Kaolinite exists in nature with a variable degree of structural disorder ranging from partially to completely disordered. Kaolinite invariably contains iron although it is not clear whether it is accommodated in the structure, contained in impurities such as iron hydroxides, or a mixture of the two. The samples studied here are the ordered and disordered international standards KGa-1 and KGa-2 with different iron contents. To obtain the partially dehydroxylated metakaolinite phase, and the completely anhydrous amorphous phase precursor of mullite, the two samples were calcined at 600°C and 900°C, respectively.
The local environment of iron was studied by X-Ray Absorption Spectroscopy (specifically the XANES region) for either natural and calcined samples. It was found that natural kaolinites KGa-1 and KGa-2 mainly contain 6-fold coordinated Fe3+. The disordered sample KGa-2 shows some 4-fold coordinated iron. Upon heating, for both kaolinites, iron goes into a 4-fold coordination. Fe3+ (and Fe2+) are substituting for Al3+ in kaolinite and thus follow the structure changes to metakaolinite and subsequent high temperature amorphous phase. It is not possible to rule out the possibility that some iron is also contained in amorphous and/or crystalline intercalated hydroxides since at 600°C there is still some Fe3+ in an octahedral environment. Notwithstanding, metakaolinite also has some Al3+ (and possibly Fe3+) retained in a distorted octahedral environment. At 900°C Fe3+ is in 4-fold and partly in 6-fold coordination as it is the case for Al3+ ill mullite.