Talk:Dickite

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I have removed the following as it was just tacked on the end with no attention to pre-existing material or formatting.

Dickite (Al2Si2O5 (OH)4) is a clay mineral with the same composition as halloysite, nacrite and kaolinite but different structures. Dickite has tetragonal or pseudo tetragonal layered structures and belongs to the kaolinite – serpentinite group. It was hard to distinguish the kaolin minerals apart but subtle differences in their looks have made it possible to differentiate them through optical means. Dickite seems to grow in somewhat better crystals than its polymorphs. Their looks range from white, rhombohedral crystals to massive white chalk-like solid. The unit cell of Dickite contains two kaolinite layers, related by a glide plane. In each layer there are four hydroxyl groups; one is situated in the plane of atoms shared by inner hydroxyls, whereas the remaining three form the outer surface of the octahedral sheet. It is formed by inter-layered bonds.

It was named after Allan Brugh Dick, a Scottish metallurgical chemist who provided an extensive description and distinction of the mineral in the island of Anglesey, Wales. It is also well known of being in Red Mountain, near Ouray, Colorado.

Dickite has molecular weight of 258.16grams. It is an aluminium silicate hydroxide chemically composed of aluminium, silicon, hydrogen and oxygen contributing 20.90%, 21.76%, 1.56%, and 55.78% each respectively. Dickite (Al2Si2O5 (OH) 4) sometimes contains impurities like iron, magnesium, calcium, sodium and potassium.

Dickite’s type location is in Pant-y-Gaseg, Amlwch, Isle of Anglesey, Wales, U.K. It was found during a 662 feet dig for copper. Kaolinite, as dickite was then addressed, was found in the mine. 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 to name a couple

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 a 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. Two forms are described below in the diagrams. It usually has a dull clay-like texture. Its hardness on Mohr’s scale is 1.5-2, basically between talc and gypsum, this is attributed to its chemical bond. It is held with hydrogen bonds which are otherwise weak like Fig. 1 diagram suggested. It leaves a white streak and it has a pearly luster. It has a density of 2.6 and is subtranslucent to opaque. The density is low due to its specific element individual weight and the atoms are not tightly packed. 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:

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.

"This article was prepared by Kelvin Bariwei for the University of Houston course Geol 3370 Introduction to Mineralogy,by Proffessor Jonathan Snow, Instructor".

The information is valid and can be integrated in with the existing material - not just tacked on the end. Please integrate and wikify folloing Wikipedia conventions. Thanks, Vsmith (talk) 04:27, 7 December 2007 (UTC)