Wollastonite - CaSiO

Wollastonite is a form of calcimn metasilicate. It is usually white, but it can exist in rocks alongside quartz, calcite, diopside, garnet, or prehnite, and when contaminated by trace elements it could be grey, brown, cream, yellow or pink. The contaminants can be removed to 

The plastics industry is wollastonite s largest single customer, accounting for well over a third of all wollastonite consumption. It is valued as a way of reinforcing plastics, improving the mechanical properties and the thermal and dimensional stability of finished products. 

The particles are needle-shaped and fairly hard, with a tendency to abrasiveness. Low and high aspect ratio grades are available but the plastics industry much prefers grades with high aspect ratios between 12 and 20. 

Wollastonite siufaces react with water and hydrolyse to form calcium hydroxide. The mineral is attacked by strong mineral acids, especially hydrochloric acid, and by certain organic acids. Siuface treatment can be carried out with silanes, titanates or zircoaluminates. 

Although wollastonite is a constituent of many types of rock, it is rarely found in an economically processable form. The majority of deposits are formed by contact metamorphism of limestone and quartz. Extraction is concentrated on distinct, fairly pure, layers of wollastonite-rich rock. Worldwide production is currently about 250,000 tonnes per year, with China, Finland, India, Mexico and USA being the main producing countries. 

Numerous surface treatments are used to improve dispersion, particle alignment and particle to polymer adhesion. They can also aid in reducing the abrasiveness of the hard particles to processing equipment, especially dies. Silane coupling agents are very effective and are widely used. Stearate coatings have also been reported to have some applications, but have not been specified by the wollastonite producers. 

The polymer applications of wollastonite are mainly based on its combination of acicularity, chemical inertness, whiteness and low water adsorption. Primary consuming polymers include thermoplastics, such as polypropylenes and polyamides, and thermosets, snch as polyester dough and sheet-moulding compounds, epoxies and phenolics. Manufacturers are predicting considerable growth in reinforced reaction injection moulding (RRIM) systems. 

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Muller, W. F., Wenk, H.-R. (1975). Transmission electron microscopic study of wollastonite (CaSiOs). Acta Crystallogr., A31, suppl. S294. 

B. The increase in resolution possible for O in wollastonite (CaSiOs) comparing static, MAS and DOR spectra from Wu, Sun and Pines (1990) with permission of the copyright owner. 

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