Brucite

(mineralogy) Mg(OH)₂ A hexagonal mineral; native magnesium hydroxide that appears gray and occurs in serpentines and impure limestones; hardness is 2.5 on Mohs scale, and specific gravity is 2.38-2.40.

A magnesium hydroxide mineral, Mg(OH)₂, crystallizing in the trigonal system. It is a member of the important Cd(OH)₂ structure type, consisting of hexagonal close-packed oxygen atoms with alternate octahedral layers occupied by Mg. The “brucite layer'”is an important structural component in the clay, mica, and chlorite mineral groups. Brucite occurs as tabular crystals and as elongated fibers (as the variety nemalite), hardness 2½ (Mohs scale), color white to greenish, and specific gravity 2.4. Fe²⁺ and Mn²⁺ commonly substitute for Mg²⁺.

Brucite often occurs in a low-temperature vein paragenesis, usually with serpentine and accessory magnesite. It is also derived by the action of water on periclase, MgO, which results from the thermal metamorphism of dolomites and limestones. Carbonate rocks rich in periclase and brucite are called predazzites. See also Dolomite; Magnesite; Serpentine.

Environment

Veins in serpentine and magnesite.

Most often in free-standing but ill-defined poor crystal plates; also foliated, massive, and fibrous.

Pearly white to pale green, yellow, or blue. Luster pearly and waxy; hardness 2Ɖ; specific gravity 2.4; cleavage micaceous. Plates flexible, nonelastic, and sectile; transparent to pearly translucent; fluoresces blue.

Magnesium hydroxide (69.0% MgO, 31.0% H ₂ O).

Infusible but flake glows brightly in the flame. Soluble in acids.

Harder than talc but a little softer than mica. The less cleavable plates are inelastic. Gypsum is far less soluble in acids. The fluorescence is probably diagnostic in most cases.

Brucite is derived from the enclosing serpentine through alteration by hot watery solutions and it can be a common constituent of such hydrothermal veins. Also in flakes scattered through marbles, derived from periclase (MgO). The world's outstanding occurrences are American. Large crystals were found in the old Tilly Foster iron mine at Brewster, New York, and comparable specimens came from Texas, Lancaster Co., Pennsylvania, where crystals 7 in. (19 cm) across were found. Long fibers of brucite resembling asbestos are found in the Quebec asbestos mine at Asbestos. Light blue-green veins are occasionally intersected at the Gabbs (Nevada) magnesite mine.

Brucite
General
Category	Mineral
Chemical formula	Mg(OH)2
Identification
Color	white, pale green, blue, gray; honey-yellow to brownish red
Crystal habit	platy or foliated masses and rosettes - fibrous to massive
Crystal system	Trigonal (Hexagonal Scalenohedral)
Cleavage	perfect 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 = 1.560 nε = 1.580
Birefringence	0.020 max.
Other characteristics	Pyroelectric

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.

Contents 1 Discovery 2 Industrial applications 3 Magnesian attack of cement and concrete 4 See also 5 References 6 External links

Discovery

Nemalite

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

Brucite is used as a flame retardant and also constitutes a significative 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 stress in the cement paste and is responsible for the formation of cracks and fissures in concrete.

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

See also

• List of minerals
• List of minerals named after people
• Cement
• Concrete
• Portlandite, Ca(OH)₂

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (November 2009)

• Lee, Hyomin; Robert D. Cody, Anita M. Cody, Paul G. Spry (2000). "Effects of various deicing chemicals on pavement concrete deterioration". Mid-Continent Transportation Symposium 2000 Proceedings. 

• Lee, Hyomin; Robert D. Cody, Anita M. Cody, Paul G. Spry (2002). "Observations on brucite formation and the role of brucite in Iowa highway concrete deterioration". Environmental and Engineering Geoscience 8 (2): 137–145. doi:10.2113/gseegeosci.8.2.137. http://eeg.geoscienceworld.org/cgi/content/abstract/8/2/137. Retrieved 2009-09-10. 

• Wies aw; Kurdowski (2004-09). "The protective layer and decalcification of C-S-H in the mechanism of chloride corrosion of cement paste". Cement and Concrete Research 34 (9): 1555–1559. doi:10.1016/j.cemconres.2004.03.023. 

• Biricik, Hasan; Fevziye Aköz, Fikret Türker, Ilhan Berktay (2000). "Resistance to magnesium sulfate and sodium sulfate attack of mortars containing wheat straw ash". Cement and Concrete Research 30 (8): 1189–1197. doi:10.1016/S0008-8846(00)00314-8. 

External links

• Mindat
• Webmineral data
• Mineral Data Publishing

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