View source for Kaolinite

{{redirect|Kaolin|its use as a food additive|Aluminium silicate}}
{{Infobox mineral
| name        = Kaolinite
| category    = [[Silicate mineral]]s, phyllosilicate subclass
| strunz      = 09.ED.05
| dana        = 71.1.1.2
| hey         = 15.8
| boxwidth    =
| boxbgcolor  =
| image       = KaoliniteUSGOV.jpg
| imagesize   = 250px
| caption     =
| formula     = Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>
| color       = White, sometimes red, blue or brown tints from impurities
| habit       = earthy
| system      = [[triclinic crystal system|triclinic]]
| symmetry    = 1
| twinning    =
| cleavage    = perfect on {001}
| fracture    = perfect
| mohs        = 2–2½
| luster      = dull and earthy
| refractive  = ''n''<sub>α</sub> = 1.553–1.565<br/>''n''<sub>β</sub> = 1.559–1.569<br/>''n''<sub>γ</sub> = 1.569–1.570
| birefringeance = ''δ'' = 0.007
| 2V          = 24°–50°<br/>calculated: 44° 
| opticalprop = biaxial
| pleochroism = colorless or pale yellow (''x'')<br/>buff or dark buff (''y'')
| streak      = white
| gravity     = 2.6
| melt        =
| fusibility  =
| diagnostic  =
| solubility  = insoluble
| references  = <ref name="WebMineral">{{WebMineral-ref | name = Kaolinite | url = http://www.webmineral.com/data/Kaolinite.shtml | accessdate = 2009-08-05}}</ref><ref name="MinDat">{{MinDat-ref | name = Kaolinite | id = 2156 | accessdate = 2009-08-05}}.</ref><ref>{{citation | last1 = Bish | first1 = D. L. | last2 = von Dreele | first2 = R. B. | title = Rietveld refinement of non-hydrogen atomic positions in kaolinite | journal = Clays Clay Minerals | volume = 37 | year = 1989 | pages = 289–96}}.</ref>
}}

'''Kaolinite''' is a [[clay mineral]] with the chemical composition Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>. It is a layered [[silicate mineral]] (''phyllosilicate''), with one [[Tetrahedron|tetrahedral]] sheet linked through [[oxygen]] atoms to one [[Octahedron|octahedral]] sheet of AlO<sub>6</sub> octahedra.<ref>{{citation | last1 = Deer | first1 = W. A. | last2 = Howie | first2 = R. A. | last3 = Zussman | first3 = J. | title = An introduction to the rock-forming minerals | edition = 2nd | publisher = Longman | location = Harlow, UK | year = 1992 | isbn = 0582300940}}.</ref> Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin.

The name is derived from Gaoling or Kao-Ling ("High Hill") in Jingdezhen, Jiangxi province, [[China]].<ref>{{citation | last = Schroeder | first = Paul | title = New Georgia Encyclopedia | contribution = Kaolin | url = http://www.georgiaencyclopedia.org/nge/Article.jsp?id=h-1178 | accessdate = 2008-08-01 | year = 2003}}.</ref> Kaolinite was first described as a [[mineral species]] in 1867 for an occurrence in the Jari River basin of [[Brazil]].<ref name="MinDat"/>

Kaolinite has a low [[shrink-swell capacity]] and a low [[cation exchange capacity]] (1–15&nbsp;meq/100&nbsp;g.) It is a soft, earthy, usually white mineral (dioctahedral phyllosilicate [[clay]]), produced by the chemical weathering of [[aluminosilicate mineral]]s like [[feldspar]]. In many parts of the world, it is colored pink-orange-red by [[iron oxide]], giving it a distinct [[rust]]y hue. Lighter concentrations yield white, yellow or light orange colours. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, [[United States|USA]].

==Structural transformations==
Kaolin-type clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure. Endothermic dehydroxylation (or alternatively, dehydration) begins at 550–600&nbsp;°C to produce disordered metakaolin, Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>, but continuous hydroxyl loss (-OH) is observed up to 900&nbsp;°C and has been attributed to gradual [[oxolation]] of the metakaolin.<ref name=b1>{{citation | last1 = Bellotto | first1 = M. | last2 = Gualtieri | first2 = A. | last3 =  Artioli | first3 = G. | last4 = Clark | first4 = S. M. | year = 1995 | title = Kinetic study of the kaolinite-mullite reaction sequence. Part I: kaolinite dehydroxylation | journal = Phys. Chem. Minerals | volume = 22 | pages = 207–14}}.</ref> Because of historic disagreement concerning the nature of the metakaolin phase, extensive research has led to general consensus that metakaolin is not a simple mixture of amorphous silica (SiO<sub>2</sub>) and alumina (Al<sub>2</sub>O<sub>3</sub>), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers <ref name=b1/>. 
:2Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> → 2Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> + 4 H<sub>2</sub>O

