Purified kaolins from Thai soil on diverse parent materials were characterized using analytical transmission electon microscopy, X-ray diffraction, thermogravimetric analysis and chemical analysis. The properties of Thai soil kaolins appear to be more diverse than Indonesian and Western Australian soil kaolins investigated using the same analytical procedures; this difference may reflect the greater range of parent materials for the Thai soils. The kaolins show a variety of crystal morphologies including euhedral hexagonal to subhedral platy crystals, tubes and laths and several morphologies were present in most samples. TEM-EDS enabled analysis of single crystals of each morphology present within a sample. Tubular or lath-shaped crystals usually have lower %Fe2O3 contents than hexagonal platy crystals in the same sample. The relationships between crystal size and Fe content within morphological populations were also examined by TEM-EDS. Generally, smaller kaolin crystals display a wider range of Fe concentration than the larger kaolin crystals in the same sample. Increasing Fe concentration in bulk samples is closely correlated to decreasing coherently scattering domain size (R2 = 0.57), increasing cation exchange capacity (R2 = 0.44) and increasing specific surface area (R2 = 0.65). However the properties of the deferrated soil kaolins, including their Fe content, are not related to forms of Fe, (total Fe, amorphous or organic) in the untreated clay fraction of the soil.
Inhibited vermiculite is a common minor constituent of these clay fractions and its average structural formula derived from EDS data indicates that it was formed by Al replacing K in muscovite. One Al3+ ion occupies the interlayer space previously occupied by three K+ ions. As the distances between these Al3+ cations in the interlayer space is large it is proposed that isolated or loosely associated hydrated Al3+ groups such as Al(OH2)3+ 6 exist that resist exchange by other cations due to hydrogen bonding with the adjacent tetrahedral oxygen surfaces.