Editing Kaolinite (section)

==Synthesis and genesis==
Difficulties are encountered when trying to explain kaolinite formation under atmospheric conditions by extrapolation of thermodynamic data from the more successful high-temperature syntheses.<ref>{{Cite book|url=https://core.ac.uk/display/29380966|title=Relative stabilities of soil minerals|vauthors=Meijer EL, van der Plas L|publisher=Veenman|year=1980|series=Mededelingen Landbouwhogeschool Wageningen|volume=80|location=Wageningen|pages=18}}</ref> La Iglesia and Van Oosterwijk-Gastuche (1978)<ref>{{Cite journal|vauthors=La Iglesia A, Van Oosterwyck-Gastuche MC|date=1978|title=Kaolinite Synthesis. I. Crystallization Conditions at Low Temperatures and Calculation of Thermodynamic Equilibria. Application to Laboratory and Field Observations|journal=Clays and Clay Minerals|volume=26|issue=6|pages=397–408|doi=10.1346/CCMN.1978.0260603|bibcode=1978CCM....26..397L|doi-access=free}}</ref> 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, [[thermodynamic equilibrium|equilibrium]] thermodynamic models appear to be satisfactory for the description of kaolinite dissolution and [[nucleation]], because the thermal energy suffices to overcome the [[energy barrier]]s involved in the [[nucleation]] 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)<ref name="Caillère-1960">{{Cite journal|vauthors=Caillère S, Hénin S|date=1960|title=Vues d'ensemble sur le problème de la synthèse des minéraux argileux à basse température|journal=Bulletin du Groupe français des argiles|language=fr|volume=12|issue=7|pages=63|doi=10.3406/argil.1960.969}}</ref> 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 the [[soil]] environment.
Fripiat and Herbillon (1971),<ref>{{Cite book|title=Soils and tropical weathering: proceedings of the Bandung Symposium 16 to 23 November 1969|vauthors=Fripiat JJ, Herbillon AJ|publisher=[[Unesco]]|year=1971|series=Natural resources research|volume=11|location=Paris|pages=15–24|chapter=Formation and transformations of clay minerals in tropical soils|oclc=421565}}</ref> in a review on the formation of kaolinite, raised the fundamental question how a [[Randomness|disordered]] material (i.e., the [[amorphous]] 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 ambient [[temperature]] (and [[pressure]]).

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;<ref>{{Cite journal|vauthors=Caillère S, Hénin S, Esquevin J|date=1957|title=Synthèse des minéraux argileux.|journal=Bulletin du Groupe français des argiles|language=fr|volume=9|issue=4|pages=67–76|doi=10.3406/argil.1957.940}}</ref> Caillère and Hénin, 1960;<ref name="Caillère-1960" /> Wey and Siffert, 1962;<ref>{{Cite journal|vauthors=Wey R, Siffert B|date=1961|title=Réactions de la silice monomoléculaire en solutions avec les ions Al3+ et Mg2+|journal=Colloques Internationaux|language=fr|publisher=Centre National des Recherches Scientifiques|volume=105|pages=11–23}}</ref> Millot, 1970<ref>{{Cite book|title=Geology of Clays|last=Millot|first=Georges|publisher=Springer-Verlag|year=1970|isbn=9783662416099|location=New York|translator-last=Paquet|translator-first=H.|doi=10.1007/978-3-662-41609-9|s2cid=128831318 |name-list-style=vanc|translator-last2=Farrand|translator-first2=W. R.}}</ref>). In order to prevent the formation of [[amorphous]] [[silica]] [[gel]]s precipitating from supersaturated solutions without reacting with the [[aluminium]] or [[magnesium]] [[cation]]s to form crystalline [[silicate]]s, the [[silicic 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.&nbsp;182<ref>{{Cite book|title=The colloid chemistry of silica and silicates|last=Iler|first=R. K.|publisher=Cornell University Press|year=1955|location=Ithaca, N.Y.|name-list-style=vanc}}</ref>)

