What is Garnet?
Garnet is the name used for a large group of rock-forming minerals. 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.
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 aluminium 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 colours and has many other uses. In the United States, the major industrial uses of garnet in 2012 were water-jet 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 colours are listed below.
|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 colour. In contrast, the iron and manganese garnets have a higher specific gravity, a greater hardness and are typically red in colour.
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 causes minerals to recrystallise 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 schist matrix. 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 rocks such 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 peridotite that 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
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.
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.
Garnet granules are also used in abrasive blasting (commonly known as "sand blasting"). In these processes, a tool propels a stream of abrasive granules (also known as "media") against a surface using a highly pressurized fluid (usually air or water) as a propellant. Abrasive blasting is done in order to smooth, clean, or remove oxidation products from metals, brick, stone, and other materials. It is usually much faster than sanding by hand or with a sanding machine. It can clean small and intricate surfaces that other cleaning methods would miss. Abrasives of various hardnesses can be used to clean a surface of greater hardness, without damaging the surface.
Garnet granules are often used as a filter media. Small garnet particles are used to fill a container through which a liquid flows. The pore spaces of the garnet are small enough to allow passage of the liquid but are too small to allow passage of some contaminant particles, which are filtered from the flow. Garnet is suited for this use because it is relatively inert and has a relatively high specific gravity. Garnet granules, crushed and graded to about 0.3 millimetres in size, can be used to filter out contaminant particles as small as a few microns in diameter. Garnet's high specific gravity and high hardness reduce bed expansion and particle abrasion during back flushing.
Garnet as a Geological Indicator Mineral
Although most of the garnets found at Earth's surface have formed within the crust, some garnets are brought up from the mantle during deep-source volcanic eruptions. These eruptions entrain pieces of mantle rock known as "xenoliths" and deliver them to the surface in a structure known as a "pipe." These xenoliths are the source of most diamonds found at or near Earth's surface.
Although xenoliths contain diamonds, they often contain a tremendous number of garnets for every diamond, and those garnets are generally larger in size. These deep-source garnets are very different from the garnets that form in the crust at shallow depth. So, a good way to prospect for diamonds is to look for these unique garnets. The garnets serve as "indicator minerals" for geologists exploring for diamond deposits. As the xenoliths weather, their garnets are liberated in large numbers. These unusual garnets then move downslope in soils and streams. Geologists who find them can follow the garnet trail to the source deposit. Some of the diamond pipes in Canada were found by following a garnet trail produced by moving ice.
Garnets as Gemstones
Garnet has been used as a gemstone for over 5000 years. It has been found in the jewellery of many Egyptian burials and was the most popular gemstone of Ancient Rome. It is a beautiful gem that is usually sold without treatment of any kind. It is also durable and common enough that it can be used in jewellery at a relatively low cost.
Garnet continues as a popular gemstone today. It serves as a birthstone for the month of January and is a traditional gem given on a second anniversary. Most people will think of a red gemstone when they hear the name "garnet" because they are not aware that garnet occurs in a variety of colours. However, gem-quality garnets occur in every colour with red being the most common and blue garnets being especially rare.
Red almandine is the red garnet most often found in jewellery because it is abundant and inexpensive. Pyrope and spessartine are reddish garnets that are commonly encountered in jewellery for the same reason. In recent decades, green demantoid garnet has become popular. It has a dispersion of 0.057 that gives it a "fire" which exceeds that of diamond's at 0.044. Green tsavorite has a bright, rich colour that is very much like emerald. It is commonly used as an alternative stone to emerald. Both of these green garnets are becoming more popular, but their price is much higher than almandine.
Properties of Garnet
|Chemical Formula||Garnet is a series of several different minerals with unique chemical formulas. See The chemical formula of Garnet above for details.|
|Colour||Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black|
|Hardness||6.5 - 8.5|
|Refractive Index||1.780 - 1.889|
|SG||3.5 - 4.3|
|Transparency||Transparent to translucent|
|Luster||Vitreous; some forms adamantine or submetallic|
|Cleavage||None. May exhibit parting.|