Clay minerals possess the peculiar characteristic that they normally occuras very small crystals. We cannot, therefore, afford to ignore thecontribution that colloid science can make to the study of clay minerals.Many colloidal systems are stable because the particles are electricallycharged. There is, of course, electrical neutrality of the system as awhole, but there is an electric separation within the system wherebyelectricity of one kind (it may be a positive or negative, depending on thesystem) is anchored for the time being to the particles while thecomplementary charge exists as ions (counter ions they are often called).Some of the counter ions must be regarded as in solution in the suspendingmedium (normally water) for it is their mobility relative to the particleswhich gives rise to electrical conductivity and to electrophoresis,electro-endosmosis and streaming potential.