A member of the kaolin group, with a chemical composition of ideally Al₂Si₂O₅(OH)₄ . x(H₂O). Varying amounts (x) of H₂O may be present in the interlayer, and the terms halloysite (7 Å) and halloysite (10 Å) were recommended for general usage to quantify the amount of H₂O present between layers. Values of x ~ 0 [halloysite (7 Å)] are near kaolinite and x ~2 is halloysite (10 Å). Gentle heating of the halloysite (10 Å) phase will produce halloysite (7 Å), and this is a non-reversible reaction. Halloysite (10 Å) requires storage in water to prevent (partial) dehydration. Halloysite commonly has considerable stacking disorder although a “well crystallized” sample may have an approximate two-layer (2M₁) structure for halloysite (7 Å) for a limited stacking sequence of 6-7 layers. Atomic coordinates for interlayer H₂O are not known, although H₂O within the silicate ring and H₂O in a discontinuous plane between the layers have been suggested. Halloysite layers may be planar, curved, rolled (tubular), and partly spherical to spherical, and these morphologies appear to be related to crystallization conditions and chemical composition. There is no way to conclusively differentiate between halloysite and kaolinite without knowing the history of the sample, although suggestions have included the evidence of the 2M₁ polytype and various treatments involving intercalation as ways to identify halloysite (7 Å).
Cf., dickite, kaolin, kaolinite, nacrite