Scheelite

Scheelite is a calcium tungstate mineral that is commonly found in various geological environments. It is an important ore of tungsten, which is a metal known for its high melting point, density, and strength. Scheelite is named after Carl Wilhelm Scheele, a Swedish chemist who discovered tungsten in 1781.

The mineral is typically found in contact-metamorphic deposits associated with granitic intrusions. It can also occur in skarns, hydrothermal veins, and greisen-type deposits. Scheelite is known for its distinctive color, which can range from white to gray, brown, or yellowish-brown. It often has a vitreous to adamantine luster and can display fluorescence under ultraviolet light.

Scheelite has a tetragonal crystal structure, with prismatic or tabular crystals being common. It has perfect cleavage on {110} planes and a high specific gravity, making it relatively heavy compared to many other minerals. Its hardness ranges from 4.5 to 5.5 on the Mohs scale, indicating moderate hardness.

One of the notable properties of scheelite is its ability to fluoresce under ultraviolet light. This phenomenon, known as fluorescence, can cause the mineral to emit a bright blue or yellow glow, depending on the impurities present. This characteristic has made scheelite a popular mineral among collectors.

From an industrial perspective, scheelite is an important source of tungsten. Tungsten has various applications, including the production of hard metals, alloys, filaments for incandescent light bulbs, and X-ray tubes. The mineral is typically processed through flotation or gravity separation methods to obtain the desired tungsten concentrate.

In summary, scheelite is a calcium tungstate mineral that serves as a significant ore of tungsten. It is known for its distinctive color, fluorescence, and occurrence in various geological environments. Its economic importance stems from the extraction of tungsten, a metal with diverse industrial applications.

Chemical composition and structure

The chemical composition of scheelite is calcium tungstate, with the chemical formula CaWO4. It consists of calcium (Ca) cations and tungstate (WO4) anions. The tungstate anion contains one tungsten atom (W) bonded to four oxygen atoms (O). The calcium cation is coordinated with oxygen atoms in the structure.

Scheelite crystallizes in the tetragonal crystal system, meaning it has four-fold rotational symmetry along the c-axis. Its crystal structure is classified as scheelite-type structure or the calcium tungstate structure. In this structure, the tungsten atoms occupy the center of the unit cell, surrounded by oxygen atoms forming a distorted octahedral coordination. The calcium atoms are located in the interstitial sites between the octahedra.

The arrangement of the tungstate and calcium ions in the structure gives scheelite its characteristic tetragonal crystal habit. The mineral commonly occurs as prismatic or tabular crystals, often with square or rectangular cross-sections. It exhibits perfect cleavage on {110} planes, which is a plane perpendicular to the c-axis.

The crystal structure of scheelite is relatively dense, contributing to its high specific gravity. The mineral has a specific gravity ranging from 5.9 to 6.1, making it significantly heavier than most common minerals.

It’s important to note that scheelite can have minor impurities and substitutions, which can affect its color and fluorescence properties. For example, the presence of molybdenum impurities can give scheelite a bluish tint, while rare earth element substitutions can contribute to its fluorescence under ultraviolet light.

In summary, scheelite has a chemical composition of calcium tungstate (CaWO4) and crystallizes in a tetragonal structure known as the scheelite-type structure. The arrangement of tungstate and calcium ions in the structure gives rise to its distinctive crystal habit and physical properties.

Occurrence and mining

Scheelite occurs in a variety of geological environments and can be found in different types of deposits. Some of the common occurrences and mining methods associated with scheelite are as follows:

