Wollastonite

Wollastonite occurs as bladed crystal masses, single crystals can show an acicular particle shape and usually it exhibits a white color, but sometimes cream, grey or very pale green. The streak of wollastonite is white, its Mohs hardness is 4.5–5 and specific gravity is 2.87–3.09. There are more than one cleavage planes for it, there is a perfect cleavage on {100}, good cleavages on {001}, and {102}, and an imperfect cleavage on {101}. It is common for wollastonite to have a twin axis [010], a composition plane (100), and rarely to have a twin axis [001]. The luster is usually vitreous to pearly. The melting point of wollastonite is about 1540 ˚C.

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.

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.

Wollastonite is a calcium siliconate material with an enormous range of industrial applications. Its bright white coloring makes it popular for ceramics and tiles. It is also used in brakes and clutches, in paint, plastics, metal making and concrete. Particularly importantly, it is a replacement for asbestos in many products.

Large deposits of wollastonite have been identified in China, Finland, India, Mexico, and the United States. Smaller, but significant, deposits have been identified in Canada, Chile, Kenya, Namibia, South Africa, Spain, Sudan, Tajikistan, Turkey, and Uzbekistan.