GARNET[GARNET IN GRANITE]

Listing description

Garnet is the name used for a large group of rock-formingminerals. These minerals 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⁺.

Detailed description

These minerals are found throughout the world in metamorphic,igneous, and sedimentary rocks. Most garnet found near Earth's surface forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce schist or gneiss. Garnet is also found in the rocks of contact metamorphism, subsurface magma chambers, lava flows, deep-source volcanic eruptions, and the soils and sediments formed when garnet-bearing rocks are weathered and eroded. 

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 Physical and Chemical Properties

 

The most commonly encountered minerals in the garnet group include almandine, pyrope, spessartine, andradite, grossular, and uvarovite. 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, stream-worn pebbles, granular aggregates, and massive occurrences. Their chemical composition, specific gravity, hardness, and colors are listed below.

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	orange to red to brown
Andradite	Ca3Fe2(SiO4)3	3.90	6.5 - 7	green, yellow, black
Grossular	Ca3Al2(SiO4)3	3.57	6.5 - 7.5	green, yellow, red, pink, clear
Uvarovite	Ca3Cr2(SiO4)3	3.85	6.5 - 7	green
The compositions listed above are for end members of several solid solution series. There are a number of 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 has a different chemical composition. There are also solid solution series between most of the garnet minerals. This wide 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 typically green in color. In contrast, the iron and manganese garnets have a higher specific gravity, a greater hardness and are typically red in color. 

Physical Properties of Garnet
Chemical Classification	silicate
Color	Typically red, but can be orange, green, yellow, purple, black or brown. Blue garnets are extremely rare.
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, specific gravity, isometric crystal habit, lack of cleavage
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?

 

Garnet in Metamorphic Rocks 

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 causesminerals to recrystallize into structures that are stable under the new temperature-pressure environment. The aluminum garnet, almandine, generally forms in this environment. 

As these rocks are metamorphosed, the garnets start as tiny grains and enlarge slowly over time as metamorphism progresses. As they grow, they displace, replace, and include the surrounding rock materials. The photo below shows a microscopic view of a garnet grain that has grown within a schistmatrix. It included a number of the host rock's mineral grains as it grew. This explains why so many garnets formed by regional metamorphism are highly included. 


The calcium garnets typically form when argillaceous limestone is altered into marble by contact metamorphism along the edges of igneous intrusions. These are andradite, grossular, and uvarovite, the slightly softer, typically green garnets with a lower specific gravity. Two calcium garnets are highly regarded in the gem trade; they are tsavorite (a bright green grossular) and demantoid (a golden-green andradite). 


Garnet in Igneous Rocks 

Garnet often occurs as an accessory mineral in igneous rockssuch as granite. Many people are familiar with almandine garnet because it is sometimes seen as dark red crystals in the igneous rocks used as granite countertops. Spessartine is an orange garnet found as crystals in granite pegmatites. Pyrope is a red garnet that is brought to Earth's surface in pieces of peridotitethat were torn from the mantle during deep-source volcanic eruptions. Garnet is also found in basaltic lava flows.


Garnet in Sedimentary Rocks and Sediments 

Garnets are relatively durable minerals. They are often found concentrated in the soils and sediments that form when garnet-bearing rocks are weathered and eroded. These alluvial garnets are often the target of mining operations because they are easy to mine and remove from the sediment/soil by mechanical processing. 

Uses of Garnet 

Garnet has been used as a gemstone for thousands of years. In the past 150 years, it has seen many additional uses as an industrial mineral. The chart below shows recent industrial uses of garnet in the United States. Garnet is also used as an indicator mineral during mineral exploration and geologic assessments.

Garnet as an Industrial Mineral

Garnet Abrasives 

The first industrial use of garnet was as an abrasive. Garnet is a relatively hard mineral with a hardness that ranges between 6.5 and 7.5 on the Mohs Scale. That allows it to be used as an effective abrasive in many types of manufacturing. When crushed, it breaks into angular pieces that provide sharp edges for cutting and sanding. Small granules of uniform size are bonded to paper to produce a reddish color sandpaper that is widely used in woodworking shops. Garnet is also crushed, screened to specific sizes, and sold as abrasive granules and powders. In the United States, New York and Idaho have been important sources of industrial garnet for abrasives.


Waterjet Cutting 

The largest industrial use of garnet in the United States is in waterjet cutting. A machine known as a waterjet cutter produces a high-pressure jet of water with entrained abrasive granules. When these are directed at a piece of metal, ceramic, or stone, a cutting action can occur that produces very little dust and cuts at a low temperature. Waterjet cutters are used in manufacturing and mining. 

PRICE

$52.8/KG OR $24/IB

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