1. Contact-Metamorphic Deposits: Scheelite is often found in contact-metamorphic deposits associated with granitic intrusions. In these deposits, the mineral forms as a result of hydrothermal activity and metasomatic replacement of pre-existing rocks. The mineralization is typically found in the vicinity of the contact zone between the granite and surrounding country rocks.
2. Skarn Deposits: Skarns are metamorphic rocks formed through the interaction of hydrothermal fluids from intrusive bodies with carbonate-rich rocks. Scheelite can be found as an important mineral within skarn deposits. Skarns often occur in proximity to granite or other intrusive bodies.
3. Hydrothermal Veins: Scheelite can also occur in hydrothermal veins, which are fractures or fissures in rocks filled with mineral-rich fluids. These veins are formed as a result of hydrothermal activity associated with magmatic processes. Hydrothermal veins can be found in various geological settings and can host scheelite mineralization.
4. Greisen-Type Deposits: Greisen-type deposits are associated with granitic intrusions and are characterized by alteration of the surrounding rocks by hydrothermal fluids. Scheelite can be present as an accessory mineral in these deposits, along with other minerals such as mica and quartz.

Mining methods for scheelite extraction vary depending on the deposit type and its geological characteristics. Some of the common mining techniques employed for scheelite include:

a. Open-Pit Mining: In situations where scheelite deposits are near the surface and the ore body is large and easily accessible, open-pit mining may be employed. This method involves the removal of overlying rocks and soil to expose the scheelite-bearing ore. The ore is then extracted using machinery and transported for further processing.

b. Underground Mining: When scheelite deposits are located at greater depths, underground mining methods may be used. This involves the creation of tunnels and shafts to access the ore body. Underground mining can be more complex and expensive than open-pit mining but is necessary for extracting scheelite from deeper deposits.

c. Flotation: Flotation is a common method used for the concentration of scheelite ore. It involves the use of chemicals to selectively separate scheelite from other minerals and impurities. The ore is crushed and ground into fine particles, and then mixed with water and flotation reagents. Air bubbles are introduced into the mixture, and the hydrophobic scheelite particles attach to the bubbles, allowing them to be collected and separated.

d. Gravity Separation: Gravity separation techniques can be used to separate scheelite from gangue minerals based on the differences in their densities. The ore is crushed and subjected to various processes such as jigging, spiraling, or shaking tables to separate the heavier scheelite particles from the lighter gangue minerals.

After the extraction process, the scheelite concentrate is further processed to obtain the desired tungsten product, which can be used in various industrial applications.

It’s worth noting that specific mining methods and techniques can vary depending on the characteristics of the deposit, economic considerations, and environmental factors.

Physical Properties of Scheelite

Scheelite possesses several distinctive physical properties, including the following:

1. Color: Scheelite can exhibit a range of colors, including white, gray, brown, and yellowish-brown. The color can be influenced by impurities present in the crystal lattice. For example, molybdenum impurities can give scheelite a bluish tint.
2. Luster: The mineral typically displays a vitreous to adamantine luster when freshly fractured. Vitreous luster refers to a glassy appearance, while adamantine luster describes a brilliant, diamond-like shine.
3. Transparency: Scheelite is generally translucent to opaque, meaning that light may pass through the mineral to varying degrees, but it is not transparent enough to see through clearly.
4. Crystal Habits: Scheelite commonly occurs as prismatic or tabular crystals, often with square or rectangular cross-sections. The crystals can be well-formed and exhibit distinct faces and edges. Other crystal habits include bladed, columnar, and granular forms.
5. Cleavage: Scheelite displays perfect cleavage on {110} planes. This means that if the mineral is carefully split or cut, it will break along these planes, resulting in smooth, flat surfaces.
6. Hardness: Scheelite has a hardness of 4.5 to 5.5 on the Mohs scale. This indicates that it can be scratched by harder minerals such as orthoclase or quartz but can scratch minerals with a lower hardness.
7. Specific Gravity: Scheelite has a relatively high specific gravity ranging from 5.9 to 6.1. This means that it is significantly denser and heavier than most common minerals.
8. Fluorescence: One of the notable characteristics of scheelite is its ability to fluoresce under ultraviolet (UV) light. When exposed to UV light, scheelite can emit a bright blue or yellow glow, depending on the impurities present. This fluorescence property has made scheelite a sought-after mineral for collectors.
9. Streak: The streak of scheelite is white. This refers to the color of the mineral’s powdered form when it is scraped across a rough surface.

These physical properties help in identifying and distinguishing scheelite from other minerals. They also contribute to its aesthetic appeal and scientific significance.

