GibbsiteAl(OH)3c 2001-2005 Mineral Data Publishing, version 1

Crystal Data: Monoclinic. Point Group: 2/m. As pseudohexagonal crystals, tabularon {001}, with {101}, {110}, to 3 cm. Also as lamellar or acicular aggregates, stalactitic, ormammillary; in enamellike coatings, compact earthy. Twinning: Common on {001}, less common
on {100} and {110}; also very common about [130] with {001} of the individuals in parallel.
Physical Properties:
Cleavage: {001}, perfect. Tenacity: Tough. Hardness = 2.5–3.5D(meas.) = 2.40 D(calc.) = [2.42] Strong argillaceous odor when breathed upon.
Optical Properties: Transparent. Color: Colorless, white, gray, pale green, pale red; colorless
to brownish in thin section. Luster: Vitreous, pearly on cleavage surfaces, dull in aggregates.
Optical Class: Biaxial (+). Orientation: X = b; Y ∧ a = 25.5◦ ; Z ∧ c ' −21◦ .
Dispersion: r > v, strong; sometimes v > r. α = 1.56–1.58 β = 1.56–1.58 γ = 1.58–1.60
2V(meas.) = 0◦ –40◦
Cell Data:
Z=8Space Group: P 21 /n. a = 8.684(1)b = 5.078(1)c = 9.736(2)β = 94.54(1)◦X-ray Powder Pattern: Langesundsfjord, Norway. (ICDD 29-41).
4.853 (100), 4.380 (36), 4.328 (18), 2.389 (16), 2.456 (12), 2.052 (12), 1.8074 (10)Chemistry:
SiO2
Al2 O3Fe2 O3

(1)
1.03
64.92
trace

(2)
65.36

MgO
CaO
H2 O

(1)
trace
0.17
34.12

(2)

34.64

Total

100.24100.00(1) Klein-Tresny, Moravia. (2) Al(OH)3 .
Polymorphism & Series:

Polymorphous with bayerite, doyleite, and nordstrandite.

Occurrence: A typical product of weathering of aluminous minerals, common in lateritic soils
and bauxite. Also formed in low-temperature hydrothermal and metamorphic environments,replacing aluminous minerals.
Association:Diaspore, böhmite, corundum, kaolinite, goethite.

Distribution: Widespread, particularly in bauxite deposits. Some prominent localities are: inthe USA, at Richmond, Berkshire Co., Massachusetts; Unionville, Chester Co., Pennsylvania; large
crystals from the Champion mine, White Mountains, Mono Co., California; from Toomsboro,Wilkinson Co., Georgia; at Tar Branch, near Winston-Salem, Forsyth Co., North Carolina.
Around Saramenha, Ouro Prêto, Minas Gerais, Brazil. From Paramaribo, Surinam. OnEikaholmen and Lille-Arø Islands, Langesundsfjord, and at Tredalen, near Larvik, Norway.
Large crystals from Schischimskaya, near Zlatoust, Ural Mountains, Russia. On the Vogelsberg,Hesse, and on the Katzenbuckel, Baden-Württemberg, Germany. In India, many localities, asat Kodikanal, Madras, and Talevadi, near Bombay, Maharashtra. From Dundas, Tasmania,Australia.
Name: Honors Colonel George Gibbs (1776–1833), a prominent mineral collector of New Haven,
Connecticut, USA.
Type Material:n.d.

References: (1) Palache, C., H. Berman, and C. Frondel (1944) Dana’s system of mineralogy,(7th edition), v. I, 663–667. (2) Deer, W.A., R.A. Howie, and J. Zussman (1962) Rock-forming
minerals, v. 5, non-silicates, 93–101. (3) Saalfeld, H. and M. Wedde (1974) Refinement of thecrystal structure of gibbsite, Al(OH)3 . Zeits. Krist., 139, 129–135. (4) Catti, M., G. Ferraris, andS. Hull (1991) Powder neutron-diffraction study of brucite and gibbsite and pressure dependenceof the structure of Mg(OD)2 . Zeits. Krist., supplement 4, 316. (5) Phillips, W.R. and D.T. Griffen(1981) Optical mineralogy, 42–43.
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