Rarely as crystals, thin plates or stacked. More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses.
Rocks that are rich in kaolinite, andhalloysite, are known askaolin (/ˈkeɪ.əlɪn/) orchina clay.[10] In many parts of the world kaolin is colored pink-orange-red byiron oxide, giving it a distinctrust hue. Lower concentrations of iron oxide yield the white, yellow, or light orange colors of kaolin. Alternating lighter and darker layers are sometimes found, as atProvidence Canyon State Park in Georgia, United States.
Kaolin is an importantraw material in many industries and applications. Commercial grades of kaolin are supplied and transported as powder, lumps, semi-dried noodle orslurry. Global production of kaolin in 2021 was estimated to be 45 million tonnes,[11] with a total market value of US $4.24 billion.[12]
Kaolinite is also occasionally discussed under thearchaic nameslithomarge and lithomarga fromLatinlithomarga, a combination oflitho- (Ancient Greek:λίθος，líthos, "stone") andmarga ("marl"). In more proper modern use, lithomarge now refers specifically to a compacted and massive form of kaolin.[16]
Thechemical formula for kaolinite as written inmineralogy isAl2Si2O5(OH)4,[4] however, inceramics applications the same formula is typically written in terms of oxides, thus givingAl2O3·2SiO2·2H2O.[17]
Kaolinite structure, showing the interlayer hydrogen bonds
Compared with other clay minerals, kaolinite is chemically and structurally simple. It is described as a 1:1 orTO clay mineral because its crystals consist of stackedTO layers. EachTO layer consists of a tetrahedral (T) sheet composed of silicon and oxygen ions bonded to an octahedral (O) sheet composed of oxygen, aluminium, and hydroxyl ions. TheT sheet is so called because each silicon ion is surrounded by four oxygen ions forming a tetrahedron. TheO sheet is so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. The two sheets in each layer are strongly bonded together via shared oxygen ions, while layers are bonded viahydrogen bonding between oxygen on the outer face of theT sheet of one layer and hydroxyl on the outer face of theO sheet of the next layer.[18]
View of the structure of the tetrahedral (T) sheet of kaolinite
View of the structure of the octahedral (O) sheet of kaolinite
Kaolinite crystal structure looking along the layers
A kaolinite layer has no net electrical charge and so there are no large cations (such as calcium, sodium, or potassium) between layers as with most other clay minerals. This accounts for kaolinite's relatively low ion exchange capacity. The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's nonswelling character.[18]
When moistened, the tiny platelike crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being molded, but strong enough to hold the plates in place and allow the molded clay to retain its shape. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic.[19]
High-energy milling of kaolin results in the formation of a mechanochemically amorphized phase similar tometakaolin, although the properties of this solid are quite different.[20] The high-energy milling process is highly inefficient and consumes a large amount of energy.[21]
Below 100 °C, exposure to low humidity air will result in the slow evaporation of any liquid water in the kaolin. At low moisture content the mass can be describedleather dry, and at near 0% moisture it is referred to asbone dry.
Above 100 °C any remaining free water is lost. Above around 400 °C hydroxyl ions (OH−) are lost from the kaolinite crystal structure in the form of water: the material cannot now be plasticised by absorbing water.[22] This is irreversible, as are subsequent transformations; this is referred to ascalcination.
Endothermic dehydration of kaolinite begins at 550–600 °C producing disorderedmetakaolin, but continuoushydroxyl loss is observed up to 900 °C (1,650 °F).[23] Although historically there was much disagreement concerning the nature of the metakaolin phase, extensive research has led to a general consensus that metakaolin is not a simple mixture of amorphous silica (SiO2) and alumina (Al2O3), but rather a complex amorphous structure that retains some longer-range order (but notstrictly crystalline) due to stacking of its hexagonal layers.[23]
Finally, at 1400 °C the "needle" form ofmullite appears, offering substantial increases in structural strength and heat resistance. This is a structural but not chemical transformation. Seestoneware for more information on this form.
