NMR relaxation and diffusion measurements on iron(III)-Doped Kaolin clay

Kaolin clay samples were mixed with various amounts of Fe2O3 powder. The in fluence of this magnetic impurity on NMR relaxation and diffusion measureme nts on the water in this porous material was investigated. The NMR relaxati on measurements showed a nearly mono-exponential decay, leading to the conc lusion that the pore size distribution of the clay samples is either narrow and/or that the pores are interconnected very well. Both the longitudinal and the transverse relaxation rate depend linearly on the concentration of the Fe2O3 impurity. The NMR diffusion measurements revealed that the Fe2O3 causes internal magnetic field gradients that largely exceed the maximum ex ternal gradient that could be applied by our NMR apparatus (0.3 T/m). Addit ional SQUID measurements yielded the magnetization and magnetic susceptibil ity of the samples at the magnetic field strength used in the NMR measureme nts (0.8 T). A theoretical estimate of the internal magnetic field gradient s leads to the conclusion that the water in the porous clay samples cannot be described by the commonly observed motional averaging regime. Probably a n intermediate or a localization regime is induced by the large internal gr adients, which are estimated to be on the order of 1 to 10 T/m in the pore volume and may exceed 1000 T/m at the pore surface. (C) 2001 Academic Press .