Brucite

Brucite
General
Category Oxide mineral
Formula
(repeating unit)		 Mg(OH)2
Strunz classification 4.FE.05
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group P3m1
Unit cell a = 3.142(1) Å, c = 4.766(2) Å; Z = 1
Identification
Color White, pale green, blue, gray; honey-yellow to brownish red
Crystal habit Tabular crystals; platy or foliated masses and rosettes – fibrous to massive
Cleavage Perfect on {0001}
Fracture Irregular
Tenacity Sectile
Mohs scale hardness 2.5 to 3
Luster Vitreous to pearly
Streak White
Diaphaneity Transparent
Specific gravity 2.39 to 2.40
Optical properties Uniaxial (+)
Refractive index nω = 1.56–1.59
nε = 1.58–1.60
Birefringence 0.02
Other characteristics Pyroelectric
References [1][2][3]

Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg(OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites. Brucite is often found in association with serpentine, calcite, aragonite, dolomite, magnesite, hydromagnesite, artinite, talc and chrysotile.

Notable locations include Wood's Chrome Mine, Cedar Hill Quarry, Lancaster County, Pennsylvania, US.

Discovery

Nemalite
Brucite crystals from the Sverdlovsk Region, Urals, Russia (size: 10.5 x 7.8 x 7.4 cm

Brucite was first described in 1824 and named for the discoverer, American mineralogist, Archibald Bruce (1777–1818). A fibrous variety of Brucite is called Nemalite. It occurs in fibers or laths, usually elongated along [1010], but sometimes [1120] crystalline directions.

Industrial applications

Crystal structure

Brucite is used as a flame retardant because it thermally decomposes to release water in a similar way to aluminium hydroxide and mixtures of huntite and hydromagnesite.[4][5] It also constitutes a significant source of magnesium for industry.

Magnesian attack of cement and concrete

When cement or concrete are exposed to non negligible concentration of Mg2+, e.g. when these materials are left in prolonged contact with sea water or brines, Mg(OH)2 precipitates under the high pH conditions prevailing in the cement porewater. The neoformation of brucite, an expansive material, induces mechanical stress in the hardened cement paste and is responsible for the formation of cracks and fissures in concrete.

The use of dolomite as aggregate in concrete can also cause the magnesian attack and should be avoided.

See also

References

  1. ↑ Brucite on Mindat.org
  2. ↑ Handbook of Mineralogy
  3. ↑ Brucite on Webmineral
  4. ↑ Hollingbery, LA; Hull TR (2010). "The Thermal Decomposition of Huntite and Hydromagnesite - A Review". Thermochimica Acta. 509 (1-2): 1–11. doi:10.1016/j.tca.2010.06.012.
  5. ↑ Hollingbery, LA; Hull TR (2010). "The Fire Retardant Behaviour of Huntite and Hydromagnesite - A Review". Polymer Degradation and Stability. 95 (12): 2213–2225. doi:10.1016/j.polymdegradstab.2010.08.019.
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