Further heating to 925–950&nbsp;°C converts metakaolin to a defect aluminium–silicon [[spinel]], Si<sub>3</sub>Al<sub>4</sub>O<sub>12</sub>, which is sometimes also referred to as a gamma-alumina type structure:
:2Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> → Si<sub>3</sub>Al<sub>4</sub>O<sub>12</sub> + SiO<sub>2</sub>

Upon calcination to ~1050&nbsp;°C, the spinel phase (Si<sub>3</sub>Al<sub>4</sub>O<sub>12</sub>) nucleates and transforms to [[mullite]], 3Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>, and highly crystalline [[cristobalite]], SiO<sub>2</sub>: 
:3Si<sub>3</sub>Al<sub>4</sub>O<sub>12</sub> → 2Si<sub>2</sub>Al<sub>6</sub>O<sub>13</sub> + 5SiO<sub>2</sub>

==Occurrence==
[[Image:NGruev.kaolin.mine.bulgaria.jpg|thumb|375px|A kaolin mine in [[Ruse Province]], [[Bulgaria]]]]
Kaolinite is one of the most common minerals; it is mined, as kaolin, in [[Brazil]], [[Bulgaria]], [[France]], [[United Kingdom]], [[Germany]], [[India]], [[Australia]], [[Korea]], the [[People's Republic of China]], the [[Czech Republic]], and the [[United States]]. 

===Predominance in tropical soils===
Kaolinite clay occurs in abundance in [[soil]]s that have formed from the chemical [[weathering]] of rocks in hot, moist [[climate]]s, for example in [[tropical rainforest]] areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolonite decreases, while the proportion of other clay minerals such as [[illite]] (in cooler climates) or [[smectite]] (in drier climates) increases. Such climatically-related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved.

==Uses==
Kaolin is used in [[ceramic]]s, [[medicine]], [[coated paper]], as a [[food additive]], in [[toothpaste]], as a light diffusing material in white incandescent [[light bulbs]], and in [[cosmetics]]. It is generally the main component in [[porcelain]].

It is also used in [[paint]] to extend [[titanium dioxide]] (TiO<sub>2</sub>) and modify gloss levels; in [[rubber]] for semi-reinforcing properties; and in adhesives to modify [[rheology]].<ref>{{citation | title = Kaolin (China Clay) | url = http://www.imerys-perfmins.com/kaolin/eu/kaolin.htm | publisher = Imerys Performance Minerals | accessdate = 2008-08-01}}.</ref>

Kaolin was also used in the production of common pipes for centuries in Europe and Asia.  The practice of making and using kaolin pipes was brought to the colonies and reproduced once adequate sources of kaolin were discovered.  (These distinctive pipes with unusually long stems when new are seen regularly in Renaissance paintings of après-hunt scenes or portraits of people relaxing with a kaolin pipe in one hand and the white stem stretching across the canvas.)  The plain, long-stemmed, slender, small-bowled pipes were typically stored on the mantle.  A member of the household would break off a 2&nbsp;cm piece of the stem to provide a "fresh" mouthpiece before filling and lighting.  (As a sign of hospitality, hosts would offer guests his/her own pipe from the mantle and break off the mouthpiece for them, ensuring a completely fresh pipe.)  Though most pipes were undecorated, a few had monograms or coats of arms either embossed onto or pressed into the wet paste before firing.  Some were even pressed into molds of human or animal heads.  Archaeologists have been using kaolin pipes to date sites for decades now ever since the inverse relationship between time and bore diameter was confirmed.  As ceramic technology improved through time, the diameter of the hole in the stem leading to the bowl decreased.  The change has been found to be so constant and measurable as to allow dating of sites to specific decades.<ref>See ''Martin's Hundred'' and ''Flowerdew Hundred,'' both by Ivor Noël Hume as well as dozens of references and samples in the Archaeology Lab at the University of New Orleans.</ref>  Kaolin pipes grew to be obsolete when paper began to be used to roll tobacco into cigarettes.

The largest use is in the production of paper, including ensuring the gloss on some grades of paper. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle or as liquid [[slurry]]. 

