Wollastonite

Wollastonite
General
Category	Silicate mineral
Formula (repeating unit)	Calcium silicate, CaSiO3
Strunz classification	09.DG.05
Crystal symmetry	Triclinic pinacoidal H-M symbol: 1 Space group: P1 (1A polytype)
Unit cell	a = 7.925 Å, b = 7.32 Å, c = 7.065 Å; α = 90.055°, β = 95.217°, γ = 103.42°
Identification
Color	White, colorless or gray
Crystal habit	Rare as tabular crystals—commonly massive in lamellar, radiating, compact and fibrous aggregates.
Crystal system	Triclinic, monoclinic polytype exists
Twinning	Common
Cleavage	Perfect in two directions at near 90°
Fracture	Splintery to uneven
Mohs scale hardness	4.5 to 5.0
Luster	Vitreous or dull to pearly on cleavage surfaces
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.86–3.09
Optical properties	Biaxial (-)
Refractive index	nα = 1.616–1.640 nβ = 1.628–1.650 nγ = 1.631–1.653
Birefringence	δ = 0.015 max
2V angle	Measured: 36° to 60°
Melting point	1540 °C
Solubility	Soluble in HCl, insoluble in water
References

Wollastonite is a calcium inosilicate mineral (CaSiO₃) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids as in skarns or contact metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Some of the properties that make wollastonite so useful are its high brightness and whiteness, low moisture and oil absorption, and low volatile content. Wollastonite is used primarily in ceramics, friction products (brakes and clutches), metalmaking, paint filler, and plastics.

Despite its chemical similarity to the compositional spectrum of the pyroxene group of minerals—where magnesium and iron substitution for calcium ends with diopside and hedenbergite respectively—it is structurally very different, with a third SiO₄ tetrahedron in the linked chain (as opposed to two in the pyroxenes).

Production trends

Wollastonite output in 2005

World production data for wollastonite is not available for many countries and those that are available frequently are 2 to 3 years old. Estimated world production of crude wollastonite ore was in the range of 530,000 to 550,000 tonnes in 2010. World reserves of wollastonite were estimated to exceed 90 million tonnes, with probable reserves of about 270 million tonnes. However, many large deposits have not been surveyed yet.

In 2010, the major producers were China (300,000 tonnes), India (120,000 t), United States (67,000 t), Mexico (30,000 t) and Finland (16,000). Finland has long been a major European supplier of wollastonite, but in 2003 it was joined by Spain with comparable production volumes. In the United States, wollastonite is mined in Willsboro, New York and Gouverneur, New York. Deposits have also been mined commercially in North Western Mexico.

Uses

Wollastonite has industrial importance worldwide. It is used in many industries, mostly by tile factories which have incorporated it into the manufacturing of ceramic to improve many aspects, and this is due to its fluxing properties, freedom from volatile constituents, whiteness, and acicular particle shape. 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. 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. As an additive in paint, it 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. 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. As a substitute for asbestos in floor tiles, friction products, insulating board and panels, paint, plastics, and roofing products, wollastonite is resistant to chemical attack, inert, stable at high temperatures, and improves flexural and tensile strength. In some industries, it is used in different percentages of impurities, such as its use as a fabricator of mineral wool insulation, or as an ornamental building material.

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. 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.

Substitutes

White acicular crystals of wollastonite (field of view 8 mm) from the Central Bohemia Region, Czech Republic

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 skarn with diopside (green), andradite garnet (red) and vesuvianite (dark brown) from the Stanisław mine near Szklarska Poręba, Izerskie Mountains, Lower Silesia, Poland.

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

Unit cell of triclinic wollastonite-1A

Tetrahedra arrangement within the chains in pyroxenes compared to wollastonite

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 neighbour 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.

Physical and optical properties

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.

See also

• List of minerals
• List of minerals named after people

References

 This article incorporates public domain material from the United States Geological Survey document: 

• http://www.minsocam.org/ammin/am79/am79_134.pdf

External links

• Oxford University MSDS sheet