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Formula: Mg3Si4O10(OH)2
Phyllosilicate (sheet silicate),
pyrophyllite-talc group
Steatite is a massive variety of talc with a greasy feeling
Crystal System: Triclinic
Specific gravity: 2.58 - 2.83 measured, 2.78 calculated
Hardness: 1
Streak: White
Colour: White, grey, yellow, brown, green
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Common impurities: Ni,Fe,Al,Ca,Na,H2O
Environments:
Metamorphic environments
Hydrothermal environments
Talc is a secondary mineral formed by the
alteration of magnesium silicates, such as
olivine,
pyroxene and
amphibole, and may be found as pseudomorphs after these minerals. It occurs
characteristically in low-grade metamorphic rocks such as talc
schist, through
hydrothermal alteration of mafic rocks. It also forms by low temperature
metamorphism of siliceous
dolostone, where it may make up nearly the entire rock mass.
Talc may be found in
dolostone and
skarn.
Common associates are serpentine,
and forsterite.
Talc is a mineral of the
albite-epidote-hornfels facies.
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Talc is a primary silicate that has been found with
chlorite, tremolite and
hedenbergite at the Two Mile deposit, and with
andradite and in marble at the
Three Mile deposit
(AJM 22.1.36).
At Bahia, Brazil, quartz pseudomorphs
after talc have been found
(KL p254).
At Gopfersgrun, Germany, talc pseudomorphs after
quartz have been found
(KL p235).
At Greenwood, Oxford ounty, Maine, USA, talc pseudomorphs after
spodumene have been found
(KL p234).
At the Dafoe property, Pierrepont, St. Lawrence county, New York, USA, talc occurs rarely as exceptional
primary crystals to about 1 cm and sometimes to 4 cm. They are
a silver to greenish-white colour and, unless closely inspected, would pass for a white
mica. Talc is also commonly found as white coatings and encrustations on,
and pseudomorphs after, first-generation Tessin-habit
quartz crystals
(R&M 97.3.250).
At Rose Road, Pitcairn, St. Lawrence county, New York, USA, in the skarn
deposit, talc pseudomorphs after
wollastonite are uncommon and have smooth surfaces with mottled
colours of white, pale grey and medium grey.
The geochemistry of wollastonite alteration at this location has several
interesting features. The mineralising fluids available as the alteration to
diopside or talc proceeded must have carried sufficient magnesium to
form those two minerals because the original wollastonite is
magnesium-free. At the same time, calcium from the original wollastonite
must have been carried away by these fluids, probably to form
secondary calcite in
the cases of alteration to quartz or to talc. Additionally,
mineralising fluids involved in the alteration of wollastonite must have
been enriched in iron over those that formed the
primary green diopside
crystals because the acicular yellow crystals commonly forming tan rinds on
wollastonite have a higher weight percent FeO than the original green
primary diopside.
Talc that intermittently coats green diopside and white
albite crystals fluoresces a bright white under both short wave and long wave
UV. Talc that formed as a replacement of wollastonite has little
to no fluorescence
(R&M 97.5.434-444).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Talc occurs as thin white coatings on some scapolite crystals
(R&M 96.6.552).
Alteration
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
Talc is stable from 100oC up to between 500oC and 700oC depending on pressure. With
increasing temperature talc first forms
anthophyllite, then anthophyllite reacts to form
enstatite
(MOM).
antigorite to
forsterite, talc and H2O
5Mg3Si2O5(OH)4 = 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
This reaction may occur in olivine-diopside-
antigorite schist within
the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which
produces forsterite and tremolite
(DHZ 1A p258).
At 10 kbar pressure the equilibrium temperature is about 600oC
(amphibolite facies), with equilibrium to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(SERC).
dolomite and
chert to talc,
calcite and H2O
3CaMg(CO3)2 + 4SiO2 + H2O →
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Metamorphism of siliceous carbonate rocks causes the formation of hydrous phases such as talc and
tremolite.
(DHZ 5B p127).
