Almandine garnet

Almandine (Garnet)

Almandine is the most common member of the Garnet group of minerals. It is an Iron Aluminum Silicate mineral. This Garnet is also a popular gemstone and the most widely used in the gem trade. More gemstones are faceted from Almandine than any other type of Garnet. Only a small amount of Almandine crystals are transparent and light enough for gemstone use. Most of the Almandine found is rough and opaque and not gem quality. Some Almandine Garnets display asterism when polished as cabochons, and are known as “Star Garnets”. Other varieties of Almandine include “Common Garnet” (dark, brownish-red opaque), “Precious Garnet” (deep red transparent), “Syrian Garnet” (slightly purple).

Forms and Formation

Almandine most commonly forms at convergent plate boundaries where regional metamorphism has occurred. It is often found within a schist matrix that has formed from shale during the regional metamorphic process. Garnet can be found in metamorphic rocks such as gneiss, schist and eclogite. It can also sometimes be found in igneous rocks such as granite pegmatites and as a sedimentary mineral in alluvial deposits.

Almandine garnet
set of Almandine (Almandite, garnet) gemstones

Almandine crystal forms are usually dodecahedron (12 faces) and trapezohedron (24 faces), and sometimes in modified combinations of the two. Crystals may be striated or with stepped growth layers and are sometimes warped into rounded ball-like forms. Also in dodecahedral crystal aggregates, grainy, massive, and as rounded water worn crystals.

Sometimes, Almandine will form something called a snowball garnet. A snowball garnet shows mineral inclusions that seems to be rotated into the mineral as it was forming, much like one would form the base of a snowman. Snowball garnets that are found in the Appalachian Mountains of Vermont, for example, have the amazingly unique ability to help geologists understand how the Appalachian Mountains were built and determine the rate at which rocks have folded in this area.

Uses of Almandine

Almandine Garnet has been used as a gemstone for over 5,000 years. It has appeared in the archaeological record of Ancient Egyptian burial sites and was a very popular gemstone in Ancient Roman culture. Almandine is the most popular garnet due to how abundant it is compared to other varieties of garnet. This gemstone is used in all types of jewelry. It is sometimes put into a rock tumbler to make smooth beads for necklaces and bracelets or is cut and polished into a cabochon. In the United States, Almandine is used for water jet cutting, abrasive blasting, water filtration, and as an abrasive powder.

Crystalline Structure

Almandine belongs to the Isometric Crystal System, Hexoctahedral Class. Hardness is 7.0 – 7.5 on the Mohs Scale. It has no cleavage and is not fluorescent. Color is dark red, reddish-brown, dark brown, black, or rarely purple. Luster is vitreous to subadamantine.

Almandine forms a series with Pyrope, with Almandine being the iron end member and Pyrope being the magnesium end member. This garnet also forms a series with Spessartine, with Spessartine being the manganese end member.

Noteworthy Locations of Almandine

Some of the best crystallized Almandine embedded in mica schist come from the classic locality of the Ziller valley, in the North Tyrol, Austria. Also, high up in the Alps, in an occurrence spanning two countries, is the Granatenkogel Mountain, with the northern slope in the Ötztal, North Tyrol, Austria, and the southern slope in the Passiria Valley, Bolzano Province, Italy.

Other important worldwide occurrences include Šumperk, Moravia, Czech Republic; the Altay Mine in the Koktokay pegmatite field, Xinjiang Autonomous Region, China; the Thackaringa District, Yancowinna County, New South Wales, Australia; and Serrote Redondo, Pedra Lavrada, Paraíba, Brazil.

In the United States, perhaps the most well-known occurrences are Garnet Ledge and the Sitkine River on Wrangell Island, Alaska. This locality produces excellent crystals embedded in a shiny mica schist matrix. The Barton Garnet Mine, in Gore Mountain, North River, Warren County, New York, touts itself as the world’s largest Garnet mine, producing extensive amounts of Almandine for use as garnet paper. Very large crystals have come from there but are all crude and incomplete. Large and historic Almandine crystals were found in various construction projects on the island of Manhattan in New York City, New York over the past two centuries. In fact, one the largest complete Almandine crystals ever found in the U.S. originated from Midtown Manhattan and is dubbed the “Subway Garnet”.

The New England states have several important Almandine occurrences, including Green’s Farm, Roxbury, Litchfield County, Connecticut; the Nathan Hall Quarry, East Hampton, Middlesex County, Connecticut; the Russell Garnet mine, Russell, Hampden County, Massachusetts; Greenwood, Oxford County, Maine; and Mt. Apatite, Auburn, Androscoggin County, Maine. Excellent trapezohedral crystals came from the Hedgehog Hill Quarry, Peru, Oxford County, Maine.

Enormous Almandine crystals were found in the Sedalia Mine, Salida, Chaffee County, Colorado, often coated with a mica layer; and lustrous dark crystals come from Garnet Hill, Ely, White Pine County, Nevada. North Carolina has several localities. The most noteworthy is Spruce Pine, Mitchell County, NC. Large Almandine crystals, including those that display asterism, are found at Emerald Creek, Latah County and Fernwood, Benewah County, Idaho.

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By Bill Jones, Sidewinder Minerals

Almandine garnet
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