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Formula: CaSiO3
Inosilicate (chain silicate), single width chains,
wollastonite group
Crystal System: Triclinic
Specific gravity: 2.86 to 3.09 measured, 2.90 calculated
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).
Wollastonite typically forms at a high temperature and low-to-moderate pressure at, or near,
a contact between an intrusive and a preexisting carbonate rock
(R&M 97.5.434-444).
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).
Several minerals occur at the skarn deposit as
pseudomorphs after wollastonite; they include
diopside, quartz and
talc. Almost all the pseudomorphs
observed are alterations or replacements of primary
wollastonite. The wollastonite and resulting pseudomorphs
are fairly sharply crystallised and occur either as isolated crystals in areas of coarsely crystallised
calcite or as crystals lining the walls of a
diopside-albite rock that faces into
coarsely crystallised calcite
(R&M 97.5.434-444).
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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