Industrial Minerals of the United States Wollastonite–A Versatile Industrial Mineral What is Wollastonite? Wollastonite is a chemically simple mineral named in honor of English mineralogist and chemist Sir W.H. Wollaston (1766–1828). It is composed of calcium (Ca) and silicon and oxygen (SiO2, silica) with the chemical formula CaSiO3. Although much wollastonite is relatively pure CaSiO3, it can contain some iron, magnesium, manganese, aluminum, potassium, sodium, or strontium substituting for calcium in the mineral structure. Pure wollastonite is bright white; the geologic conditions during formation What Makes Wollastonite and host rock composition. The type and amount of impurities can Useful? produce gray, cream, brown, pale- Lewis Deposit, mined by NYCO green, or red colors.� Minerals, Inc., in the Adirondack Wollastonite has several physical Mountains in Essex County, was � properties that make it useful as an formed by the recrystallization of industrial mineral: Geology of U.S. Precambrian carbonate rocks inter- Wollastonite Deposits layered with high-grade metamor Wollastonite is largely inert, phic rocks. Nearby reserves are although it will dissolve in concen Wollastonite is formed by two contained in the Oak Hill and trated hydrochloric acid. It will not processes. The first occurs when Deerhead deposits. The ore bodies react with other components of silica and limestone are raised to a consist of the minerals wollastonite, manufactured products either during temperature of 400°–450°C, either garnet, and diopside with as much as or after the manufacturing process. because of deep burial (regional 60 percent of the bodies being During crushing, wollastonite metamorphism) or by being baked wollastonite. The heat required to breaks into lath- or needle-shaped because of their proximity to an recrystallize the carbonate rocks has (acicular) particles because of its igneous intrusion (skarn deposits), been attributed to either regional unique cleavage properties. This forming wollastonite and giving off metamorphism accompanying deep tendency is measured by the ratio of carbon dioxide: burial or thermal metamorphism the length to diameter of the parti SiO2 + CaCO3 = CaSiO3 + CO2 related to a nearby igneous intrusion. cles (aspect ratio). Small-aspect-ratio Silica + limestone = wollastonite + carbon dioxide In either case, the recrystallization particles of wollastonite have ratios The second way wollastonite was primarily of thermal origin with of around 3:1. High-aspect-ratio forms is by direct crystallization only minor movement of material particles reach ratios of 20:1. Par from molten rock (magma) that is into or out of the system (meta ticles with different aspect ratios unusually high in carbon content. somatism). have different applications. Being The origin of these magmas is The other currently commercial acicular, wollastonite (1) acts to controversial, but current thinking is wollastonite deposit is mined by the reinforce products such as plastics, that they probably originate in the R.T. Vanderbilt Co. in the Gouver paint films, and green ceramic bodies lower crust and upper mantle. The neur District, Lewis County, N.Y. much like reinforcing rods do in con rocks they form, called carbonatites, The deposit was formed by the crete and (2) promotes early release are scattered throughout the U.S., but recrystallization of siliceous Precam of gases in ceramic production. they have not been exploited for brian carbonate rocks by the contact The brightness and whiteness of commercial wollastonite. metamorphism and metasomatism pure wollastonite are important for The only two currently commer accompanying an igneous intrusion. some applications. Commercial cial wollastonite deposits in the U.S. The ore bodies consist of large pods wollastonite has brightness (reflect are in New York. These deposits of wollastonite, calcite, and graphite ance) values equal to 85–90 percent differ in their basic mineralogy, with accessory prehnite, magnetite, that of common reflectance stand reflecting both differences in and diopside. ards like magnesium oxide and barium sulfate. Being bright and replacement for asbestos in products, recognition in the 1970's of health white, wollastonite has a minimal including insulating board and panels, issues associated with asbestos, effect on a product's color, but it can paint, plastics, roofing tiles, and in wollastonite became a substitute for be used as a white pigment. friction devices such as brakes and