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Formula: Mg(OH)2
Hydroxide
Crystal System: Trigonal
Specific gravity: 2.39 measured, 2.368 calculated
Hardness: 2½
Streak: White
Colour: White, grey, greenish, bluish, yellow to brown
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Common impurities: Fe, Mn, more rarely Zn, Ni (R&M 94.5.428-432).
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).
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