This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and
250oC
(MOM).
enstatite and H2O to serpentine
and talc
3Mg2Si2O6 + 3H2O →
Mg3Si2O5(OH)4 + Mg3Si4O10(OH)2
Talc may occur as a pseudomorph after
enstatite by the above reaction
(DHZ 3 p185).
forsterite and talc to
anthophyllite and H2O
4Mg2SiO4 + 9Mg3Si4O10(OH)2 ⇌
5Mg2Mg5Si8O22(OH)2 + 4H2O
At 2 kbar pressure the equilibrium temperature is about 650oC
(pyroxene-hornfels facies), with equilibrium
to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(DHZ 1A p258, SERC).
forsterite and talc to
enstatite and H2O
Mg2SiO4 + Mg3Si4O10(OH)2 ⇌ 5MgSiO32O
(JVW p103)
At 10 kbar pressure the equilibrium temperature is about 680oC
(amphibolite facies), with equilibrium to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(SERC).
serpentine to forsterite,
talc and H2O
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
At a pressure of 10 kbar the equilibrium temperature is about 600oC. Increasing temperature favours the forward reaction
(SERC).
serpentine (chrysotile) and CO2 to talc,
magnesite and H2O
2Mg3Si2O5(OH)4 + 3CO2 →
Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O
Serpentine (chrysotile) is not stable in the presence of carbon dioxide and reacts with it according
to the above equation
(R&M 90.6.521).
serpentine and diopside to
forsterite, talc and H2O
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
In olivine-diopside-
antigorite schist within
the aureole of the tonalite in the southern Bergell Alps, Italy, this reaction occurs at a higher grade of metamorphism
than that which
produces forsterite and tremolite
talc to anthophyllite,
quartz and H2O
7Mg3Si4O10(OH)2 →
3☐Mg2Mg5Si8O22(OH)2 + 4SiO2 + 4H2O
At 10 kbar pressure the equilibrium temperature is about 790oC
(granulite facies). The equilibrium moves to the right at
higher temperatures and to the left at lower temperatures
(http://www.tulane.edu/~sanelson/eens212/metaminerals.htm, SERC).
talc to enstatite, quartz and H2O
2Mg3Si4O10(OH)2 ⇌ 3Mg2Si2O6 +
2SiO2 + 2H2O
At 10 kbar pressure the equilibrium temperature is about 790oC
(granulite facies). The equilibrium moves to the right at
higher temperatures and to the left at lower temperatures
(SERC).
talc and CO2 to magnesite,
quartz and H2O
Mg3Si4O10(OH)2 + 3CO2 → 3Mg(CO3)
+ 4SiO2 + H2O
At low temperatures talc is unstable in the presence of excess CO2, and
is replaced by magnesite
(DHZ 3 p128).
talc and calcite and CO2 to
dolomite, quartz and H2O
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 ⇌
3CaMg(CO3)2 + 4SiO2 + H2O
(JVW p144)
talc and calcite to tremolite,
dolomite, CO2 and H2O
2Mg3Si4O10(OH)2 + 3CaCO3 +4SiO2 →
Ca2Mg5Si8O22(OH)2 +
CaMg(CO3)2 + CO2 +H2O
This is a low-grade metamorphic change, occurring at temperature between about 250oC and 450oC
(MOM).
talc, calcite and quartz to
tremolite, CO2 and H2O
5Mg3Si4O10(OH)2 + 6CaCO3 +4SiO2 →
3Ca2Mg5Si8O22(OH)2 + 6CO2 +2H2O
Metamorphism of siliceous carbonate rock causes the formation of hydrous phases such as talc and
tremolite
(DHZ 5B p127, p213).
talc and enstatite to anthophyllite
Mg3Si4O10(OH)2 + 2Mg2Si2O6 ⇌
☐Mg2Mg5Si8O22(OH)2
At 10 kbar pressure the equilibrium temperature is 750oC
(granulite facies).
talc and forsterite to
anthophyllite and H2O
9Mg3Si4O10(OH)2 + 4Mg2SiO4 =
5Mg2Mg5Si8O22(OH)2 + 4H2O
This reaction occurs at a higher metamorphic grade than that forming forsterite
and talc from
serpentine
(DHZ 1A p258).
talc and kyanite to cordierite,
corundum and H2O
2Mg3Si4O10(OH)2 + 7Al2OSiO4 ⇌
3Mg2Al4Si5O18 + Al2O3 + 2H2O
(DHZ 2A p635)
tremolite and CO2 to dolomite,
talc and SiO2
☐Ca2Mg5Si2O22(OH)2 + CO2
⇌ 2CaMg(CO3)2 + Mg3Si4O10(OH)2
+ 4SiO2
In the greenschist facies
tremolite may be converted to
talc according to the above reaction
(DHZ 3 p127).
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