wollastonite

Wollastonite
General
Category	Silicate mineral
Chemical formula	Calcium silicate, CaSiO3
Identification
Color	white, colorless or gray
Crystal habit	rare tabular crystals - commonly massive in lamellar, radiating, compact and fibrous aggregates.
Crystal system	triclinic 1
Twinning	common
Cleavage	perfect in two directions at near 90 degrees
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
Melting point	1540 °C
Solubility	soluble in HCl, insoluble in water
References	[1][2][3][4]

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^[5] in the linked chain (as opposed to two in the pyroxenes).

Production trends

Wollastonite output in 2005

In 2005, China was the top producer of wollastonite with at least 50% world share followed by India and the USA, reports the British Geological Survey.

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.^[6] It also can be used in paint, paper, and vinyl tile manufacture. 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.^[7]

Composition

There are two main constituents that form the mineral wollastonite: CaO and SiO₂. In a pure CaSiO₃, each component forms nearly half of the mineral by weight percentage. The CaO has a weight percentage of 48.3 %, and the SiO₂ has a weight percentage of 51.7%. In some cases, small amounts of iron (Fe), and manganese (Mn), and lesser amounts of magnesium (Mg) substitute for calcium (Ca) in the mineral formule (e.g., rhodonite).^[7] Wollastonite can form a series of solid solutions in the system CaSiO₃ - FeSiO₃, or hydrothermal synthesis of phases in the system MnSiO₃ - CaSiO₃.^[6]

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 ^[6]

Wollastonite may also be produced in a diffusion reaction in skarn, it develops when limestone within a sandstone is metamorphosed by a dyke, which results in the formation of wollastonite in the sandstone as a result of outward migration of Ca.^[6]

Structure

Unit cell of triclinic wollastonite-1A

Tetrahedra arrangement within the chains in pyroxenes compared to wollastonite

Wollastonite crystallizes triclinically in space group 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.^[8] 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.^[6] 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.

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, and its hardness on mohs' scale ranges between 4.5-5. Its specific gravity ranges between 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 1,540˚C.

See also

• List of minerals
• List of minerals named after people

References

External links

• Oxford University MSDS sheet