Mantles of kaolinite are common in Western and Northern Europe. The ages of these mantles areMesozoic to Early Cenozoic.[24]
Kaolinite clay occurs in abundance insoils that have formed from the chemicalweathering of rocks in hot, moistclimates; for example intropical rainforest areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolinite decreases, while the proportion of other clay minerals such asillite (in cooler climates) orsmectite (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.[25]
In the United States, the main kaolin deposits are found in centralGeorgia, on a stretch of theAtlantic Seaboard fall line betweenAugusta andMacon. This area of thirteen counties is called the "white gold" belt;Sandersville is known as the "Kaolin Capital of the World" due to its abundance of kaolin.[27][28][29] In the late 1800s, an active kaolin surface-mining industry existed in the extreme southeast corner of Pennsylvania, near the towns ofLandenberg andKaolin, and in what is present-day White Clay Creek Preserve. The product was brought by train toNewark, Delaware, on theNewark-Pomeroy line, along which can still be seen many open-pit clay mines. The deposits were formed between the lateCretaceous and earlyPaleogene, about 100 to 45 million years ago, in sediments derived from weatheredigneous and metakaolin rocks.[14] Kaolin production in the United States during 2011 was 5.5 million tons.[30]
ABuell dryer in the UK, which is used to dry processed kaolin
Difficulties are encountered when trying to explain kaolinite formation under atmospheric conditions by extrapolation of thermodynamic data from the more successful high-temperature syntheses.[32] La Iglesia and Van Oosterwijk-Gastuche (1978)[33] thought that the conditions under which kaolinite will nucleate can be deduced from stability diagrams, based as they are on dissolution data. Because of a lack of convincing results in their own experiments, La Iglesia and Van Oosterwijk-Gastuche (1978) had to conclude, however, that there were other, still unknown, factors involved in the low-temperature nucleation of kaolinite. Because of the observed very slow crystallization rates of kaolinite from solution at room temperature Fripiat and Herbillon (1971) postulated the existence of high activation energies in the low-temperature nucleation of kaolinite.
At high temperatures,equilibrium thermodynamic models appear to be satisfactory for the description of kaolinite dissolution andnucleation, because the thermal energy suffices to overcome theenergy barriers involved in thenucleation process. The importance of syntheses at ambient temperature and atmospheric pressure towards the understanding of the mechanism involved in the nucleation of clay minerals lies in overcoming these energy barriers. As indicated by Caillère and Hénin (1960)[34] the processes involved will have to be studied in well-defined experiments, because it is virtually impossible to isolate the factors involved by mere deduction from complex natural physico-chemical systems such as thesoil environment.Fripiat and Herbillon (1971),[35] in a review on the formation of kaolinite, raised the fundamental question how adisordered material (i.e., theamorphous fraction of tropical soils) could ever be transformed into a corresponding ordered structure. This transformation seems to take place in soils without major changes in the environment, in a relatively short period of time, and at ambienttemperature (andpressure).
Low-temperature synthesis of clay minerals (with kaolinite as an example) has several aspects. In the first place the silicic acid to be supplied to the growing crystal must be in a monomeric form, i.e., silica should be present in very dilute solution (Caillère et al., 1957;[36] Caillère and Hénin, 1960;[34] Wey and Siffert, 1962;[37] Millot, 1970[38]). In order to prevent the formation ofamorphoussilicagels precipitating from supersaturated solutions without reacting with thealuminium ormagnesiumcations to form crystallinesilicates, thesilicic acid must be present in concentrations below the maximum solubility of amorphous silica. The principle behind this prerequisite can be found in structural chemistry: "Since the polysilicate ions are not of uniform size, they cannot arrange themselves along with the metal ions into a regular crystal lattice." (Iler, 1955, p. 182[39])
The second aspect of the low-temperature synthesis of kaolinite is that thealuminium cations must be hexacoordinated with respect tooxygen (Caillère and Hénin, 1947;[40] Caillère et al., 1953;[41] Hénin and Robichet, 1955[42]). Gastuche et al. (1962)[43] and Caillère and Hénin (1962) have concluded that kaolinite can only ever be formed when the aluminium hydroxide is in the form ofgibbsite. Otherwise, the precipitate formed will be a "mixed alumino-silicic gel" (as Millot, 1970, p. 343 put it). If it were the only requirement, large amounts of kaolinite could be harvested simply by adding gibbsite powder to a silica solution. Undoubtedly a marked degree of adsorption of the silica in solution by the gibbsite surfaces will take place, but, as stated before, mere adsorption does not create the layer lattice typical of kaolinite crystals.
