The Greeks and Egyptians also used garnet ornamentally. During the Middle Ages, garnet was used for medicinal purposes.
Obtaining Garnet
High-quality garnets are cut as semiprecious gemstones and made into jewelry. The transparent red almandine is the most common garnet gemstone, but the most valuable is the brilliant green-colored demantoid garnet.
Uses of Garnet
The major use of garnet is as an abrasive. Its hardness and brittle fracturing allow garnet particles to undergo little chemical or structural change when crushed or ground into a powder. Abrasive uses include the finishing of wood furniture, the production of plastic, and the processing of sheet aluminum for the aircraft and shipbuilding industries. Garnet is also used in the petroleum industry, in filtration media, in ceramics and glass, and as an electronic component. The United States is one of the dominant producers and consumers of abrasive garnet. Industrial garnets are also used in waterjet cutting and water filtration media. Gem-quality garnets are used in jewelry.
Bibliography
U.S. Geological Survey.