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Wollastonite

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Wollastonite
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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 bar 1
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]

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

Production trendsEditar

Archivo:2005wollastonite.PNG

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.

UsesEditar

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.[5]> 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.[6]

CompositionEditar

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 molecules.[6] Wollastonite can form a series of solid solutions in the system CaSiO3-FeSiO3, or hydrothermal synthesis of phases in the system MnSiO3-CaSiO3.[5]

Geologic OccurrenceEditar

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 reaction: CaCO3+SiO2=>CaSiO3+CO2. [5] Wollastonite could also develop in a diffusion reaction 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.[5]

StructureEditar

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 unit cell of wollastonite differs from minerals in the pyroxene group, and they classified this mineral within a group known as the pyroxenoids.[5] 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 consist of infinite chains, parallel to the c axis, with a unit repeat of three twisted tetrahedra. The smallest repeat distance of the unit cell in the wollastonite chains is 7.1Å.[7]

Physical & Optical PropertiesEditar

Wollastonite has a triclinic crystal structure, with α=90˚, β=95˚16', γ=103˚22'. It also has bladed crystal masses, and acicular particles shape. Wollastonite usually 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 alsoEditar

ReferencesEditar

  1. http://www.mindat.org/min-4323.html Mindat
  2. http://webmineral.com/data/Wollastonite-1A.shtml Webmineral
  3. http://mineral.galleries.com/minerals/silicate/wollasto/wollasto.htm Mineral galleries
  4. Deer, Howie & Zussman (1966) An Introduction to the Rock Forming Minerals, Longman 528pp + xii, ISBN 0-582-44210-9
  5. 5,0 5,1 5,2 5,3 5,4 Deer, Howie and Zussman. Rock Forming Minerals; Single Chain Silicates, Vol. 2A, Second Edition, London, The geological society, 1997.
  6. 6,0 6,1 Andrews, R. W. Wollastonite. London, Her majesty's stationary office, 1970.
  7. Klein, Cornelis, and Barbara Dutrow. The Manual of Mineral Science. New York : J. Wiley, 2002.

External linksEditar

et:Vollastoniit es:Wollastonita eu:Wollastonita hr:Wollastonit he:וולאסטוניט nl:Wollastoniet ja:珪灰石 pl:Wollastonit ro:Wolastonit ru:Волластонит sr:Воластонит fi:Wollastoniitti

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