Talc

phyllosilicate

serpentine

Formula: Mg3Si4O10(OH)2 phyllosilicate (sheet silicate)
Specific gravity: 2.7 to 2.8
Hardness: 1
Streak: White
Colour: White, grey, yellow, brown, green
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
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.

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 (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).

Common impurities: Ni,Fe,Al,Ca,Na,H2O

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