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Dickite () is a phyllosilicate
clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...
named after the metallurgical chemist Allan Brugh Dick, who first described it. It is chemically composed of 20.90%
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
, 21.76% silicon, 1.56% hydrogen and 55.78% oxygen. It has the same composition as kaolinite,
nacrite Nacrite Al2 Si2 O5( OH)4 is a clay mineral that is polymorphous (or polytypic) with kaolinite. It crystallizes in the monoclinic system. X-ray diffraction analysis is required for positive identification. Nacrite was first described in 1807 f ...
, and
halloysite Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are oxygen (55.78%), silicon (21.76%), aluminium (20.90%), and hydrogen (1.56%). Halloysite typically forms by hydrothermal alteration o ...
, but with a different crystal structure ( polymorph). Dickite sometimes contains impurities such as titanium, iron, magnesium,
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
, sodium and potassium. Dickite occurs with other clays and requires x-ray diffraction for its positive identification. Dickite is an important alteration indicator in
hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water ( Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with t ...
systems as well as occurring in soils and shales. Dickite's type location is in Pant-y-Gaseg,
Amlwch Amlwch (; ) is a port town and community in Wales. It is situated on the north coast of the Isle of Anglesey, on the A5025 which connects it to Holyhead and to Menai Bridge. As well as Amlwch town and Amlwch Port, other settlements within the ...
, Isle of Anglesey, Wales, United Kingdom, where it was first described in 1888. Dickite appears in locations with similar qualities and is found in China, Jamaica, France, Germany, United Kingdom, United States, Italy, Belgium and Canada.


History

In 1888, Allan Brugh Dick (1833–1926), a Scottish metallurgical chemist, was on the island of
Anglesey Anglesey (; cy, (Ynys) Môn ) is an island off the north-west coast of Wales. It forms a principal area known as the Isle of Anglesey, that includes Holy Island across the narrow Cymyran Strait and some islets and skerries. Anglesey island ...
to conduct research on kaolin. He performed various experiments describing the
clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...
. It was not until 1931 that Clarence S. Ross and Paul F. Kerr looked closer at the mineral and concluded that it was different from the known minerals of kaolinite and
nacrite Nacrite Al2 Si2 O5( OH)4 is a clay mineral that is polymorphous (or polytypic) with kaolinite. It crystallizes in the monoclinic system. X-ray diffraction analysis is required for positive identification. Nacrite was first described in 1807 f ...
. They named it after the first person to describe the mineral.


Composition

is the chemical formula of dickite. The calculated percent abundances are very close when compared to other kaolin minerals. Chemical composition of dickite: * 46.54% * 39.50% * 13.96% Dickite and other kaolin minerals are commonly developed by weathering of feldspars and
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...
. Through its evolution, dickite, a phyllosilicate mineral, maintains the aluminium and silicon elements influencing the formation of hexagonal sheets common to clay minerals. The problem of mistaken identity arises when comparing dickite to other kaolin minerals due to the fact that kaolinite, dickite, and nacrite all have the same formula but different molecular structures. The only way to determine the true identity of the mineral is through powder x-ray diffraction and optical means.


