Wollastonite

minerals

titanite

monticellite

danburite

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Formula: CaSiO3
Inosilicate (chain silicate) wollastonite group
Specific gravity: 2.86 to 3.09
Hardness: 4½ to 5
Streak: White
Colour: White, green, brown
Solubility: Slightly soluble in hydrochloric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments

Wollastonite is found in contact and regional metamorphic environments.
It may be found in skarn associated with calcite, diopside, andradite, grossular, tremolite, plagioclase feldspar feldspar, vesuvianite and epidote.
In contact metamorphic rocks it may be associated with danburite. It is a mineral of the pyroxene-hornfels, amphibolite and granulite facies.
Hydrothermal wollastonite is found in thermally and regionally metamorphosed carbonates, associated with calcite, garnet, clinopyroxene, quartz and tremolite. Its formation is dependent on fluid and host-rock composition, fluid CO2 content, temperature and pressure. Carbonates that contain too much magnesium or iron alter to tremolite or clinopyroxene instead of wollastonite (AofA).

Localities

At Rose Road, New York State, USA, wollastonite occurs in skarn associated with diopside, albite, titanite, quartz, fluorapatite and calcite. This occurrence pre-dates regional granulite facies metamorphism in the area. Wollastonite crystals alter to an outer zone of granular diopside and quartz and an inner zone of orientated acicular diopside and compact calcite, sometimes with an inner core of unaltered wollastonite (R&M 84.2.167-168).

Alteration

During the progressive metamorphism of silica-rich dolostone the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite
As åkermanite cools from its melting point it is stable down to 1,345oC, when the stable mixture is åkermanite and wollastonite. From 1,240oC down to 1,050oC a mixture of åkermanite, wollastonite and diopside is stable, and at lower temperatures åkermanite dissociates to form wollastonite and monticellite (DHZ 1B p307).

åkermanite to monticellite and wollastonite
Ca2MgSi2O7 → CaMg(SiO4) + CaSiO3

grossular to anorthite, gehlenite and wollastonite
2Ca3Al2(SiO4)3 ⇌ CaAl2Si2O8 + Ca2Al2SiO7 + 3CaSiO3
The equilibrium temperature for this reaction at 5 kbar pressure is 1,110oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular and quartz to anorthite and wollastonite
Ca3Al2(SiO4)3 + SiO2 ⇌ CaAl2Si2O8 + 2CaSiO3
The equilibrium temperature for this reaction at 5 kbar pressure is 730oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC, MM 48.206).

quartz and calcite to wollastonite and CO2
3SiO2 + 3CaCO3 ⇌ Ca3Si3O9 + 3CO2 (gaseous)
This is a contact metamorphic change occurring at temperatures from about 600°C such as in the immediate border zone of an igneous intrusion into limestone (MOM p486, KB p417). The reaction occurs at about 800oC at pressure of 0.3 Pa and increases rapidly with increasing pressure. However, temperatures as low as 400oC allow the formation of wollastonite, if the water activity is high (R&M 94.6.549). At 10 kbar pressure the equilibrium temperature is about 1,070oC (granulite facies) (SERC). High pressure inhibits the forward reaction by suppressing the formation of gaseous CO2 (KB p18).

wollastonite and calcite to tilleyite and CO2
2CaSiO3 + 3CaCO3 ⇌ Ca5Si2O7(CO3)2+ CO2
Higher temperatures favour the forward reaction (MM 34.1.1-16).

wollastonite and spurrite to rankinite and CO2
4CaSiO3 + Ca5(SiO4)2(CO3) ⇌ 3Ca3Si2O7 + CO2
Higher temperatures favour the forward reaction (MM 34.1.1-16).

xonotlite to wollastonite and H2O
Ca6Si6O17(OH)2 ⇌ 6CaSiO3 + H2O
Higher temperatures favour the forward reaction (MM 34.1.1-16).

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