Mica slurries were found to be flocculated over the whole pH range of between 2 and 12. These slurries of different solid loading appeared to obey the yield stress-DLVO force model producing a single value for the deflocculation point zeta potential or the critical zeta potential (at point of flocculated-disperse state transition) of ~. 48. mV. Such a high value indicates the presence of an additional attractive force in addition to the van der Waals force. This force is attributed to edge-face unlike charge attraction. The use of pyrophosphate additive to minimize edge-face interactions via positive edge charge neutralization caused a large reduction in the critical zeta potential, to 22. mV. With this new value, the Hamaker constant of mica in water was determined to be ~. 13. zJ. This falls within the range of values reported for mica. Low Ca(II) kaolin suspension displayed a lower critical zeta potential of 41. mV and the use of pyrophosphate additive at 0.2 and 0.4. dwb% appeared not to decrease the magnitude of the critical zeta potential. This pyrophosphate-influence critical zeta potential is also independent of Ca(II) content in the kaolin. The established model of edge-face attraction for kaolin suspension at low pH may not be completely correct. A face-face interaction model may be more appropriate. A power law model relating the maximum yield stress with volume fraction showed a very high exponent value of 8 for mica slurries, only 3.1 for the low Ca(II) kaolin and 3.6-3.9 for the high Ca(II) kaolin suspensions. This exponent value may reflect the predominant particle-particle interaction configuration in the clay suspensions. © 2014 Elsevier B.V.