Wollastonite

Wollastonite is a group of innosilicate mineral, formula is CaSiO3 that may inlude small amount of magnesium, manganese and iron substituting for calcium. A valuable industrial mineral, wollastonite is white, gray, or pale green in color. It occurs as rare, tabular crystals or massive, coarse-bladed, foliated, or fibrous masses. Its crystals are usually triclinic, although its structure has seven variants, one of which is monoclinic. These variations are however, indistinguishable in hand specimens. Wollastonite forms as a result of the contact metamorphism of limestones and in igneous rocks that are contaminated by carbon-rich inclusions. It can be accompanied by other calcium containing silicates, such as diopside, tremolite, epidote, and grossular garnet. Wollastonite also appears in regionally metamorphosed rocks in schists, slates, and phyllites. It forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns or in contact with metamorphic rocks.

Name: For William Hyde Wollaston (1766{1828), English chemist and mineralogist.

Association: Calcite, grossular, diopside, vesuvianite, akermanite, merwinite, larnite, spurrite

Polymorphism & Series: 1A, 2M, 3A, 4A, 5A, 7A polytypes

Mineral Group: Wollastonite Group

Chemical Properties

Chemical Classification	Inosilicate mineral
Formula	CaSiO3

Wollastonite Physical Properties

Color	White, gray-white, light green, pinkish, brown, red, yellow
Crystal habit	Rare as tabular crystals—commonly massive in lamellar, radiating, compact and fibrous aggregates.
Streak	White
Luster	Vitreous, Pearly
Cleavage	Perfect Perfect on {001} Good on {001} and {102}
Diaphaneity	Transparent, Translucent
Mohs Hardness	4,5 – 5
Crystal System	Triclinic
Tenacity	Brittle
Density	2.86 – 3.09 g/cm3 (Measured)    2.9 g/cm3 (Calculated)
Fracture	Irregular/Uneven
Other characteristics	Heat of Formation (@298): -89.61kJ Gibbs Free Energy: 41.78kJ
Melting point	1540 °C

Wollastonite Optical Properties

2V:	Measured: 36° to 60°
RI values:	nα = 1.616 – 1.640 nβ = 1.628 – 1.650 nγ = 1.631 – 1.653
Optic Sign	Biaxial (-)
Birefringence	δ = 0.015
Relief	Moderate
Dispersion:	weak r>v
Twinning	Common

Occurrence of Wollastonite

Common in thermally metamorphosed siliceous carbonates, the intruding igneous rock, and skarn deposits along their contact; also in some alkalic igneous rocks and and carbonatites.

Wollastonite Uses Area

• Wollastonite has industrial importance worldwide. In many sectors, it is often used involved in the manufacture of ceramics to improve many performance parameters, and this is due to yield properties, release from volatile components, whiteness and acicular particle form.
• In ceramics, wollastonite reduces shrinkage and gas formation during firing, increases green and firepower, maintains brightness during firing, allows rapid firing and reduces crazing, cracking and glaze defects.
• In metallurgical applications, wollastonite is used to protect the surface of the molten metal during a flux for welding, a source of calcium oxide.
• As an additive to the paint, it increases the durability of the paint film, acts as a pH buffer, increases weather resistance, reduces gloss, reduces pigment consumption and acts as a smoothing and suspending agent.
• In plastics, wollastonite increases tensile and bending strength, reduces resin consumption and increases thermal and dimensional stability at high temperatures. Surface treatments are used to improve adhesion between wollastonite and the polymers to which it is added.
• Instead of asbestos in floor tiles, friction products, insulation boards and panels, paint, plastic and roofing products, wollastonite is resistant to chemical attack, is stable at high temperatures and increases bending and tensile strength.
• In some industries, it is used at different impurity rates, such as being used as a manufacturer of mineral wool insulation or as an ornamental building material.

The price of raw wollastonite varied in 2008 between US$80 and US$500 per tonne depending on the country and size and shape of the powder particles.

Composition

In a pure CaSiO3, each component constitutes almost half of the mineral: 48.3% CaO and 51.7% SiO2. In some cases, a small amount of iron (Fe) and manganese (Mn) and a smaller amount of magnesium (Mg) are used instead of calcium (Ca) in the mineral formula (eg, Rhodonite). [9] Wollastonite may form a series of solid solutions in the hydrothermal synthesis of phases in the CaSiO3-FeSiO3 system or in the MnSiO3-CaSiO3 system.

Production

World production data for Wollastonite are not available in most countries and are often available from 2 to 3 years of age. Estimated production of crude wollastonite ore in the world in 2016 ranged from 700,000 to 720,000 tons. The world’s wollastonite reserve is estimated to exceed 100 million tons. However, many large deposits have not yet been explored.

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

In 2016, major producers were China (425,000 tons), India (185,000 tons), the United States (for commercial reasons third place), Mexico (67,000 tons) and Finland (16,000).

The price of raw wollastonite varied in 2008 between US$80 and US$500 per tonne depending on the country and size and shape of the powder particles.

Distribution of Wollastonite

A widely distributed mineral; some prominent localities are:

• in Romania, at Dognecea (Dognaczka) and Csiklova, Banat.
• In Italy, at Sarrabus, Sardinia, and from Monte Somma and Vesuvius, Campania. In Ireland, at Dunmorehead, Mourne Mountains, and Scawt Hill, near Larne, Co. Antrim.
• From Kongsberg, Norway.
• At GÄockum, Sweden.
• In Germany, at Harzburg, Harz Mountains, and Auerbach, Odenwald, Hesse.
• In the USA, at Natural Bridge and Diana, Lewis Co., New York; from Crestmore, Riverside Co., and Darwin, Inyo Co., California; in a large deposit two miles southeast of Gilbert, Esmeralda Co., Nevada.
• In Canada, at Oka and Asbestos, Quebec; at Outlet Post, Leeds Co., Ontario.
• From Pichucalo, Chiapas, and in the Pilares deposit, 55 km north of Hermosillo, Sonora, Mexico.
• At Hiiagiyama, Ibaragi Prefecture; Ishiyamadera, Shiga Prefecture; and Kushiro, Hiroshima Prefecture, Japan.
• Large crystals from Belafa, Madagascar.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].