One of the present authors (B. T.) previously made studies on the colloid-chemical and rheological properties of the concentrated suspension of sericite, one of the common clay minerals produced in Japan, in water, aqueous salt solution and organic solvent (Ref. 2). The present paper reports a study of flow properties, namely, viscosity and streaming double refraction under various velocity gradients, of the dilute suspension of the same mineral in water and aqueous salt solution. For the viscosity measurement a COUETTE-type rotational viscometer is used and for the measurement of streaming double refraction an EDSALL-type apparatus is applied.The suspension of the mineral in water or in aqueous salt solution exhibits anomalous viscosity as shown in Fig. 1 and Fig. 2. The relative viscosity is remarkably increased by adding salt. Moreover, a hysteresis phenomenon is observed in the course of the viscosity measurement for the suspension in salt solution. The angle of extinction is found to be indendent of the sericite concentration in the range of 0.001-0.03% as shown in Fig. 3 and Fig. 4, while the double refraction increases with the concentration and it becomes almost constant for each concentration, when velocity gradient reaches the value of 500sec-1, as indicated in Fig. 5.These experimental results suggest that the sericite particles behave almost independently to each other and orientate in the direction of the shear force in the above concentration range. However, the effect of the added salt is very remarkable, and for the suspension in salt solution a regular relation between angle of extinction and velocity gradient can not be obtained as shown in Fig. 6. This may be due to a partial aggregation of the particles.From electron microscopic measurement the sericite particle is known to have a blade-like shape (Photo. 1). Assuming an oblate ellipsoide model for the particle, the rotational diffusion coefficient and accordingly, the longer axis of the particle is calculated from the value of dχ/dG. The result obtained is found in good agreement with that of the electron microscopic measurement.The flow properties determined for dilute sericite suspensions are further discussed in relation to the rheological properties of concentrated suspensions. The rheological properties of the sericite suspension are compared with those of suspensions of other clay minerals (Table 1).

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