Kaolinite has also seen some use in [[organic farming]], as a [[Kaolin spray|spray]] applied to crops to deter [[Codling moth|insect]] damage, and in the case of apples, to prevent sun scald.

A [[folk medicine]] use is to soothe an upset [[stomach]], similar to the way [[parrot]]s (and later, humans) in [[South America]] originally used it.<ref>{{citation | title = Evolutionary Biology: Dirty eating for healthy living | last = Diamond | first = Jared M. | journal = Nature | volume = 400 | pages = 120–21 | url = http://cogweb.ucla.edu/Abstracts/Diamond_99.html}}.</ref>

Kaolin is, or has been, used as the active substance in liquid anti-[[diarrhea]] medicines such as Kaomagma and [[Kaopectate]]. Such medicines were changed away from [[aluminium]] substances due to a scare over [[Alzheimer's disease]], but have since changed back to compounds containing aluminium as they are more effective. Kaolin is known in [[Traditional Chinese Medicine]] as an herb under the name 赤石脂 (chì shí zhī), "crimson stone resin" in a direct translation. Its taste is sweet, astringent and warm. In [[traditional Chinese medicine]], it's used for restricting leakage from intestines and stopping diarrhea, blood containment and stopping bleeding, wounds healing.

In April 2008, the [[Naval Medical Research Center]] announced the successful use of a Kaolinite-derived [[aluminosilicate]] [[nanoparticles|nanoparticle]] infusion in traditional [[gauze]], known commercially as QuikClot Combat Gauze.<ref>{{citation | last = Rowe | first = Aaron | title = Nanoparticles Help Gauze Stop Gushing Wounds | url = http://www.wired.com/medtech/health/news/2008/04/blood_clotting | publisher = Wired.com | accessdate = 2009-08-05}}.</ref>

When heated to between 650 and 900&nbsp;°C kaolinite dehydroxylates to form [[metakaolin]]. According to the American National Precast Concrete Association this is a [[supplementary cementitious material]] (SCM). When added to a concrete mix, metakaolin affects the acceleration of [[Portland cement]] hydration when replacing Portland cement by 20% by weight.

In [[ceramics]] or [[pottery]] applications, the formula is typically written in terms of oxides, thus the formula for kaolinite is Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>·2H<sub>2</sub>O.

This format is also useful for describing the firing process of clay as the kaolin loses the 2 water molecules, termed the [[chemical water]], when fired to a high enough temperature. This is different from clay's physical water which will be lost simply due to [[evaporation]] and is not a part of the [[chemical formula]].

==See also==
[[Image:KaolinUSGOV.jpg|thumb|right|Kaolin. (''unknown scale'')]]
*[[Silicate minerals]]
*[[Dickite]]
*[[Geology of Cornwall]]
*[[Halloysite]]
*[[Nacrite]]
*[[Medicinal clay]]
*[[Porcelain]]
*[[Clay pit]]
*[[Kaolin Deposits of Charentes Basin, France]]

==References==
{{reflist}}

===Further reading===
*{{citation | last1 = Deer | first1 = W. A. | last2 = Howie | first2 = R. A. | last3 = Zussman | first3 = J. | year = 1992 | title = An introduction to the rock-forming minerals | edition = 2nd | location = Harlow, UK | publisher = Longman | isbn = 0-582-30094-0}}.
*{{citation | last1 = Hurlbut | first1 = Cornelius S. | last2 = Klein | first2 = Cornelis | year = 1985 | title = Manual of Mineralogy, after J.&nbsp;D.&nbsp;Dana | edition = 20th | publisher = Wiley | location = New York | pages = 428–29 | isbn = 0-471-80580-7}}.
*{{citation | last = Breck | first = D. W. | year = 1984 | title = Zeolite Molecular Sieves | publisher = Robert E. Brieger | location = Malabar, FL | pages = 314–15 | isbn = 0-89874-648-5}}.

==External links==
*[http://www.wheal-martyn.com/ China Clay Museum]
*[http://mineral.galleries.com/minerals/silicate/kaolinit/kaolinit.htm The Mineral KAOLINITE - Mineral Galleries]
*[http://www.gelighting.com/na/business_lighting/education_resources/environmental/downloads/msd/msds_incandescent_lamps.pdf MSDS: Incandescent Light Bulb - GE]

[[Category:Aluminosilicate minerals]]
[[Category:Phyllosilicate minerals]]

{{Imported from Wikipedia|name=Kaolinite|id=330270382}}