The second aspect of the low-temperature synthesis of kaolinite is that the [[aluminium]] cations must be hexacoordinated with respect to [[oxygen]] (Caillère and Hénin, 1947;<ref>{{Cite journal|vauthors=Caillère S, Hénin S|date=1947|title=Formation d'une phyllite du type kaolinique par traitement d'une montmorillonite|journal=Comptes Rendus de l'Académie des Sciences de Paris|volume=224|issue=1|pages=53–55}}</ref> Caillère et al., 1953;<ref>{{Cite journal|vauthors=Caillère S, Hénin S, Esquevin J|date=1953|title=Recherches sur la synthèse des minéraux argileux|journal=Bulletin de la Société française de Minéralogie et de Cristallographie|language=fr|volume=76|issue=7|pages=300–314|doi=10.3406/bulmi.1953.4841}}</ref> Hénin and Robichet, 1955<ref>{{Cite journal|vauthors=Hénin S, Robichet O|date=1955|title=Résultats obtenus au cours de nouveaux essais de synthèse de minéraux argileux|journal=Bulletin du Groupe français des argiles|language=fr|volume=6|issue=1|pages=19–22|doi=10.3406/argil.1955.1257}}</ref>). Gastuche et al. (1962)<ref>{{Cite journal|vauthors=Gastuche MC, Fripiat JJ, DeKimpe C|date=1962|title=La genèse des minéraux argileux de la famille du kaolin. I. – Aspect colloidal|journal=Colloque C.N.R.S.|volume=105|pages=57–65}}</ref> and Caillère and Hénin (1962) have concluded that kaolinite can only ever be formed when the aluminium hydroxide is in the form of [[gibbsite]]. Otherwise, the precipitate formed will be a "mixed alumino-silicic gel" (as Millot, 1970, p.&nbsp;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)<ref>{{Cite journal|vauthors=Gastuche MC, DeKimpe C|date=1962|title=La genèse des minéraux argileux de la famille du kaolin. II. Aspect cristallin|journal=Colloque C.N.R.S.|volume=105|pages=75–88}}</ref> for kaolinite formation
:<chem>2Al(OH)3 + 2H4SiO4 -> Si2O5 . Al2(OH)4 + 5H2O</chem>
it can be seen that five molecules of water must be removed from the reaction for every [[molecule]] 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 studying [[soil formation]] on a [[basalt]]ic rock in [[Kivu]] ([[Zaïre]]), they noted how the occurrence of kaolinite depended on the {{lang|fr|"degrée de drainage"}} of the area involved. A clear distinction was found between areas with good [[drainage]] (i.e., areas with a marked difference between wet and dry seasons) and those areas with poor [[drainage]] (i.e., [[perennial]]ly [[swamp]]y 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 of [[allophane]] into kaolinite has been stressed by Tamura and Jackson (1953).<ref>{{Cite journal|last=Tamura T, Jackson ML|date=1953|title=Structural and Energy Relationships in the Formation of Iron and Aluminum Oxides, Hydroxides, and Silicates|journal=[[Science (journal)|Science]]|volume=117|issue=3041|pages=381–383|doi=10.1126/science.117.3041.381|pmid=17749950|bibcode=1953Sci...117..381T}}</ref> The role of alternations between wetting and drying on the formation of kaolinite has also been noted by Moore (1964).<ref>{{Cite journal|vauthors=Moore LR|date=1964|title=The in Situ Formation and Development of Some Kaolinite Macrocrystals|url=https://www.cambridge.org/core/journals/clay-minerals-bulletin/article/in-situ-formation-and-development-of-some-kaolinite-macrocrystals/3B15F40C24CBDF3AF6128FC3C0963C2D|journal=[[Clay Minerals]]|volume=5|issue=31|pages=338–352|doi=10.1180/claymin.1964.005.31.02|bibcode=1964ClMin...5..338M}}</ref>