Geologic occurrence

Dickite was first discovered in Almwch, Island of
Anglesey Anglesey (; cy, (Ynys) Môn ) is an island off the north-west coast of Wales. It forms a principal area known as the Isle of Anglesey, that includes Holy Island across the narrow Cymyran Strait and some islets and skerries. Anglesey island ...
, Wales, UK. Dickite is scattered across Wales forming occurrences in vein assemblages and as a rock-forming mineral. This area and others where dickite can be found all share similar characteristics. Pockets in phylloid
algal Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
limestones, in interstices of biocalcarenites and sandstone are a suitable environment for dickite. Very low pressure and high temperatures are the ideal environment for the formation of dickite. The more perfected crystallization of dickite occurs in porous algal limestones in the form of a white powder. The more disordered dickites can be found in less porous rocks. 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
breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s containing cream to pinkish and purplish fragments composed largely of dickite with subordinate
anatase Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) ...
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 is found worldwide in locations such as Ouray,
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...
, US; San Juanito,
Chihuahua Chihuahua may refer to: Places * Chihuahua (state), a Mexican state **Chihuahua (dog), a breed of dog named after the state **Chihuahua cheese, a type of cheese originating in the state **Chihuahua City, the capital city of the state **Chihuahua Mu ...
, Mexico in a silicified zone among the
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
area; and in St. George, Utah, US, where the mineral is thought to be associated with volcanic rock. An extensive study was done on dickite pertaining to its location in Pennsylvanian limestones of southeastern Kansas, US. In the dickite deposits of southeast Kansas the distribution is dependent on the following: the
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...
alternation of limestones and shales, westward regional dip, thick deposits of highly porous algal limestones, and
igneous intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and ...
s. It was found that groundwaters substantially heated along with
magmatic water Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust and is released to the atmosphere during a volcanic eruption. It plays a key role in ass ...
s which made its way up-dip and through the intrusions in the conduit-like algal mounds which allowed the dickite to be deposited in this area and it might be conclusive to say that this trend follows elsewhere in other locations around the world.


Physical properties

Dickite takes on the appearance of a white, brown earthy color and is often found embedded in many other minerals such as quartz. Dickite has perfect cleavage in the (001) direction. Its color varies from blue, gray, white to colorless. It usually has a dull
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay parti ...
-like texture. Its hardness on the
Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by ...
is 1.5-2, basically between talc and
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...
. 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringe ...
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). The atomic structure of dickite, being very similar to that of kaolinite and other kaolin type minerals, has a very specific arrangement that differs slightly enough to set its physical appearance and other physical properties apart from that of its family members kaolinite and nacrite. In a comparison of the family of minerals through experiments examined by Ross and Kerr the similarities between them are clearly evident and can, depending on the samples, be indistinguishable by optical means. The hexagonal structure and the stacking of the atoms influence the physical properties in many ways including the color, hardness, cleavage, density, and luster. Another important factor in influencing physical properties of minerals is the presence of bonding between atoms. Within dickite there exists dominant O-H bonding, a type of strong ionic bonding.


Structure

Dickite has a
monoclinic crystal system In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
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 An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
and an oxygen atom coordinated to another cation. The reaction is predominantly electrostatic; hence an ionic bonding model is appropriate. Its
axial ratio Axial ratio, for any structure or shape with two or more axes, is the ratio of the length (or magnitude) of those axes to each other - the longer axis divided by the shorter. In ''chemistry'' or ''materials science'', the axial ratio (symbol P) i ...
is a=0.576, b=1, c=1.6135. The hexagonal network of Si-O tetrahedra along with the superimposed layer of Al-O, OH octahedra make up the kaolin layer found in dickite. Dickite is composed of regular sequences of one, two and six kaolin layers. Analysis of the dickite structure reveals the space group to be C4s-Cc. The a and c axis both lie on the glide plane of symmetry. Dickite's structure is made up of a shared layer of corner-sharing tetrahedra filled by a plane of oxygens and hydroxyls along with a sheet of edge-sharing octahedra with every third site left empty. An experiment was conducted using a pseudo-hexagonal crystal of dickite to determine the unit cell information and the layers that exist within dickite. It was found that there are six layers within the kaolin layer within dickite. This is evidenced in the following findings. There is an oxygen atom from the all oxygen layer that lies at the center. The atoms of the O layer, the Si layer and the O, (OH) layer are situated for the ideal kaolin layer. X-ray experiments were performed by C. J. Ksanda and Tom F. W. Barth and it was concluded that dickite is composed of tiny layers of
cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
and
anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...
which are parallel to the a-b plane stacked on top of one another which they found to be exactly as Gruner had described. It was also concluded that the two dimensional arrangement of some of the atoms are not as Gruner described.


References

* {{Clay minerals Phyllosilicates Aluminium minerals Clay minerals group Monoclinic minerals Minerals in space group 9