Pure Natural Minerals
Clay & Minerals
Wollastonite
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.
- 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.