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Home » Minerals » Garnet

Garnet


Best known as a red gemstone, this mineral has many other uses and occurs in many other colors


What is Garnet?



Garnet is the name for a group of very common rock-forming minerals. These minerals are found throughout the world in metamorphic, igneous and sedimentary rocks. Most garnet forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce mica schist or gneiss.

Most people associate the word "garnet" with a red gemstone; however, they are often surprised to learn that garnet occurs in many other colors and has many other uses. In the United States the major industrial uses of garnet in 2012 were: waterjet cutting (35%), abrasive blasting media (30%), water filtration granules (20%) and abrasive powders (10%).


Garnet Composition and Properties



Minerals in the garnet group share a common crystal structure and a generalized chemical composition of A3B2(SiO4)3. In that composition A can be Ca, Mg, Fe2+ or Mn2+, and B can be Al, Fe3+, Mn3+, V3+ or Cr3+.

They also share a number of properties. They all have a vitreous luster, a transparent to translucent diaphaneity, a brittle tenacity and a lack of cleavage. They can be found as individual crystals, granular aggregates and massive occurrences.

Common Garnet Minerals

Mineral Composition Specific Gravity Hardness Colors
Almandine Fe3Al2(SiO4)3 4.20 7 - 7.5 red, brown
Pyrope Mg3Al2(SiO4)3 3.56 7 - 7.5 red to purple
Spessartine Mn3Al2(SiO4)3 4.18 6.5 - 7.5 red-orange-brown
Andradite Ca3Fe2(SiO4)3 3.90 6.5 - 7 green, yellow
Grossular Ca3Al2(SiO4)3 3.57 6.5 - 7.5 green, yellow, red
Uvarovite Ca3Cr2(SiO4)3 3.85 6.5 - 7 green
There are a number other garnet minerals that are less frequently encountered and not as important in industrial use. They include: goldmanite, kimzeyite, morimotoite, schorlomite, hydrogrossular, hibschite, katoite, knorringite, majorite and calderite.

As seen above, there are a variety of different types of garnet and each of them has a different chemical composition. This variation in chemistry determines many of their physical properties. As an example, the calcium garnets generally have a lower specific gravity, a lower hardness and are green in color, while the iron and manganese garnets have a higher specific gravity, a greater hardness and are red in color.

Physical Properties of Garnet

Chemical Classification silicate
Color typically red, but can be almost any color with orange, green, yellow and purple being common
Streak colorless
Luster vitreous
Diaphaneity transparent to translucent
Cleavage none
Mohs Hardness 6.5 to 7.5
Specific Gravity 3.5 to 4.3
Diagnostic Properties hardness, color, specific gravity, isometric crystal habit
Chemical Composition general formula X3Y2(SiO4)3
Crystal System isometric
Uses waterjet cutting granules, abrasive blasting granules, filtration granules, abrasive grits and powders, gemstones


How Does Garnet Form?



Most garnet forms at convergent plate boundaries where shale is being acted upon by regional metamorphism. The heat and pressure of metamorphism breaks chemical bonds and causes minerals to recrystalize into structures that are stable under the new temperature-pressure environment. The aluminum garnets, almandine and pyrope, generally form in this environment.

The garnets start as tiny grains and enlarge slowly over time as metamorphism progresses. As they grow then displace, replace and include the surrounding rock materials. A photo in the right column of this page shows a microscopic view of a garnet grain that has grown within a schist matrix. It has included a number of host rock grains as it grew.

The calcium garnets typically form when argillaceous limestone is altered into marble by contact metamorphism. These are andradite, grossular and uvarovite, the slightly softer, green garnets with a lower specific gravity. Two of the most high-priced gem garnets are tsavorite (a bright green grossularite) and demantiod (a golden green andradite).


Uses of Garnet



Garnet from Canada
Almandite, a variety of garnet from River Valley, Ontario, Canada. This specimen is a nice euhedral crystal approximately 2 inches (5 centimeters) across.


Almadite Garnet
Almandite, a variety of garnet from Lount Township, Ontario, Canada. This is a granular specimen approximately 4-1/2 inches (11.4 centimeters) across.


Rhodolite Garnet
Rhodolite, a variety of garnet, in mica schist from Jackson County, North Carolina. Specimen is approximately 3-1/2 inches (8.9 centimeters) across.


