Metakaolins formed by total dehydroxylation of kaolinite, dickite and nacrite were hydrothermally treated at 240°C, for 2, 21 or 182 days, at initial pH = 5–6 or 1. Kaolinite was the only mineral of the kaolin subgroup crystallized in these treatments. Kaolinite is less ordered for pH = 4–6 than for pH = 1. This can be attributed to the influence of specific chemical, morphological, and/or crystalline properties of the metakaolins. Two different processes of kaolinite formation and crystal growth must be involved depending on the pH and metakaolin. The first process implies the dissolution of the metakaolin and crystallization of thin prismatic or dendritic kaolinite particles which curl, probably due to surface tension. The second process implies the rapid formation of small pseudohexagonal kaolinite particles which would aggregate and coalesce to form bigger particles.

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