Dickite

Dickite
General
Category	Phyllosilicate
Chemical formula	Al2Si2O5(OH)4
Strunz classification	09.ED.05
Dana classification	71.01.01.01
Identification
Color	White, with coloration from impurities
Crystal habit	pseudohexagonal crystals, aggregates of platelets and compact massive
Crystal system	Monoclinic - domatic, H-M Symbol (m) Space Group: Cc
Cleavage	Perfect on {001}
Tenacity	Flexible but inelastic
Mohs scale hardness	1.5 - 2
Luster	Satiny to pearly
Streak	White
Diaphaneity	Transparent
Specific gravity	2.6
Optical properties	Biaxial (+)
Refractive index	nα = 1.561 - 1.564 nβ = 1.561 - 1.566 nγ = 1.566 - 1.570
References	[1][2][3]

Dickite (Al₂Si₂O₅(OH)₄) has molecular weight of 258.16 grams. It is a phyllosilicate clay mineral chemically composed of aluminium, silicon, hydrogen and oxygen contributing 20.90%, 21.76%, 1.56%, and 55.78% each respectively. It has the same composition as kaolinite, nacrite, and halloysite, but with a different crystal structure (polymorph). Dickite sometimes contains impurities such as titanium, iron, magnesium, calcium, sodium and potassium.^[2]

Dickite occurs with other clays and requires x-ray diffraction for its positive identification. Dickite is an important alteration indicator in hydrothermal systems as well as occurring in soils and shales. It takes its name from Allan Brugh Dick (1833-1926), the Scottish metallurgical chemist who first described it.

Dickite’s type location is in Pant-y-Gaseg, Amlwch, Isle of Anglesey, Wales, U.K., where it was first described in 1960.^[2] Dickite is scattered across Wales forming occurrences in vein assemblages and as a rock forming mineral. Another occurrence spot, as indicated by Brindley and Porter of the American Mineralogists journal, is the Northerly dickite-bearing zone in Jamaica. The dickite in this zone ranges from indurate breccias containing cream to pinkish and purplish fragments composed largely of dickite with subordinate anatase set in a matrix of greenish dickite, to discrete veins and surface coatings of white, cream and translucent dickite. It appears that dickite in the northerly zone were formed by hot ascending waters from an uncertain origin. Dickite occurrence is well spread among countries like China, Jamaica, France, Germany, U.K., U.S.A, Italy, Belgium and Canada.

Dickite has a monoclinic crystal system and its crystal class is domatic (m). This crystal system contains two non-equal axes (a and b) that are perpendicular to each other and a third axis (c) that is inclined with respect to the an axis. The a and c axes lie in a plane. Dickite involves an interlayer bonding with at least 3 identifiable bonds: an ionic type interaction due to net unbalanced charges on the layers, van der waals forces between layers and hydrogen bonds between oxygen atoms on the surface of one layer and hydroxyl groups on the opposing surface. A hydrogen bond, as the term is used here, involves a long range interaction between hydrogen of a hydroxyl group coordinated to a cation and an oxygen atom coordinated to another cation. The reaction is predominantly electrostatic; hence an ionic bonding model is appropriate. Its axial ratio is a=0.576, b=1, c=1.6135.

Dickite has perfect cleavage in the (001) direction. Its color varies from blue, gray, white to colorless. It usually has a dull clay-like texture. Its hardness on the Mohs scale is 1.5-2, basically between talc and gypsum. This is attributed to its loose chemical bonds. It is held with hydrogen bonds, which are otherwise weak. It leaves a white streak and it has a pearly luster. It has a density of 2.6. Dickite is biaxial, its birefringence is between 0.0050-0.0090, its surface relief is low and it has no dispersion. The plane of the optical axis is normal to the plane of symmetry and inclined 160, rear to the normal to (0,0,1).

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/dickite.pdf Handbook of Mineralogy
2. ^ ^{a b c} http://www.mindat.org/min-1287.html Mindat.org
3. ^ http://webmineral.com/data/Dickite.shtml Webmineral data

• Dickite stability PDF
• Dick, Allan B., On Kaolinite: Mineral Mag., vol. 8, pp 15–27, 1888.
• Adams, J. M. and Hewart, A. W. (1981) Hydrogen positions in Dickite: Clays and Clay Minerals, vol. 32, No. 6, 483-485, 1984.
• Brindley, G. W. and Porter, R. D. (1975) Occurrence of dickite in Jamaica-ordered and disordered varieties: American Mineralogists, vol. 63, pp. 554-562, 1978.
• Ksanda, C. J. and Barth, F.W., Note on the structure of Dickite and other clay minerals: American Mineralogist, vol. 19, pp. 557-575, 1934.