Wollastonite (mineral)
Wollastonite is a naturally occurring mineral composed of calcium, silicon, and oxygen, with the chemical formula CaSiO3. It typically forms through contact metamorphism, particularly when granite intrudes into limestone or dolomite, resulting in skarn deposits. Significant deposits of wollastonite are found in regions including the United States, Mexico, Canada, China, and India, making it widely accessible for industrial use. This mineral is known for its distinctive needle-like crystals and is primarily utilized as a functional filler in various applications, such as ceramics, plastic polymers, and paints.
The industrial versatility of wollastonite has increased, especially following the decline of asbestos use due to health concerns. It enhances the strength, durability, and corrosion resistance of products in the ceramics and plastics industries, while also being utilized in construction materials and metallurgical processes. Additionally, wollastonite has medical applications, particularly in the production of bone cements and dental implants, as it supports natural bone growth. Its unique properties, including low ignition loss and potential fluorescence, make it a valuable mineral in both industrial and scientific contexts.
Wollastonite (mineral)
Where Found
Wollastonite is a common mineral; large deposits worldwide have been formed by contact metamorphism where granite intrudes into limestone rocks. Deposits formed in this way exist in the United States (California and New York), Mexico, Canada (Quebec and Ontario), China, and India. Wollastonite is found in volcanic ash and is formed by some volcanoes as molten lava interacts with limestone strata.
Primary Uses
Wollastonite is widely used in industry as functional filler. Most commercially mined wollastonite is used in the production of plastic polymers and ceramics. It is used in production of nylon 6 and polypropylene. Wollastonite is used in paints, construction products, metallurgical processes, and the production of medical bone cements and dental implants. Ground wollastonite is used in remedial treatment of soils contaminated by acid deposition. Wollastonite has also been used in friction products, including brake-drum linings.
Technical Definition
Wollastonite is a mineral composed of calcium, oxygen, and silicon, with a chemical composition CaSiO3, sometimes called calcium inosilicate or calcium silicate. Wollastonite has triclinic symmetry, forming long, needlelike crystals with splintery or uneven fractures. It has a Mohs hardness of 5, similar to the mineral apatite and harder than fluorite. It is soluble in concentrated hydrochloric acid. Some wollastonite specimens fluoresce yellow or orange under short-wave ultraviolet light. It has a very low loss on ignition (LOI). Pure wollastonite may be transparent and colorless or translucent and white; impurities cause a pink, brown, or green coloration. Its streak coloration is white.
Description, Distribution, and Forms
Wollastonite is a contact metamorphic mineral formed when granitic magma intrudes into limestone or dolomite strata, creating what geologists call a “skarn” deposit. Skarn deposits of wollastonite usually contain garnets and other minerals which are removed and discarded during ore processing. Wollastonite is a common mineral, with exploitable deposits known from many locales around the world. At the close of the twentieth century, most of the world’s supply was mined in China, the United States, and India, with an unknown amount mined and used in Russia. Mining operations by NYCO in Mexico were expected to supply large amounts of wollastonite to the world market in the early twenty-first century.
The mineral wollastonite is composed of long, single chains of silicates, sometimes referred to as “pyroxenoid” structures. Tetrahedral groups of silica are linked together by calcium cations to form a long, crystal chain. Occasionally atoms of iron, manganese, or magnesium replace calcium in the crystal matrix. These impurities cause pastel colorations of yellow, pink, and green.
Excellent wollastonite crystals are on display in most large science museums worldwide. Many specimens displayed in museums have grown in vugs (cavities inside rocks), allowing clear, perfect, large crystals of wollastonite to form. Certain volcanic localities, including Monte Somma/Vesuvius, are renowned for wollastonite-filled vugs. Wollastonite chunks are often used in museum displays of fluorescent minerals, where under ultraviolet light (“black light”) an otherwise unimpressive rock produces a bright orange coloration. Wollastonite specimens from Franklin, New Jersey, are especially well known from U.S. museum displays and sought by private collectors because of their orange fluorescence.
