Brucite

hydroxide

periclase

Formula: Mg(OH)2 hydroxide
Specific gravity: 2.03
Hardness: 2½
Streak: White
Colour: White, grey, greenish, bluish, yellow to brown
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments

Brucite is found in limestone, marble, dolostone and slate, as an alteration product of serpentine. At Palabora, Limpopo Province, South Africa, it is found in carbonatite (R&M 92.5.435).
It is associated with serpentine, dolomite, magnesite and chromite.
It is a mineral of the blueschist, greenschist and amphibolite facies.

Alteration

brucite to periclase and H2O
Mg(OH)2 ⇌ MgO + H2O (JVW p102).
The equilibrium temperature for this reaction at 10 kbar pressure is about 840oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (for the same pressure) (SERC).

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).

forsterite and H2O to serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4 + Mg(OH)2
The forward reaction is highly exothermic. At 5 kbar pressure the equilibrium temperature is about 420°C (greenschist facies) (WJ).

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