Wollastonite is a common mineral in skarns or contact metamorphic rocks. Skarns can sometimes produce some wonderfully rare and exotic minerals with very unusual chemistries. However, wollastonite has no unusual elements in its chemistry and it is somewhat common and not considered very exotic among collectors. Wollastonite forms from the interaction of limestones, that contain calcite, CaCO3, with the silica, SiO2, in hot magmas. This happens when hot magmas intrude into and/or around limestones or from limestones chunks that are broken off into the magma tubes under volcanoes and then blown out of themAlthough not an "exotic" mineral, wollastonite has its uses. It is an important constituent in refractory ceramics (those ceramics that are resistant to heat) such as refractory tile and as a filler for paints. It is easily mined in some places where it is the major component of the metamorphosed rock.
Plastics
In plastics, wollastonite improves the durability of the composite due to its acicular or needle-like structure. It also enhances electrical insulating properties, adds fire resistance, and improves dimensional stability. Finer particle size grades provide improved scratch and impact resistance compared to other materials. The application of surface treatments like silanes on the wollastonite substrate changes the mineral from a utilitarian filler to a functional component of a polymer composite. This in turn adds performance values which the base resin alone does not possess. Optimum performance is created by properly matching the correct coupling agent at the right concentration level to the polymer system. Overall, the benefits of a surface modified wollastonite are improved physical properties, improved processing along with improved dispersion of the resin.
Construction
Wollastonite has found application as a substitute for asbestos in fire-resistant building products used in the construction industry. As a functional additive, wollastonite improves flexural and impact strengths. Its low thermal conductivity and high aspect ratio structure also makes wollastonite an attractive addition for applications requiring fire resistance. Wollastonite finds application in interior and exterior construction boards, roof tiles, shaped insulation products, sheets, panels and sidings.
Paints and Coatings
In coatings, fine acicular particles act as a good flattening agent and allow paint to settle out after application to produce a dry film of uniform thickness, and the interlocking particles improve toughness and durability of the coating with excellent tint retention, scrub, and weather resistance. High brightness and whiteness reduce pigment load and typically very low oil absorption, reduces the volume of binder required and contributes to reduced pigment costs. As noted earlier, wollastonite can also act as a pH buffer for improved in-can paint stability over long periods of time. The acicular structure and alkaline nature of wollastonite also make it an ideal auxiliary pigment in industrial coatings and primers for improved corrosion resistance.
Ceramics
Wollastonite can be used in a variety of ceramic applications including ceramic glazes and bodies, enamels, frits, fluxes and in sanitary-ware. This industrial mineral is a source of CaO to alkaline glaze formulations to improve the strength of the glaze. Wollastonite also has a low LOI (< 1%) which gives minimal gas evolution during firing resulting in a smooth surface with diminished pinholing. Due to its acicular structure, wollastonite improves green strength and reduces crazing and checking especially during rapid heating and cooling. It has a low sintering temperature (approximately 991-1196C) which is comparable to that of most natural frits.
Metallurgical
In metallurgical applications, wollastonite is used due to its low water solubility, low loss on ignition and its Ca-Si ratio. Wollastonite is commonly added to formulated powders for steel casting and welding. A CaO/SiO2 ratio of 1 readily absorbs Al2O3 that is detrimental to finished steels. The addition of wollastonite to metallurgical fluxes provides ready fusibility, good insulating qualities and low viscosity. Because wollastonite is a natural, low temperature flux, it has found acceptance in fluxing formulations used in the continuous casting of steel. When molten steel is poured continuously from a ladle or tundish, a casting powder is applied to maintain surface defects, prevents oxidation of the steel, lubricates the mold wall and absorbs harmful inclusions.
Friction
Due to its physical structure and non-hazardous properties, wollastonite is used as a reinforcing additive in many friction applications. It is also a partial/full replacement for asbestos, milled fibres, chopped glass and synthetic materials. Its primary application is in semi-metallic and nonasbestos formulations for truck blocks, drum linings, disc pads and friction paper. Benefits of using wollastonite in a friction formulation include improved physical and mechanical properties, reduction or elimination of cracking, improved pre-form strength, improved friction stability, noise reduction and reduced raw material and unit cost (density).