The aspect (length-to-width) ratio of wollastonite crystals determines the mineral’s application and hence value. Powders, with low aspect ratios of 3:1, are used in metallurgy and in paints and coatings. Wollastonite with high aspect ratios (ranging between 10:1 and 20:1) is more expensive than low-aspect wollastonite.
History
Prior to 1822, the mineral wollastonite was known in England as table spar or tabular spar. It was renamed at that time for William Hyde Wollaston, who had investigated its physical and chemical properties in the late eighteenth century.
Mineral specimens of wollastonite for scientific display and comparison were collected in the nineteenth century, when geologists used both “wollastonite” and “table spar” to refer to field specimens. Bustamite (CaMnSi2O6)—a pink or red-colored inosilicate mineral similar to wollastonite, but with manganese alternating with calcium in the mineral lattice—was referred to as “manganese wollastonite” by mineralogists during the nineteenth and early twentieth centuries. Bustamite may be found with wollastonite in metamorphic rocks.
In the United States, wollastonite was commercially mined beginning in 1933, in California, to use as mineral wool. Following the Korean War, the building boom required increasing amounts of wollastonite-containing construction products.
In 2006, about 450,000 metric tons of refined wollastonite were sold worldwide, with China the world’s largest producer, followed by the United States and India. Demand for wollastonite grew during the last several decades of the twentieth century, because it became a substitute for asbestos (which posed health risks and hence was not mined in the United States after 2002) and because of its versatility as inexpensive functional filler in paints, coatings, polymers, and ceramics. In the early twenty-first century, interest in using wollastonite in bone cements and other biomedical applications grew.
Obtaining Wollastonite
The extensive wollastonite deposits commercially exploited were all formed when granite intruded into limestone or dolomite, causing large-scale contact metamorphism. Wollastonite is commonly strip-mined, with some large open-pit operations and a few underground mines using drill-and-blast methods.
Small amounts of synthetic wollastonite are produced by sintering ground silica and calcite in rotary kilns. The wollastonite crystals produced are short (have low aspect ratios) and powdery. Synthetic wollastonite costs more to produce than the natural mineral, and its use is usually restricted to metallurgical and ceramic processes that require very pure and extremely uniform crystals.
Uses of Wollastonite
Wollastonite is a versatile nonmetallic mineral with many different industrial uses. Its industrial applications expanded dramatically following the discovery of the carcinogenicity of the mineral asbestos, which had been banned in most countries by the mid-1980’s. The U.S. Occupational Safety and Health Administration (OSHA) recognizes wollastonite dust particles as an irritant, associated with reduced pulmonary function when inhaled, but not as a carcinogen. Addition of wollastonite to products increases their strength and alkalinity, enhancing corrosion resistance.
The ceramics industry accounts for about 25 to 30 percent of the wollastonite used within the United States. The needlelike crystalline structure, LOI, and white coloration are important attributes that wollastonite contributes to ceramics. It is widely used as filler in ceramics, and in ceramic glazes, frits, and enamels, where it is well known for minimizing shrinkage and crazing. Wollastonite is used in the production of ceramic electrical insulators.
Wollastonite is widely used in plastic polymers and elastomers, where it adds structural strength. Within the United States, about 35 to 40 percent of wollastonite is consumed by the plastics industry. It is used in automotive plastics.
In metallurgical applications, wollastonite is used in molds for casting aluminum and continuous casting of steel, where it absorbs impurities. Wollastonite is also used as a welding flux and as a slag conditioner.
In the construction industry, high-aspect-ratio wollastonite is used in to enhance strength and durability. It is used as filler in portland cement and in the production of wallboard. It is used in roofing materials. It is also used as backing for linoleum. Addition of wollastonite to latex paint increases mildew resistance. Wollastonite used in paint and coatings is sometimes treated with silane.
Wollastonite-containing medical products are able to act as a substrate for natural bone growth. Wollastonite-containing ceramics, including Bioglass® and Cerabone® AW, can bond with living bone and are increasingly used in implants.
Applications of wollastonite have been explored for tertiary wastewater treatment and for treating acidified soils. A high-pH slurry of water and ground wollastonite applied to environments contaminated by high levels of acid deposition increases foliage growth and promotes plant germination.
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