Kyanite, andalusite and sillimanite are polymorphic aluminum-silicate minerals with the same chemical formula, Al2SiO5, but different atomic structures and physical properties. Mullite is a closely related mineral with the chemical formula Al6Si2O13 that occurs rarely in nature, but can be synthesized from kyanite and other alumina- and silica-enriched minerals using a thermal treatment process referred to as calcination. Collectively, these minerals along with topaz (Al2SiOF2) and dumortierite (Al7(BO3)(SiO4)3O3) belong to the sillimanite minerals group, which are valued as industrial raw materials for the manufacture of heat and acid refractories for ceramics industries, precision castings, refractory additives and fillers, and other applications in the ferrous and non-ferrous foundry industries. Due to the qualities of hardness, durability, and resistance to heat and chemical corrosion, exceptional specimens of the sillimanite minerals group are often marketed in the gemstone and jewelry industry.
The sillimanite minerals are common constituents in metamorphic rocks, typically making up a percent or two of the mineral composition of peraluminous gneisses and schists. Economic deposits are found in a variety of geologic settings that include massive segregations in metamorphosed aluminous sediments, as stratiform replacements within foliated and nonfoliated quartzose rocks often associated with meta-volcanic strata, mineralized quartz veins and pegmatites, and in residual soils and placers. Andalusite also occurs in hornfels and other thermally-altered pelitic rocks within contact metamorphic aureoles adjacent to intrusive stocks. In the United States, economic and sub-economic concentrations of sillimanite minerals are known to occur in the western states of Alaska, California, Idaho, Nevada, and New Mexico as well as the Appalachian regions of Georgia, North Carolina, South Carolina and Virginia. Significant occurrences of kyanite and sillimanite are found in heavy mineral sand deposits in Florida. Outside of the United States, mining and/or potentially significant resources of sillimanite minerals have been reported in Australia, Brazil, Canada, China, France, India, Kenya, Nigeria, Norway, Peru, Russia, South Africa, Ukraine, and Zimbabwe.
The sillimanite minerals are valued in the manufacture of refractories and ceramic products due to their high alumina content (>55 percent Al2O3), volumetric stability at high temperatures, resistance to thermal shock, low thermal conductivity, resistance to chemical corrosion, electrical insulation capabilities, among other properties. Each of the minerals in the group is characterized by specific expansion and thermal properties when calcined to produce a mixture of mullite and residual silica. At decomposition temperatures ranging from as low as 1,250 °C for kyanite up to 1,650°C for sillimanite the percent volume expansion is both predictable and irreversible. Calcined kyanite will expand in volume by an amount that is dependent on initial particle size. Very fine particles (325-mesh) increase volumetrically by about 2 percent, while coarser particle fractions (35-mesh) can potentially expand up to about 25 percent in volume. Calcined andalusite and sillimanite expand in volume by about 6 percent and 4 percent, respectively. Below temperatures of decomposition, the minerals are characterized by relatively low coefficients of thermal expansion.
Iron- and steel-manufacturing industries are the principal consumers of refractories manufactured from the sillimanite minerals. These industries require high temperature refractory linings for metallurgical furnaces and other high performance heat and corrosion resistant materials. Refractory products include monolithics, firebrick, mortars, kiln furniture, and investment casting shell molds. In the steel and foundry industries, as well as other metallurgical and glass applications requiring extreme durability, temperature, and corrosion resistance, mullite and calcined kyanite products are often used exclusively. Other important end products include glazes and chamotte for porcelain and sanitary ware, electrical insulators, heating elements, ceramic tiles, brake shoes, and spark plugs.
Kyanite Mining Corporation (KMC), a privately owned company based in Dillwyn, Virginia is the world’s largest producer of industrial kyanite and calcined kyanite (mullite). The company mines kyanite-bearing quartzites associated with felsic and mafic volcanic rocks of the Chopawamsic Formation in the central Virginia Piedmont region. Of the vast resources located at Willis Mountain, kyanite generally makes up 10-40 percent of the host quartzite rock. Current surface mining operations and processing facilities are located in eastern Buckingham County near the town of Dillwyn. KMC states the annual production capacity at the Virginia operations to be about 150,000 tons (short tons) of commercial grade kyanite concentrates (>57 percent Al2O3, <0.75 percent Fe2O3), and 30,000 tons of calcined kyanite. The company markets a range of milled kyanite and mullite products that are shipped by truck and rail to a wide variety of domestic and international customers. Exports currently account for about one-half of KMC’s business. In the annual production report to the Department of Mines, Minerals and Energy, KMC reported about 88,000 tons of combined kyanite and calcined kyanite in 2016.
Annual production reported by mine operator
One of Kyanite Mining Corporation’s processing facilities at Willis Mountain, Buckingham County.
Selected References:Conley, J. F., and Marr, J. D., 1980, Evidence for the correlation of the kyanite quartzites of Willis and Woods mountains with the Arvonia Formation: Virginia Division of Mineral Resources Publication 27, p. 1-11.
Dixon, G. B., Jr., 1980, Kyanite mining in Virginia: Virginia Division of Mineral Resources, Virginia Minerals, v. 26, n. 1, p. 12.
Johnson, S. S., 1967, Virginia’s contribution to the kyanite – mullite industry: Virginia Division of Mineral Resources, Virginia Minerals, v. 13, n. 1, p. 1-7.
Jonas, A. I., and Watkins, J. H., 1932, Kyanite in Virginia, (Including) Geology of the kyanite belt of Virginia: Virginia Division of Mineral Resources Bulletin 38, 52 p.
Marr, J. D., 1990, Geology of the Kyanite Deposits at Willis Mountain, Virginia, in Proceedings 26th Forum on the Geology of Industrial Minerals: Virginia Division of Mineral Resources Publication 119, p. 129-134.
Tanner, A. O., 2013, Kyanite and Related Minerals (Advance Release): U.S. Geological Survey 2012 Minerals Yearbook, August 2013.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.