Wollastonite

Uses

Wollastonite is among the fastest reacting silicates, but may have high costs associated with carbon storage. Addition of wollastonite to soil stimulates organic carbon mineralization.

Ceramics

Wollastonite has industrial importance in ceramics manufacturing as an additive.

In ceramics, wollastonite decreases shrinkage and gas evolution during firing, increases green and fired strength, maintains brightness during firing, permits fast firing, and reduces crazing, cracking, and glaze defects.

Construction

Wollastonite can serve as a substitute for asbestos in floor tiles, friction products, insulating board and panels, paint, plastics, and roofing products. Similar to asbestos, wollastonite is resistant to chemical attack, stable at high temperatures, and improves flexural and tensile strength in composites. In some industries, wollastonite is used in different percentages of impurities, such as its use as a fabricator of mineral wool insulation, or as an ornamental building material. Wollastonite is used in a cement announced in 2019 which "reduces the overall carbon footprint in precast concrete by 70%."

Wollastonite has been studied for carbon mineralization for storage of carbon dioxide (CO₂) according to the following reaction:

CaSiO₃ + CO₂ → CaCO₃ + SiO₂

Metallurgy

In metallurgical applications, wollastonite serves as a flux for welding, a source for calcium oxide, a slag conditioner, and to protect the surface of molten metal during the continuous casting of steel.

Paint

As an additive in paint, wollastonite improves the durability of the paint film, acts as a pH buffer, improves its resistance to weathering, reduces gloss, reduces pigment consumption, and acts as a flatting and suspending agent.

Plastic

In plastics, wollastonite improves tensile and flexural strength, reduces resin consumption, and improves thermal and dimensional stability at elevated temperatures. Surface treatments are used to improve the adhesion between the wollastonite and the polymers to which it is added.

Plastics and rubber applications were estimated to account for 25% to 35% of U.S. sales in 2009, followed by ceramics with 20% to 25%; paint, 10% to 15%; metallurgical applications, 10% to 15%; friction products, 10% to 15%; and miscellaneous, 10% to 15%. Ceramic applications probably account for 30% to 40% of wollastonite sales worldwide, followed by polymers (plastics and rubber) with 30% to 35% of sales, and paint with 10% to 15% of sales. The remaining sales were for construction, friction products, and metallurgical applications.

Hydrogen production

Wollastonite is used in a dark fermentation process that produces hydrogen gas from organic substrates under oxygen-free conditions. The mineral acts as a dual-function agent to enhance yield while capturing CO
₂.

Substitutes

The acicular nature of many wollastonite products allows it to compete with other acicular materials, such as ceramic fiber, glass fiber, steel fiber, and several organic fibers, such as aramid, polyethylene, polypropylene, and polytetrafluoroethylene in products where improvements in dimensional stability, flexural modulus, and heat deflection are sought.

Wollastonite also competes with several nonfibrous minerals or rocks, such as kaolin, mica, and talc, which are added to plastics to increase flexural strength, and such minerals as barite, calcium carbonate, gypsum, and talc, which impart dimensional stability to plastics.

In ceramics, wollastonite competes with carbonates, feldspar, lime, and silica as a source of calcium and silicon. Its use in ceramics depends on the formulation of the ceramic body and the firing method.

Composition

In a pure CaSiO₃, each component forms nearly half of the mineral by weight: 48.3% of CaO and 51.7% of SiO₂. In some cases, small amounts of iron (Fe), and manganese (Mn), and lesser amounts of magnesium (Mg) substitute for calcium (Ca) in the mineral formula (e.g., rhodonite). Wollastonite can form a series of solid solutions in the system CaSiO₃-FeSiO₃, or hydrothermal synthesis of phases in the system MnSiO₃-CaSiO₃.

Geologic occurrence

Wollastonite usually occurs as a common constituent of a thermally metamorphosed impure limestone, it also could occur when the silicon is due to metamorphism in contact altered calcareous sediments, or to contamination in the invading igneous rock. In most of these occurrences it is the result of the following reaction between calcite and silica with the loss of carbon dioxide:

CaCO₃ + SiO₂ → CaSiO₃ + CO₂

Wollastonite may also be produced in a diffusion reaction in skarn, it develops when limestone within a sandstone is metamorphosed by a dike, which results in the formation of wollastonite in the sandstone as a result of outward migration of Ca.

Structure

Wollastonite crystallizes triclinically in space group P1 with the lattice constants a = 7.94 Å, b = 7.32 Å, c = 7.07 Å; α = 90,03°, β = 95,37°, γ = 103,43° and six formula units per unit cell. Wollastonite was once classed structurally among the pyroxene group, because both of these groups have a ratio of Si:O = 1:3. In 1931, Warren and Biscoe showed that the crystal structure of wollastonite differs from minerals of the pyroxene group, and they classified this mineral within a group known as the pyroxenoids. It has been shown that the pyroxenoid chains are more kinked than those of pyroxene group, and exhibit longer repeat distance. The structure of wollastonite contains infinite chains of [SiO₄] tetrahedra sharing common vertices, running parallel to the b-axis. The chain motif in wollastonite repeats after three tetrahedra, whereas in pyroxenes only two are needed. The repeat distance in the wollastonite chains is 7.32 Å and equals the length of the crystallographic b-axis.

Molten CaSiO₃ maintains a tetrahedral SiO₄ local structure at temperatures up to 2000 °C. The nearest neighbor Ca-O coordination decreases from 6.0(2) in the room temperature glass to 5.0(2) in the 1700 °C liquid, coincident with an increasing number of longer Ca-O neighbors.