Editing Dickite

{{Short description|Phyllosilicate mineral}}
{{infobox mineral
| name        = Dickite
| category    = [[Phyllosilicates]] <br />Kaolinite-[[serpentine group]]
| boxwidth    = 
| image       = Dickite-d06-179a.jpg
| alt         = 
| caption     = 
| formula     = {{Chem2|Al2Si2O5(OH)4}}
| IMAsymbol   = Dck<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>  
| molweight   = 
| strunz      = 9.ED.05
| dana        = 71.01.01.01
| system      = [[Monoclinic]]
| class       = Domatic (m) <br/><small>(same [[H-M symbol]])</small>
| symmetry    = ''Cc''
| unit cell   = a = 5.150, b = 8.940 <br/>c = 14.424&nbsp;[Å]; β = 96.8°; Z&nbsp;=&nbsp;4
| color       = White, with coloration from impurities
| colour      = 
| habit       = Pseudohexagonal crystals, aggregates of platelets and compact massive
| twinning    = 
| cleavage    = Perfect on {001}
| fracture    = 
| tenacity    = Flexible but inelastic
| mohs        = 1.5–2
| luster      = Satiny to pearly
| streak      = White
| diaphaneity = Transparent
| gravity     = 2.6
| density     = 
| polish      = 
| opticalprop = Biaxial (+)
| refractive  = n<sub>α</sub> = 1.561 – 1.564 n<sub>β</sub> = 1.561 – 1.566 n<sub>γ</sub> = 1.566 – 1.570
| birefringence = δ = 0.005 – 0.006
| pleochroism = 
| 2V          = Measured: 50° to 80°
| dispersion  = 
| extinction  = 
| length fast/slow =
| fluorescence= 
| absorption  =
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| other       = 
| alteration  = 
| references  = <ref name=Handbook>{{Cite book|title=Handbook of mineralogy|vauthors=Anthony JW, Bideaux RA, Bladh KW, Nichols MC|publisher=Mineral Data Publishing|year=1995|isbn=9780962209734|location=Tucson, Ariz.|chapter=Dickite|oclc=20759166|display-authors=3|chapter-url=http://rruff.info/doclib/hom/dickite.pdf}}</ref><ref name=Mindat/><ref name=Webmin>{{Cite web|url=http://webmineral.com/data/Dickite.shtml|title=Dickite Mineral Data|website=webmineral.com|access-date=27 Mar 2019}}</ref><ref name=Dick/><ref name=barth/><ref name=cruz/><ref name=ross/><ref name=hayes/><ref name=Brind/><ref name=Main/><ref name=porter/>
}}

'''Dickite''' ({{Chem2|Al2Si2O5(OH)4}}) is a [[Silicate minerals|phyllosilicate]] [[clay mineral]] named after the [[metallurgy|metallurgical]] chemist Allan Brugh Dick, who first described it. It is [[chemical composition|chemically composed]] of 20.90% [[aluminium]], 21.76% [[silicon]], 1.56% [[hydrogen]] and 55.78% [[oxygen]]. It has the same composition as [[kaolinite]], [[nacrite]], and [[halloysite]], but with a different crystal structure ([[polymorphism (materials science)|polymorph]]). Dickite sometimes contains impurities such as [[titanium]], [[iron]], [[magnesium]], [[calcium]], [[sodium]] and [[potassium]].<ref name="Mindat">{{Cite web|url=https://www.mindat.org/min-1287.html|title=Dickite: Mineral information, data and localities|website=MinDat.org|access-date=27 Mar 2019}}</ref>

Dickite occurs with other clays and requires x-ray diffraction for its positive identification. Dickite is an important alteration indicator{{clarify|date=February 2016}} in [[hydrothermal]] systems as well as occurring in soils and shales.

Dickite's [[Type locality (geology)|type location]] is in [[Pant-y-Gaseg]], [[Amlwch]], [[Isle of Anglesey]], [[Wales]], United Kingdom, where it was first described in 1888.<ref name=Mindat/> Dickite appears in locations with similar qualities and is found in [[China]], [[Jamaica]], [[France]], [[Germany]], [[United Kingdom]], [[United States]], [[Italy]], [[Belgium]] and [[Canada]].<ref name="porter">{{Cite journal|vauthors=Brindley GW, Porter AR|s2cid=41328124|date=1978|title=Occurrence of dickite in Jamaica-ordered and disordered varieties|journal=[[American Mineralogist|Am. Mineral.]]|volume=63|issue=5–6|pages=554–562}}</ref>

==History==
In 1888, Allan Brugh Dick (1833–1926), a Scottish [[metallurgy|metallurgical]] chemist, was on the island of [[Anglesey]] to conduct research on kaolin. He performed various experiments describing the [[clay mineral]].<ref name="ross">{{Cite journal|vauthors=Ross C, Kerr PF|date=1931|title=Dickite, a Kaolin Mineral|url=http://www.minsocam.org/ammin/AM15/AM15_34.pdf|journal=[[American Mineralogist|Am. Mineral.]]|volume=15|issue=1|pages=34–39}}</ref> 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]]. They named it after the first person to describe the mineral.