Geological Formation

Scheelite typically forms in various geological settings through hydrothermal processes involving the interaction of hot fluids with specific rock types. The formation of scheelite involves several stages and conditions. Here is a general overview of the geological formation of scheelite:

1. Magmatic Intrusion: The initial stage of scheelite formation often involves the emplacement of a granitic intrusion. Magma, rich in tungsten and other elements, rises towards the Earth’s crust and intrudes into the surrounding rocks. The granitic intrusion serves as a heat and fluid source for subsequent hydrothermal activity.
2. Hydrothermal Fluids: As the granitic intrusion cools, hydrothermal fluids are generated. These fluids consist of hot water and various dissolved minerals, including tungsten, calcium, and other elements required for scheelite formation. The hydrothermal fluids are rich in complex ions, such as tungstate (WO42-), which is crucial for the subsequent precipitation of scheelite.
3. Fluid Migration: The hydrothermal fluids migrate through fractures, faults, and other permeable pathways in the surrounding rocks. The fluids may be driven by pressure differentials, gravity, or other geological processes. As the fluids move, they interact with the host rocks, exchanging ions and minerals.
4. Metasomatism and Replacement: When the hydrothermal fluids encounter suitable host rocks, metasomatism occurs. Metasomatism refers to the alteration of the host rocks due to the introduction of new chemical constituents from the hydrothermal fluids. In the case of scheelite, the tungsten-rich fluids react with calcium-bearing rocks, such as limestones or skarns, resulting in the replacement of calcium minerals with scheelite.
5. Precipitation: Under specific conditions of temperature, pressure, and chemical composition, scheelite begins to precipitate from the hydrothermal fluids. The tungstate ions combine with calcium ions in the fluids to form the solid scheelite mineral. This precipitation occurs within open spaces, fractures, or cavities in the host rocks, often forming distinctive crystal habits and aggregates.
6. Post-Depositional Processes: After the initial formation, scheelite may undergo subsequent geological processes such as recrystallization, deformation, and uplift due to tectonic activity. These processes can modify the appearance and distribution of scheelite within the host rocks.

It’s important to note that the specific geological formations and environments where scheelite occurs can vary. Scheelite is commonly associated with contact-metamorphic deposits, skarns, hydrothermal veins, and greisen-type deposits. The geological context and conditions during formation play a significant role in determining the characteristics of scheelite deposits and their economic viability for mining.

Worldwide distribution of scheelite deposits

Scheelite deposits can be found in various regions around the world. While tungsten-bearing deposits exist in many countries, not all of them have significant scheelite resources. Here are some notable regions with significant scheelite deposits:

1. China: China is the largest producer of tungsten and scheelite in the world. The country has several major scheelite deposits, including those in Jiangxi, Hunan, and Yunnan provinces. The Shizhuyuan Mine in Hunan is one of the largest scheelite mines globally.
2. Russia: Russia is another significant producer of scheelite. The major scheelite deposits are located in the Primorsky and Krasnoyarsk regions. The Tyrnyauz Deposit in the Kabardino-Balkaria Republic is one of the largest scheelite deposits in Russia.
3. Bolivia: Bolivia has notable scheelite deposits in the departments of La Paz, Cochabamba, and Oruro. The Pasto Bueno Mine in the Potosí department is one of the significant scheelite mines in Bolivia.
4. South Korea: South Korea has scheelite deposits in various regions, including Gangwon Province and North Gyeongsang Province. The Sangdong Mine in Gangwon Province is a historically important scheelite mine.
5. Australia: Australia has scheelite deposits in Western Australia, Queensland, and New South Wales. The King Island Scheelite Mine in Tasmania was one of the largest scheelite mines in the world during its operation.
6. Peru: Peru has scheelite deposits in different regions, including Pasco, Junin, and Huancavelica. The Pasto Bueno Mine, mentioned earlier, is located in Peru but extends into Bolivia.
7. Canada: Canada has scheelite deposits in the Northwest Territories and Yukon. The Cantung Mine in the Northwest Territories is one of the largest and highest-grade scheelite deposits in North America.
8. United States: The United States has scheelite deposits in several states, including California, Nevada, Colorado, and Arizona. The Pine Creek Mine in California and the King Island Scheelite Mine in Tasmania are operated by a U.S.-based company.
9. Other Countries: Scheelite deposits can also be found in other countries, including Austria, Portugal, Germany, Myanmar, Brazil, Uzbekistan, and Thailand, among others. However, the production levels and significance of these deposits may vary.