The third aspect is that these two initial components must be incorporated into one mixed crystal with a layer structure. From the following equation (as given by Gastuche and DeKimpe, 1962)[44] for kaolinite formation
2Al(OH)3 + 2H4SiO4 → Si2O5•Al2(OH)4 + 5H2O
it can be seen that five molecules of water must be removed from the reaction for everymolecule of kaolinite formed. Field evidence illustrating the importance of the removal of water from the kaolinite reaction has been supplied by Gastuche and DeKimpe (1962). While studyingsoil formation on abasaltic rock inKivu (Zaïre), they noted how the occurrence of kaolinite depended on the"degrée de drainage" of the area involved. A clear distinction was found between areas with gooddrainage (i.e., areas with a marked difference between wet and dry seasons) and those areas with poordrainage (i.e.,perenniallyswampy areas). Kaolinite was only found in the areas with distinct seasonal alternations between wet and dry. The possible significance of alternating wet and dry conditions on the transition ofallophane into kaolinite has been stressed by Tamura and Jackson (1953).[45] The role of alternations between wetting and drying on the formation of kaolinite has also been noted by Moore (1964).[46]
Syntheses of kaolinite at high temperatures (more than 100 °C [212 °F]) are relatively well known. There are for example the syntheses of Van Nieuwenberg and Pieters (1929);[47] Noll (1934);[48] Noll (1936);[49] Norton (1939);[50] Roy and Osborn (1954);[51] Roy (1961);[52] Hawkins and Roy (1962);[53] Tomura et al. (1985);[54] Satokawa et al. (1994)[55] and Huertas et al. (1999).[56]Relatively few low-temperature syntheses have become known (cf. Brindley and DeKimpe (1961);[57] DeKimpe (1969);[58] Bogatyrev et al. (1997)[59]).
Laboratory syntheses of kaolinite at room temperature and atmospheric pressure have been described by DeKimpe et al. (1961).[60] From those tests the role of periodicity becomes convincingly clear. DeKimpe et al. (1961) had used daily additions ofalumina (asAlCl3·6 H2O) andsilica (in the form ofethyl silicate) during at least two months. In addition, adjustments of the pH took place every day by way of adding eitherhydrochloric acid orsodium hydroxide. Such daily additions of Si and Al to the solution in combination with the daily titrations withhydrochloric acid orsodium hydroxide during at least 60 days will have introduced the necessary element of periodicity. Only now the actual role of what has been described as the "aging" (Alterung) of amorphous alumino-silicates (as for example Harder, 1978[61] had noted) can be fully understood. As such, time is not bringing about any change in a closed system at equilibrium; but a series of alternations of periodically changing conditions (by definition, taking place in an open system) will bring about the low-temperature formation of more and more of the stable phase kaolinite instead of (ill-defined) amorphous alumino-silicates.
In 2009, up to 70% of kaolin was used in the production ofpaper. Following reduced demand from the paper industry, resulting from both competing minerals and the effect of digital media, in 2016 the market share was reported to be: paper, 36%; ceramics, 31%; paint, 7% and other, 26%.[62][63] According to theUSGS, in 2021 the global production of kaolin was estimated to be around 45 million tonnes.[64]
Paper applications require high-brightness, low abrasion and delaminated kaolins. For paper coatings it is used to enhance the gloss, brilliance, smoothness and receptability to inks; it can account for 25% of mass of the paper. As a paper filler it is used as a pulp extender, and to increase opacity; it can account for 15% of mass.[65][66][67]
In whitewareceramic bodies, kaolin can constitute up to 50% of the raw materials. In unfired bodies it contributes to the green strength, plasticity and rheological properties, such as the casting rate. During firing it reacts with other body components to form the crystal and glass phases. With suitable firing schedules it is key to the formation ofmullite. The most valued grades have low contents of chromophoric oxides such that the fired material has high whiteness.[68][66][69][70] In glazes it is primarily used as a rheology control agent, but also contributes some green strength. In both glazes and frits it contributes some SiO2 as a glass network former, and Al2O3 as both a network former and modifier.[71]
To soothe an upsetstomach, similar to the wayparrots (and later, humans) inSouth America originally used it[74] (more recently, industrially produced).
Kaolin-based preparations are used for treatment ofdiarrhea.
Humans sometimes eat kaolin for pleasure or to suppress hunger,[77] a practice known asgeophagy. In Africa, kaolin used for such purposes is known askalaba (inGabon[78] andCameroon[77]),calaba, andcalabachop (inEquatorial Guinea). Consumption is greater among women, especially during pregnancy,[79] and its use is sometimes said by women of the region to be a habit analogous to cigarette smoking among men. The practice has also been observed within a small population of African-American women in theSouthern United States, especiallyGeorgia, likely brought with the traditions of the aforementioned Africans viaslavery.[80][81] There, the kaolin is calledwhite dirt,chalk orwhite clay.[80]
Research results show that the utilization of kaolinite ingeotechnical engineering can be alternatively replaced by safer illite, especially if its presence is less than 10.8% of the total rock mass.[82]
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