Sepiolite is a natural clay mineral characterized by a nanofiber structure, unique crystal morphology and composition, and high surface area. It is capable of producing stable suspensions of high viscosity at lower solid concentrations. Dispersion of sepiolite fibers in water can increase the inner and outer surface areas of fibers in the form of a network which enables adsorption of water molecules within the inter particles resulting in a significant increase on the viscosity of the suspension. The viscosity of 3% (w/w) sepiolite suspension prepared at 21,000 rpm remarkably increased with increasing the stirring time from 1 to 3 min. Sepiolite particles are expected to disperse in water to nanosizes. Towards this aim, an Atomic Force Microscopy (AFM) study was undertaken to determine the dimensions of the fibers against the stirring time. The sepiolite suspensions stirred for 1 min showed that the fibers remained in the form of bundles. An increase in the stirring time to 3 min caused the fibers to break into pieces on all dimensions but less effective on the length. However, in the case of 5 min of stirring time, those broken fiber pieces could not organize themselves in a randomly establishing network and thus led to a significant viscosity reduction. The AFM study revealed that the average fiber dimensions at the highest viscosity were determined as 249 x 1127 x 29 nm (width x length x height). The size distribution of fibers is elaborated in order to define an optimum strategy for fiber disintegration. (C) 2009 Elsevier B.V. All rights reserved.