It’s important to note that the availability and accessibility of scheelite deposits can change over time due to factors such as mining operations, economic considerations, and geological exploration. Therefore, the distribution and importance of scheelite deposits may evolve as new discoveries are made and mining activities continue.

Industrial Uses of Scheelite

Scheelite is primarily valued for its high tungsten content, and tungsten has a wide range of industrial applications due to its exceptional physical properties. Some of the key industrial uses of scheelite and tungsten derived from scheelite include:

1. Hard Metals and Alloys: Tungsten is a crucial component in the production of hard metals, such as tungsten carbide (WC). Tungsten carbide is exceptionally hard and wear-resistant, making it ideal for applications like cutting tools, drilling bits, mining tools, and metalworking dies. Scheelite serves as a vital source of tungsten for the production of these materials.
2. Steel and Alloys: Tungsten is alloyed with steel to enhance its strength, hardness, and heat resistance. Tungsten steel alloys find applications in the manufacturing of high-speed steels, tool steels, and structural steels. These alloys are used in cutting tools, aircraft parts, armor-piercing projectiles, and high-temperature applications.
3. Filaments and Electrodes: Tungsten is commonly used in the production of filaments for incandescent light bulbs, fluorescent lamps, and electron guns in television and X-ray tubes. The high melting point and electrical conductivity of tungsten make it suitable for these applications. Scheelite serves as a significant source of tungsten for these industries.
4. Electrical Contacts: Scheelite-derived tungsten is utilized in electrical contacts, particularly in the form of tungsten alloys. Tungsten-based contact materials are valued for their high electrical conductivity, low contact resistance, and excellent resistance to arcing and welding. These materials are used in switches, relays, circuit breakers, and other electrical devices.
5. Radiation Shielding: Tungsten has excellent radiation shielding properties due to its high density and atomic number. Scheelite-derived tungsten is utilized in the production of radiation shielding materials, such as collimators, radiation therapy equipment, and nuclear shielding components.
6. Aerospace and Defense Applications: Tungsten alloys derived from scheelite find applications in the aerospace and defense industries. These alloys are used in the production of missile components, armor plating, turbine blades, and high-temperature structures where strength and durability are essential.
7. Catalysts: Tungsten compounds derived from scheelite are utilized as catalysts in various chemical processes. For example, tungsten oxide catalysts are used in the production of sulfuric acid and in selective oxidation reactions.

These are just a few of the prominent industrial uses of scheelite and tungsten. The unique properties of tungsten derived from scheelite make it indispensable in several key industries, ranging from manufacturing and engineering to electronics and defense.

Scheelite Gemstone

Scheelite, with its attractive colors and unique fluorescence, can be used as a gemstone. While it is not as well-known or widely used as other gemstones, scheelite has its own appeal and beauty. Here are some key aspects related to scheelite as a gemstone:

1. Color and Luster: Scheelite can exhibit various colors, including white, gray, brown, and yellowish-brown. Its vitreous to adamantine luster gives it a brilliant shine when cut and polished.
2. Transparency: Scheelite is generally translucent to opaque, meaning that light may pass through the mineral to varying degrees, but it is not transparent enough to see through clearly.
3. Fluorescence: One of the most distinctive features of scheelite as a gemstone is its fluorescence under ultraviolet (UV) light. Scheelite can emit a bright blue or yellow glow when exposed to UV light. This fluorescence property can enhance the visual appeal of scheelite gemstones and make them particularly intriguing.
4. Cut and Shape: Scheelite gemstones are typically cut into various faceted shapes to enhance their brilliance and light reflection. The gemstones can be fashioned into traditional cuts such as round, oval, pear, or emerald cuts, as well as more unique and customized shapes.
5. Durability: Scheelite has a hardness of 4.5 to 5.5 on the Mohs scale, which makes it relatively durable for everyday wear. However, compared to gemstones with higher hardness, such as sapphires or diamonds, scheelite may be more susceptible to scratches and abrasion.
6. Rarity: While scheelite is not as rare as some other gemstones, high-quality scheelite gemstones with desirable color and fluorescence can still be relatively uncommon to find in the market. This rarity can add to the allure and value of scheelite as a gemstone.

