Editing Kaolinite (section)

===Structural transformations===
Kaolinite group clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure.

====Milling====
High-energy milling of kaolin results in the formation of a mechanochemically amorphized phase similar to [[metakaolin]], although the properties of this solid are quite different.<ref name="kaol">{{cite journal|vauthors=Kasa E, Szabados M, Baan K, Konya Z, Kukovecz A, Kutus B, Palinko I, Sipos P|year=2021|title=The dissolution kinetics of raw and mechanochemically treated kaolinites in industrial spent liquor – The effect of the physico-chemical properties of the solids|journal=[[Applied Clay Science|Appl. Clay Sci.]]|volume=203|page=105994|doi=10.1016/j.clay.2021.105994|bibcode=2021ApCS..20305994K |doi-access=free|hdl=21.11116/0000-0008-06AA-2|hdl-access=free}}</ref> The high-energy milling process is highly inefficient and consumes a large amount of energy.<ref>{{cite book |last1=Baláž |first1=Peter |chapter=High-Energy Milling |title=Mechanochemistry in Nanoscience and Minerals Engineering |date=2008 |pages=103–132 |doi=10.1007/978-3-540-74855-7_2|isbn=978-3-540-74854-0 }}</ref>

====Drying====
{{See also|Buell dryer}}
Below 100&nbsp;°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 described ''leather dry'', and at near 0% moisture it is referred to as ''bone dry''.

Above 100&nbsp;°C any remaining free water is lost. Above around 400&nbsp;°C hydroxyl ions (OH<sup>−</sup>) are lost from the kaolinite crystal structure in the form of water: the material cannot now be plasticised by absorbing water.<ref>'Ceramics Are More Than Clay Alone - Raw Materials, Products, Applications' P. Bormans. Cambridge International Science Publishing, 2004. pg. 180</ref> This is irreversible, as are subsequent transformations; this is referred to as ''calcination''.

====Metakaolin====
Endothermic dehydration of kaolinite begins at 550–600&nbsp;°C producing disordered [[metakaolin]], but continuous [[hydroxyl]] loss is observed up to {{convert|900|C}}.<ref name="b1">{{cite journal|display-authors=3|vauthors=Bellotto M, Gualtieri A, Artioli G, Clark SM|year=1995|title=Kinetic study of the kaolinite-mullite reaction sequence. Part I: kaolinite dehydroxylation|journal=[[Physics and Chemistry of Minerals|Phys. Chem. Miner.]]|volume=22|issue=4|pages=207–214|bibcode=1995PCM....22..207B|doi=10.1007/BF00202253|s2cid=95897543}}</ref> 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 ({{Chem2|SiO2}}) and alumina ({{Chem2|Al2O3}}), but rather a complex amorphous structure that retains some longer-range order (but not [[quasicrystal|strictly crystalline]]) due to stacking of its hexagonal layers.<ref name=b1/>
:<chem>Al2Si2O5(OH)4 -> Al2Si2O7 + 2 H2O</chem>

====Spinel====
Further heating to 925–950&nbsp;°C converts metakaolin to an aluminium-silicon [[spinel]] which is sometimes also referred to as a gamma-alumina type structure:
:<chem>2 Al2Si2O7 -> Si3Al4O12 + SiO2</chem>

====Platelet mullite====
Upon calcination above 1050&nbsp;°C, the spinel phase nucleates and transforms to [[mullite|platelet mullite]] and highly crystalline [[cristobalite]]:
:<chem>3 Si3Al4O12 -> 2 (3 Al2O3 . 2 SiO2) + 5 SiO2</chem>

====Needle mullite====
Finally, at 1400&nbsp;°C the "needle" form of [[mullite]] appears, offering substantial increases in structural strength and heat resistance. This is a structural but not chemical transformation. See [[stoneware]] for more information on this form.