Andradite Garnet
Andradite, a variety of garnet, massive with wollastonite from Willsboro, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.


Spessartite Garnet cabochon
A cabochon of spessartite garnet from Namibia. This gem is 1.8 carats, and approximately 7 millimeters by 6 millimeters.


Garnet cabochons
Garnet cabochons. The 9 on the left are from Africa, and the single on the right is from Montana. All of these gems are approximately 7 millimeters across.



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Garnet
Garnet is best known for its use as a gemstone. It occurs in a wide range of colors with red stones being traditional and most common. The faceted stones in this photo are clockwise from the top left: a round red almandiite of 1.47 carats; a trillion green tsavorite (a grossularite)of 0.26 carats; a round orange spessartite or 0.74 carats; a round golden green grossularite of 0.49 carats; a round golden green demantoid of 0.32 carats and an oval rasperry red rhodolite (a pyrope) of 0.87 carats.


Uses of Garnet
The pie diagram above shows the most common industrial uses of garnet minerals. Almandine is the most common mineral used in industry.




garnet mica schist in thin section
This is a microscopic view of a garnet grain that has grown in a schist. The large black grain is the garnet, the red elongate grains are mica flakes (if you look closely you will see lots of them compressed between the other grains of the rock. The black, gray and white grains are mostly silt or smaller size grains of quartz and feldspar. The garnet has grown by replacing, displacing and including the mineral grains of the surrounding rock. You can see that many of these grains have been included in the garnet. From this photo it is easy to understand why clean, gem-quality garnets with no inclusions are very hard to find. What's hard to understand is how garnet so often grows into nice euhedral crystals under these conditions. Photo by Jackdann88, used here under a Creative Commons License.


Mineral Information
 Anhydrite
 Apatite
 Arsenopyrite
 Augite
 Azurite

 Barite
 Bauxite
 Beryl
 Biotite
 Bornite

 Calcite
 Cassiterite
 Chalcocite
 Chalcopyrite
 Chlorite
 Chromite
 Chrysoberyl
 Cinnabar
 Clinozoisite
 Copper
 Cordierite
 Corundum
 Cuprite
 Diamond
 Diopside
 Dolomite

 Enstatite
 Epidote

 Fluorite

 Galena
 Garnet
 Glauconite
 Gold
 Graphite
 Gypsum

 Halite
 Hematite
 Hornblende

 Ilmenite

 Jadeite

 Kyanite
 Limonite

 Magnesite
 Magnetite
 Malachite
 Marcasite
 Molybdenite
 Monazite
 Muscovite

 Nepheline
 Nephrite

 Olivine
 Orthoclase

 Plagioclase
 Prehnite
 Pyrite
 Pyrophyllite
 Pyrrhotite

 Quartz

 Rhodochrosite
 Rhodonite
 Rutile
 Scapolite
 Serpentine
 Siderite
 Sillimanite
 Silver
 Sodalite
 Sphalerite
 Spinel
 Spodumene
 Staurolite
 Sulfur
 Sylvite

 Talc
 Titanite
 Topaz
 Tourmaline
 Turquoise

 Uraninite

 Witherite
 Wollastonite

 Zircon
 Zoisite


crushed and graded garnet abrasive granules
This photo shows garnet granules that have been graded for use as an abrassives or filter medai. They can be used in waterjet cutting, "sand" blasting, sandpaper, water filtration and a number of other uses. Photo by the United States Geological Survey.


Garnet from Mexico
Grossularite garnet from Chihuahua, Mexico. Specimen is approximately 1-1/2 inches (3.8 centimeters) across.


Rosolite Garnet
Grossularite garnet (or Rosolite) with Idocrase, from Xalostoc, Mexico. Specimen is approximately 4 inches (10 centimeters) across.


Garnet essonite
Grossularite garnet (or Essonite) from Eden Mills, Vermont. Specimen is approximately 3 inches (7.6 centimeters) across.


Grossularite Garnet
Massive grossularite garnet (or Essonite) from Bancroft, Ontario, Canada. Specimen is approximately 1-1/2 inches (3.8 centimeters) across.


Spessartite Garnet
Spessartite garnet from Amelia, Virginia. Specimens are approximately 1/2 inch (1.3 centimeters) across.


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