Antigorite

antigorite

chondrodite

serpentine

brucite

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Formula: Mg3Si2O5(OH)4
Phyllosilicate (sheet silicate), serpentine subgroup
Specific gravity: 2.0 to 2.6
Hardness: 2½ to 4
Streak: White
Colour: Green, grey to black, white, brownish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Common impurities: Ni,Al,Mn
Environments:

Metamorphic environments

Antigorite forms in reactions at temperatures that can exceed 600°C during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. It is common in regional and contact metamorphosed serpentinite.
It is a mineral of the blueschist and greenschist facies.

Localities

Amity, Town of Warwick, Orange county, New York, USA, is an area of granite intrusions into marble and associated gneiss. The marble is mostly composed of white crystalline calcite that often has small flakes or spheres of graphite and phlogopite. Antigorite is a high-temperature mineral that occurs in contact and fault zones in thin sheets, and is also found in massive form with spinel, uvite, phlogopite and clinochlore (R&M 96.5.434).

At the Tilly Foster mine, Brewster, Putnam county, New York, USA, antigorite pseudomorphs after chondrodite have been found with magnetite on kaolinite (KL p233).

Alteration

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).

antigorite and albite to glaucophane and H2O
Mg3Si2O5(OH)4 + 2Na(AlSi3O8) → ☐Na2(Mg3Al2)Si8O22(OH)2 + H2O
This is a metamorphic reaction (DHZ 3 p156).

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist. At at about 3 kbar pressure and 400 to 500oC (greenschist facies) (DHZ 1A p263).

antigorite and magnesite to forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4 + CO2 + 2H2O (DHZ 1A p263).

antigorite and muscovite to phlogopite, amesite, SiO2 and H2O
5Mg3Si2O5(OH)4 + 3KAl2(AlSi3O10)(OH)2 → 3KMg3(AlSi3O10)(OH)2 + 3Mg2Al(AlSiO5)(OH)4 + 7SiO2 + 4H2O (DHZ 3 p23).

brucite and antigorite to forsterite and H2O
Mg(OH)2 + Mg3Si2O5(OH)4 ⇌ 2Mg2SiO4 + 3H2O
The equilibrium temperature for this reaction at 8 kbar pressure is about 450oC (greenschist facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures. The reaction also may occur in the albite-epidote-hornfels and blueschist facies (SERC).

diopside and antigorite to forsterite, Mg-rich tremolite and H2O
2CaMgSi2O6 + 5Mg3Si2O5(OH)4 ⇌ 6Mg2SIO4 + Ca2Mg5Si8O22(OH)2 + 9H2O
At 10 kbar pressure the equilibrium temperature is about 580oC (amphibolite facies) (SERC).

See also results for serpentine, which is a group of minerals including antigorite, chrysotile and lizardite, all of which share the same formula, although they have slightly different structures.

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