It’s important to note that due to its lower hardness and sensitivity to certain chemicals, scheelite may require extra care and precautions when it comes to cleaning, storing, and wearing as a gemstone. Consulting with a professional gemologist or jeweler can provide specific guidance on caring for scheelite gemstones.

Overall, scheelite’s unique colors, fluorescence, and relative rarity contribute to its appeal as a gemstone, offering a distinctive and eye-catching alternative for those seeking something less conventional in their jewelry collection.

Conclusion

In conclusion, scheelite is a fascinating mineral with a range of notable characteristics and applications. Its chemical composition, primarily consisting of calcium tungstate, gives it a high tungsten content, making it an essential source of this valuable metal. Scheelite is primarily known for its industrial uses, particularly in the production of hard metals, steel alloys, filaments, electrical contacts, radiation shielding, and catalysts.

Beyond its industrial significance, scheelite can also be appreciated as a gemstone. With its diverse colors, vitreous luster, and unique fluorescence under UV light, scheelite gemstones possess their own aesthetic appeal. Although not as widely recognized as other gemstones, scheelite’s rarity and distinctive features make it an intriguing option for jewelry enthusiasts seeking something different.

The geological formation of scheelite involves hydrothermal processes and the interaction of hot fluids with specific rock types. Scheelite deposits can be found worldwide, with notable occurrences in China, Russia, Bolivia, South Korea, Australia, Peru, Canada, and other countries.

Understanding the chemical composition, structure, physical properties, occurrence, mining, and industrial uses of scheelite provides a comprehensive overview of this mineral. Whether it is being used in industrial applications or admired as a gemstone, scheelite continues to play a valuable role in various sectors and captivate those who appreciate its unique qualities.

FAQs

What is the chemical formula of scheelite?

The chemical formula of scheelite is CaWO4, indicating that it is composed of calcium (Ca), tungsten (W), and oxygen (O).

Can scheelite be used in jewelry?

Yes, scheelite can be used as a gemstone in jewelry. Its attractive colors and fluorescence under UV light make it an interesting choice for unique and eye-catching jewelry pieces.

How does scheelite fluoresce?

Scheelite fluoresces when exposed to ultraviolet (UV) light. It emits a bright blue or yellow glow, depending on the impurities present in the crystal lattice.

Is scheelite a rare mineral?

While scheelite is not as rare as some other minerals, high-quality scheelite specimens with desirable characteristics can be relatively uncommon to find.

What are the primary industrial uses of scheelite?

Scheelite is primarily used as a source of tungsten, which is important in the production of hard metals, steel alloys, electrical contacts, filaments, radiation shielding, catalysts, and various other industrial applications.

Where are the largest scheelite deposits found?

The largest scheelite deposits are found in China, particularly in the provinces of Jiangxi, Hunan, and Yunnan. China is the leading producer of scheelite worldwide.

How is scheelite mined?

Scheelite is typically mined through traditional underground or open-pit mining methods. The specific mining technique used depends on factors such as the deposit size, depth, and economic considerations.

What is the hardness of scheelite?

Scheelite has a hardness of 4.5 to 5.5 on the Mohs scale. This means that it can be scratched by harder minerals but can scratch minerals with a lower hardness.

Can scheelite be found in the United States?

Yes, scheelite deposits can be found in the United States. Some notable occurrences are in states such as California, Nevada, Colorado, and Arizona.