short-fiber asbestos in many applica tions, and the U.S. and worldwide The amount of volatile material clutches. In ceramics, wollastonite markets have grown ever since. (gas) driven off when a material is reduces warping and cracking during firing and increases strength. Being a Another boost to the industry came heated to 1,000°C is called Loss on with the increase in demand for Ignition (LOI). The LOI of commer calcium silicate mineral, wollastonite wollastonite as a filler and extender cial wollastonite is very low, ranging contributes calcium in ceramic glaze for the rapidly growing plastics between 0.5 and 2.0 percent. The low mixes. Wollastonite is widely used as market. Today, plastics and ceramics LOI reduces problems of gas bubble a flux in the casting of steel and in the are the leading markets, followed by entrapment and gas dispersal during production of paints and coatings. In paint and asbestos substitutes. the firing of ceramics, lowers firing paints, wollastonite provides rein costs, and reduces firing time. forcement, hardening, low oil absorp tion, and other benefits. In textured The United States is the second Wollastonite has a high pH of 9.9 coatings, like stucco, wollastonite largest producer of wollastonite after in a 10 percent water slurry that can provides crack resistance, reinforce People's Republic of China. The stabilize acidity and reduce the ment, and high brightness. Wollas U.S. accounts for about 25 percent of breakdown of polyvinyl acetate in tonite is also used in the manufacture the world's 600,000 metric tons of paints. of adhesives, joint compounds, production. In general, domestic production grew steadily through the The crystal structure and physical refractories, and rubber. All grades of wollastonite are used in the pro 1990's, although the rate of growth properties of wollastonite are stable slowed toward the end of the decade to about 1,120°C, making duction of plastics, including nylons, phenolic molding compounds, due to competition from imports. wollastonite a substitute for asbestos � in thermal insulation applications. polyesters, and polyurethanes and polyureas. References Wollastonite has a hardness of Consult these reports for more detailed 4.5–5.5 on the Mohs scale. � information. � Processing Wollastonite Bauer, R.R., Copeland, J.R., and Santini, Ken, 1994, Wollastonite, To be useful for many different in Carr, D.D., ed., Industrial applications, wollastonite must be minerals and rocks, 6th Edition: processed; the processing should Littleton, Colo., Society for produce concentrates that are 97–99 Mining, Metallurgy, and percent pure wollastonite. After Exploration, Inc., p. 1119–1128. drilling, blasting, and partial crush ing at the mine site, the ore goes to a Virta, R.L., 2000, Wollastonite, in production plant for further crushing, Minerals Yearbook, metals and beneficiation, and milling. Crushing minerals 1998, Volume I: U.S. and milling must produce a range of Geological Survey, p. 83.1–83.2. particle sizes having a range of as pect ratios. The commonly associat Information ed minerals garnet and diopside are removed by high-intensity magnetic separators. Some of the material may For more on the U.S. wollastonite industry contact: be chemically treated to improve The U.S. Wollastonite Industry binding in resin-based products. Four Robert L. Virta classes of final products are: Wollastonite has been mined in the U.S. Geological Survey U.S. for more than 70 years. The Reston, VA general grades - powders of (703) 648-7726 < 400 mesh (38 micrometers) industry first started in California, where production continued until [email protected] fine grinds - > 400 mesh 1970. Production was only a few high-aspect-ratio grades (15– For more on U.S. wollastonite geology thousand metric tons per year for contact: 20:1) decorative stone, ceramics, mineral chemically modified grades wool, and paint. Today, wollastonite is Bradley S. Van Gosen mined only in Essex and Lewis U.S. Geological Survey Denver, CO Wollastonite's Many Uses Counties, N.Y. Wollastonite has been produced in New York for more than (303) 236-1566 Until the late 1970's, the primary 40 years. [email protected] use of wollastonite was as a decor Ceramic and paint markets ative stone. Since the early 1980's, traditionally were the mainstay of the Any use of trade, product, or firm names is for one of the uses has been as a descriptive purposes only and does not imply wollastonite industry. With the endorsement by the U.S. Government.