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Wollastonite

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Wollastonite Calcium Silicate
CaSiO3

The single largest use of wollastonite is in the ceramics industry where it is used as a principal ingredient in the ceramic bodies required for the manufacture of floor and wall tiles.

The History Says

Early common names for wollastonite were "Table Spar" or "Tabular Spar." In the late 1700's, Sir William Hyde Wollaston (1766-1828), an English chemist and mineralogist brought to the attention of the scientific community, the mineralogical peculiarities and uniqueness of table spar. In 1822, one of Wollaston's fellow scientists, Dr. A. Hauy, proposed the name "Wollastonite" in honour of Wollaston's work.

The Present Scenario

During the 1960's, wollastonite became widely known as an industrial mineral with a variety of potential uses. Production in Finland and in Mexico began commercially in the late 1960's, in India and Africa in the 1970's, and in China in the 1980's. Worldwide production has more than doubled in the last ten years with an average annual growth rate of over 10% per year.

WOLLASTONITE IS one of the latest minerals added to the list of industrial minerals being mined and marketed today. Its industrial applications are gradually growing.

It is a metasilicate of calcium crystallizing in monoclinic system, occurring as aggregates of bladed or needle-like crystals. When powdered to 325 mesh, it gives a natural brightness averaging 92 on the Hunter reflectometer. Its specific gravity 2.8 to 2.9 and melting point 1512ºC. It is found as a metamorphic mineral associated with garnet in metamorphosed limestone. The industrial value of wollastonite depends upon its freedom from impurities. The commercial deposits are worked only in the USA where it is mined on a limited scale.

Hardness Associated Minerals Chemical/Typical composition Colour characteristics Luster Streak
4 to 4.5 Calcite
Quartz
Diopside
Cao......48.28% SiO2 ......51.72% White Colorless gray Soluble in hydrochloric acid and some specimens will fluoresce vitreous or dull to pearly on cleavage surfaces crystal habit, cleavage, solubility in HCl, fluorescence if present, softness and environment of formation.

Industrial Applications

The single largest use of wollastonite is in the ceramics industry where it is used as a principal ingredient in the ceramic bodies required for the manufacture of floor and wall tiles. It is also utilized as an additive in ceramic bodies and glazes. Wollastonite fires white to gray, matures at a slightly lower firing temperature than most conventional ceramic bodies and can be fired at a faster rate. The firing temperature is 991ºC to 1196ºC. It has been reported that certain wollastonite bodies can be fired along with the glazes thus eliminating a second firing.

It is used as a filller in paint and paper and many other products. In metallurgy, it is used as a welding rod coating, to control the viscosity of the slag and alloying agents. A suitable mixture of finely pulverized wollastonite with china-clay improves the properties of the clay to be used for paper-coating. It is found to increase the strength of Portland cement when mixed as an additive.

Wollastonite is still to take the chemical field though the prospects are quite good. The mineral has been found to react readily with various acids like sulphuric, phosphoric and hydrochloric acids and with alkalies such as soda ash. Certain types of pastes have been produced by dissolving the mineral in some proportions of sulphuric and phosphoric acids. These pastes possess a good hiding and oil absorption power. It is certain that many more new uses of wollastonite will be developed soon. Its latest use is in the manufacture of mineral wool.

World Resources

The chief commercial deposits are found in about 35 different localities in California and near Willsboro, Essex county, New York. The deposit is in a hilly area of pre-Cambrian limestone on the northwest flank of the Adirondack mountains. This deposit has been known since 1810. It has been systematically developed since 1951, when a milling plant was set up at Willsboro and the industrial utilization of wollastonite began.

In California it is worked only in the Big Maria and Little Maria mountains about 32 km. north of Blythe, Riverside county. The beds of wollastonite are found associated with Paleozoic limestone. The thickness of individual beds varies from a few metres to 150 metres. The deposit is exploited mainly for the manufacture of mineral wool.

A new deposit situated only 80 km. south of Nairobi in the kajiado district, Kenya, has been discovered with possible reserves of 1,000,000 tibbesm assaying from 45 to 50% wollastonite. It is found interbedded with crystalline limestone and withna calcite gangue.

Not so common in Sweden, but can be found in a number of Swedish lime mines.

Other places of occurrences

Geological Formation

Wollastonite can be formed in nature in a variety of ways, however for commercial deposits it is generally accepted that there are two methods of formation. Both involve metamorphism (heat and pressure) of limestones (calcite).

In silica (quartz) bearing limestones, silica and calcite react to form wollastonite. This commonly occurs through contact metamorphism as a result of intrusive igneous activity.

Wollastonite can also form by the passage of highly siliceous hydrothermal solutions through limestone beds or zones. These siliceous solutions generally result from local intrusive igneous activity.

Groundwater heated by the local intrusion dissolves large amounts of silicate during contact with the intrusion. Hot silicate laden water migrates into surrounding limestone beds where the silica precipitates and CO2 is carried out of the deposit. CaO.SiO2 precipitates and slowly forms characteristic wollastonite crystal structures in what was formerly limestone. This process is called metasomatism.

The recrystallization into wollastonite occurs over a long period of time (in the order of thousands of years). Subsequent geological events, which involve additional heat, can result in recrystallization and can lead to even more massive crystal structures.

The simple metamorphic reaction between silica and calcium carbonate to form wollastonite occurs at about 600OC at shallow depths. The temperature required increases with depth (pressure).

quartz + calcite <--> wollastonite + carbon dioxide

SiO2 + CaO.CO2 <--> CaO.SiO2 + CO2

If they are present, ions such as aluminium, iron, magnesium, manganese, potassium and sodium can be absorbed to a certain extent into the wollastonite structure during it's formation. Excess amounts of these ions however, will lead to the formation of other minerals such as diopside, feldspars, etc.

Alternatively other minerals may be formed during secondary alterations by subsequent passage of ground waters or intrusions from subsequent geologic events. These would include garnets, epidote, etc. Subsequent weathering of the deposit may also result in the formation of secondary minerals.

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