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
- Tennbergs
and Wikströms mine outside Ludvika.
- Algfallet
outside Kopparberg.
- Gasgruvan
lime mine, Persberg.
- Anggruvan,
Nordmark.
- Tennbetgets
lime mine, Grangesberg.
- Hönsarvsbergets
lime mine, Borlänge.
- Alnon,
Sundsvall.
- Thalainen
limestone quarry, Lappenranta, Finland. Up to 25% of the rock.
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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