Brucite

brucite

periclase

hydromagnesite

pyroaurite

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Formula: Mg(OH)2
Hydroxide
Specific gravity: 2.39
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
Hydrothermal environments

The most common occurrence of brucite is as an alteration product of periclase in contact metamorphic dolomite. It also occurs as a low temperature hydrothermal mineral in serpentinite and chlorite schist and as an accessory mineral in metasomatic skarn deposits. Much less commonly as a component of hydrothermal mineralisation in carbonatites (R&M 94.5.428-432). It is found in limestone, marble, dolostone and slate, as an alteration product of serpentine (R&M 92.5.435).
It is associated with serpentine, dolomite, magnesite and chromite.
Brucite is a mineral of the blueschist, greenschist and amphibolite facies.

Localities

At the Mount Keith deposit in Western Australia, brucite is a widespread alteration product derived from the serpentinisation of dunite. Rarely brucite has been found associated with magnesite and on a matrix of serpentine and pyroaurite (R&M 94.5.428-432).

At Palabora, Limpopo Province, South Africa, brucite is found in carbonatite (R&M 92.5.435).

At the type locality, Castle Point, Hoboken, Hudson county, New Jersey, USA, brucite occurs in serpentinite associated with hydromagnesite and serpentine group minerals (R&M 94.5.428-432).

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).

Common impurities: Fe, Mn, more rarely Zn, Ni (R&M 94.5.428-432).

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