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Mineral: Attapulgite, Palygorskite
Attapulgite is a magnesium aluminum silicate clay of very fine particle size. It is also known as Fullers Earth and is closely related to Sepiolite mineral. Unlike bentonite (or montmorillonite) attapulgite crystals are needle shaped (acicular) rather than flat or flake-like. Like bentonite they disperse well to thicken, suspend and gel suspensions without flocculation problems.
Attapulgite is not typically employed in ceramics (although it would likely be useful as a plasticizer and suspender). The high surface area of attapulgite (and sepiolite) gives it the ability to absorb large amounts of water or oil and its use is mainly driven by the oil drilling and pet litter markets and as a thixotrope for paints, coatings, adhesives, fertilizers. While attapulgite is a fairly consistent magnesium-aluminum-silicate product a down-side of its use (other than expense) is that it is non-swelling & requires high-shear mixing to fully disperse & achieve its thixotropic potential. This differs from the montmorillonite, bentonite, hectorite or other smectite-type swelling minerals. They too benefit from high-shear mixing but they disperse and build viscosity with even mild agitation & might be considered a more robust thixotrope for ceramics applications. On the other hand, attapulgite is not very sensitive to soluble salts compared with smectites & which may make attapulgite a more robust thixotrope in a paint or paste application.
Attapulgite is named after Attapulgus, Georgia, one place where it occurs. The approximate formula has been quoted as Mg5Si8O20(OH)6.4H2O and Mg5Si8O20(HO)2(OH2)44H2O. It is not clear why no Al appears in the formula when this material is known as an alumina silicate? The reason could be that is has a laminated chain structure where crystalline lattice displacement allows the presence of Na+, Ca+, Fe3+ and Al.
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By Tony Hansen