==Composition==
{{Chem2|Al2Si2O5(OH)4}} is the chemical formula of dickite. The calculated percent abundances are very close when compared to other kaolin minerals.

Chemical composition of dickite:<ref name="cruz">{{Cite journal|vauthors=Cruz MD|title=Genesis and Evolution of the Kaolin-group Minerals During the Diagenesis and the Beginning of Metamorphism|url=http://www.ehu.eus/sem/seminario_pdf/SEMINARIOS_SEM_3_41.pdf|journal=University of the Basque Country Seminar|type=seminar material|pages=41–52}}</ref>
*{{Chem2|SiO2}}	46.54%
*{{Chem2|Al2O3}}	39.50%
*{{Chem2|H2O}}	13.96%

Dickite and other kaolin minerals are commonly developed by weathering of [[feldspar]]s and [[muscovite]].<ref name=cruz/> 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]], 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]] [[limestone]]s, 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]]s 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 is found worldwide in locations such as [[Ouray, Colorado|Ouray]], [[Colorado]], US; San Juanito, [[Chihuahua (state)|Chihuahua]], Mexico, in a silicified zone among the [[rhyolite]] area; and in [[St. George, Utah|St. George]], [[Utah]], US, where the mineral is thought to be associated with [[volcanic rock]].<ref name="Main">{{Cite book|title=Occurrence and microscopic examination of reference clay Mineral specimens: Preliminary reports|vauthors=Holmes RJ|publisher=Columbia University|year=1951|veditors=Ker PF, Main MS, Hamilton PK|series=American Petroleum Institute Research Project|volume=4|location=New York|chapter=Reference Clay Localities|oclc=223495759}}</ref> An extensive study was done on dickite pertaining to its location in [[Pennsylvanian (geology)|Pennsylvanian]] limestones of southeastern [[Kansas]], US.

In the dickite deposits of southeast Kansas the distribution is dependent on the following: the [[stratigraphic]] alternation of limestones and [[shale]]s, westward regional [[Strike and dip|dip]], thick deposits of highly porous algal limestones, and [[igneous intrusion]]s. It was found that [[groundwater]]s substantially heated along with [[magmatic water]]s which made its way up-dip and through the intrusions in the conduit-like [[algal mound]]s 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.<ref name=hayes>{{Cite journal|vauthors=Schroeder RJ, Hayes JB|date=1968|title=Dickite and Kaolinite in Pennsylvanian Limestones of Southeastern Kansas|journal=Clays and Clay Minerals|volume=16|issue=1|pages=41–49|doi=10.1346/CCMN.1968.0160106|bibcode=1968CCM....16...41S|doi-access=}}</ref>

==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]]-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).

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.<ref name="Dick">{{Cite journal|vauthors=Dick AB|date=1888|title=On Kaolinite|url=https://www.minersoc.org/pages/Archive-MM/Volume_8/8-36-15.pdf|url-status=dead|journal=[[Mineralogical Magazine|Mineral. Mag.]]|volume=8|issue=36 |pages=15–27|doi=10.1180/minmag.1888.008.36.03 |bibcode=1888MinM....8...15D |archive-url=https://web.archive.org/web/20161005125030/https://www.minersoc.org/pages/Archive-MM/Volume_8/8-36-15.pdf|archive-date=5 October 2016|access-date=28 March 2019}}</ref>

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.<ref name="Brind">{{Cite journal|vauthors=Newnham RE, Brindley GW|date=1956|title=The Crystal Structure of Dickite|journal=[[Acta Crystallographica|Acta Crystallogr.]]|volume=9|issue=9|pages=759–764|doi=10.1107/S0365110X56002060|doi-access=free|bibcode=1956AcCry...9..759N }}</ref>

==Structure==
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 bond|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.

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.<ref name=Brind/> 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.<ref name=cruz/>

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.<ref name=Brind/>

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]] and [[anions]] 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.<ref name="barth">{{Cite journal|vauthors=Ksanda CJ, Barth TF|date=1935|title=Note on the Structure of Dickite and Other Clay Minerals|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/20/9/631/537647/note-on-the-structure-of-dickite-and-other-clay|journal=[[American Mineralogist|Am. Mineral.]]|volume=20|issue=9|pages=631–637}}</ref>

==References==
{{Reflist|2}}
*{{Cite journal|display-authors=3|vauthors=Fialips CI, Majzlan J, Beaufort D, Navrotsky A|date=2003|title=New thermochemical evidence on the stability of dickite vs. kaolinite|journal=[[American Mineralogist|Am. Mineral.]]|volume=88|issue=5–6|pages=837–845|doi=10.2138/am-2003-5-612|bibcode=2003AmMin..88..837F|s2cid=5174457}}

{{Phyllosilicates}}
{{Clay minerals}}

[[Category:Phyllosilicates]]
[[Category:Aluminium minerals]]
[[Category:Clay minerals group]]
[[Category:Monoclinic minerals]]
[[Category:Minerals in space group 9]]