===Laboratory syntheses===
Syntheses of kaolinite at high temperatures (more than {{convert|100|C|disp=sqbr}}) are relatively well known. There are for example the syntheses of Van Nieuwenberg and Pieters (1929);<ref>{{Cite journal|vauthors=van Nieuwenburg CJ, Pieters HA|date=1929|title=Studies on hydrated aluminium silicates: I. The rehydration of metakaolin and the synthesis of kaolin|journal=[[Recueil des Travaux Chimiques des Pays-Bas|Recl. Trav. Chim. Pays-Bas]]|volume=48|issue=1|pages=27–36|doi=10.1002/recl.19290480106}}</ref> Noll (1934);<ref>{{Cite journal|vauthors=Noll W|date=1934|title=Hydrothermale Synthese des Kaolins|journal=Zeitschrift für Kristallographie, Mineralogie und Petrographie|language=de|volume=45|issue=2–3|pages=175–190|doi=10.1007/BF02943371|bibcode=1934ZKMP...45..175N|s2cid=96869398}}</ref> Noll (1936);<ref>{{Cite journal|vauthors=Noll W|date=1936|title=Über die Bildungsbedingungen von Kaolin, Montmorillonit, Sericit, Pyrophyllit und Analcim|journal=Zeitschrift für Kristallographie, Mineralogie und Petrographie|language=de|volume=48|issue=3–4|pages=210–247|doi=10.1007/BF02939458|bibcode=1936ZKMP...48..210N|s2cid=128744123}}</ref> Norton (1939);<ref>{{Cite journal|vauthors=Norton FH|date=1939|title=Hydrothermal formation of clay minerals in the laboratory|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/24/1/1/537069/hydrothermal-formation-of-clay-minerals-in-the|journal=[[American Mineralogist|Am. Mineral.]]|volume=24|issue=1|pages=1–17}}</ref> Roy and Osborn (1954);<ref>{{Cite journal|vauthors=Roy R, Osborn EF|date=1954|title=The system Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>-H<sub>2</sub>O |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/39/11-12/853/539421/the-system-al2o3-sio2-h2o|journal=[[American Mineralogist|Am. Mineral.]]|volume=39|issue=11–12|pages=853–885}}</ref> Roy (1961);<ref>{{Cite journal|vauthors=Roy R|date=1962|title=The preparation and properties of synthetic clay minerals|journal=Colloque C.N.R.S.|volume=105|pages=83–98}}</ref> Hawkins and Roy (1962);<ref>{{Cite journal|vauthors=Hawkins DB, Roy R|date=1962|title=Electrolytic Synthesis of Kaolinite Under Hydrothermal Conditions|journal=[[Journal of the American Ceramic Society|J. Am. Ceram. Soc.]]|volume=45|issue=10|pages=507–508|doi=10.1111/j.1151-2916.1962.tb11044.x}}</ref> Tomura et al. (1985);<ref>{{Cite journal|display-authors=3|vauthors=Tomura S, Shibasaki Y, Mizuta H, Kitamura M|date=1985|title=Growth Conditions and Genesis of Spherical and Platy Kaolinite|journal=Clays and Clay Minerals|volume=33|issue=3|pages=200–206|doi=10.1346/CCMN.1985.0330305|bibcode=1985CCM....33..200T|doi-access=free}}</ref> Satokawa et al. (1994)<ref>{{Cite journal|display-authors=3|vauthors=Satokawa S, Osaki Y, Samejima S, Miyawaki R, Tomura S, Shibasaki Y, Sugahara Y|date=1994|title=Effects of the Structure of Silica-Alumina Gel on the Hydrothermal Synthesis of Kaolinite|journal=Clays and Clay Minerals|volume=42|issue=3|pages=288–297|doi=10.1346/CCMN.1994.0420307|bibcode=1994CCM....42..288S|doi-access=free}}</ref> and Huertas et al. (1999).<ref>{{Cite journal|display-authors=3|vauthors=Huertas FJ, Fiore S, Huertas F, Linares J|date=1999|title=Experimental study of the hydrothermal formation of kaolinite|journal=Chemical Geology|volume=156|issue=1–4|pages=171–190|doi=10.1016/S0009-2541(98)00180-6|bibcode=1999ChGeo.156..171H}}</ref>
Relatively few low-temperature syntheses have become known (cf. Brindley and DeKimpe (1961);<ref>{{Cite journal|vauthors=Brindley GW, De Kimpe C|date=1961|title=Attempted Low-Temperature Syntheses of Kaolin Minerals|journal=[[Nature (journal)|Nature]]|volume=190|issue=4772|pages=254|doi=10.1038/190254a0|bibcode=1961Natur.190..254B|s2cid=4149442|doi-access=free}}</ref> DeKimpe (1969);<ref>{{Cite journal|vauthors=De Kimpe CR|date=1969|title=Crystallization of kaolinite at low temperature from an alumino-silicic gel|journal=Clays and Clay Minerals|volume=17|issue=1|pages=37–38|doi=10.1346/CCMN.1969.0170107|bibcode=1969CCM....17...37D|doi-access=free}}</ref> Bogatyrev et al. (1997)<ref>{{Cite journal|display-authors=3|vauthors=Bogatyrev BA, Mateeva LA, Zhukov VV, Magazina LO|date=1997|title=Low-temperature synthesis of kaolinite and halloysite on the gibbsite – silicic acid solution system|journal=Transactions (Doklady) of the Russian Academy of Sciences|series=Earth science sections|volume=353 A|pages=403–405}}</ref>).

Laboratory syntheses of kaolinite at room temperature and atmospheric pressure have been described by DeKimpe et al. (1961).<ref>{{Cite journal|vauthors=DeKimpe CR, Gastuche MC, Brindley GW|date=1961|title=Ionic coordination in alumino-silicic acids in relation to clay mineral formation|url=http://rruff.info/doclib/am/vol46/AM46_1370.pdf|journal=[[American Mineralogist|Am. Mineral.]]|volume=46|issue=11–12|pages=1370–1381}}</ref> From those tests the role of periodicity becomes convincingly clear. DeKimpe et al. (1961) had used daily additions of [[alumina]] (as {{Chem2|AlCl3*6 H2O}}) and [[silica]] (in the form of [[ethyl silicate]]) during at least two months. In addition, adjustments of the pH took place every day by way of adding either [[hydrochloric acid]] or [[sodium hydroxide]]. Such daily additions of Si and Al to the solution in combination with the daily titrations with [[hydrochloric acid]] or [[sodium 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<ref>{{Cite journal|vauthors=Harder H|date=1978|title=Synthesen von Tonmineralen unter spezieller Berücksichtigung festländischer Bedingungen|journal=Schriftenreihe für geologische Wissenschaften (Berlin)|language=de|volume=11|pages=51–78}}</ref> 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.