Inosilicate (chain silicate) wollastonite group
Specific gravity: 2.86 to 3.09
Hardness: 4½ to 5
Colour: White, green, brown
Solubility: Slightly soluble in hydrochloric acid
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).
During the progressive metamorphism of silica-rich dolostones 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 1345oC, when the stable mixture is åkermanite and wollastonite. From 1240oC down to 1050oC a mixture of åkermanite, wollastonite and diopside is stable, and at lower temperatures åkermanite dissociates to form wollastonite and monticellite (DHZ 1B p307).
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).
åkermanite to monticellite and wollastonite
Ca2MgSi2O7 → CaMg(SiO4) + CaSiO3
calcite and quartz to wollastonite and CO2
CaCO3 + SiO2 → CaSiO3 + CO2
This is a contact metamorphic change occurring at very high temperature, such as in the immediate border zone of an igneous intrusion into limestone (MOM p486). High pressure inhibits the forward reaction by suppressing the formation of gaseous CO2 (KB p18). At 10 kbar pressure the equilibrium temperature is about 1,070oC (granulite facies) (SERC).
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). High pressure inhibits the forward reaction by suppressing the formation of gaseous CO2 (KB p18).
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