XANES study of the local environment of iron in natural kaolinites

Kaolinite exists in nature with a variable degree of structural disorder ra nging from partially to completely disordered. Kaolinite invariably contain s 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 stan dards 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 degrees C and 900 degrees 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 co ordinated Fe3+. The disordered sample KGa-2 shows some 4-fold coordinated i ron. Upon heating, for both kaolinites, iron goes into a 4-fold coordinatio n. Fe3+ (and Fe2+) are substituting for Al3+ in kaolinite and thus follow t he structure changes to metakaolinite and subsequent high temperature amorp hous phase. It is not possible to rule out the possibility that some iron i s also contained in amorphous and/or crystalline intercalated hydroxides si nce at 600 degrees C there is still some Fe3+ in an octahedral environment. Notwithstanding, metakaolinite also has some Al3+ (and possibly Fe3+) reta ined in a distorted octahedral environment. At 900 degrees C Fe3+ is in 4-f old and partly in 6-fold coordination as it is the case for Al3+ in mullite .