SIZE OF MICA DOMAINS AND DISTRIBUTION OF THE ADSORBED NA-CA IONS

Mica domains have received less attention in the literature than smect ite quasi-crystals. This study was conducted to determine whether mica crystals form domains in suspension, the conditions in which those do mains exist, and the distribution of adsorbed Na and Ca ions in the do mains. Particle size distributions and electrophoretic mobilities (EM) of Silver Hill illite in suspension densities of 0.5 g liter-1 were d etermined by photon correlation spectroscopy (PCS). Solutions at salt concentration from 2 to 10 mmol(c) liter-1, sodium adsorption ratio (S AR) from 0 to infinity (mmol liter-1)0.5, and pH values 5, 7, and 9 we re used to prepare the clay suspensions. The particle size of Silver H ill illite suspensions showed a bimodal distribution. Through PCS meas urements at low angles, the second peak of the bimodal distribution of the illite was found to be associated with the rotational movement of the b-dimension of the particles. Illite domains broke down in the ra nge of SAR 10 to 15 (mmol liter-1)0.5 equivalent to exchangeable sodiu m percentages (ESP) of 13 to 18. Illite thus demonstrates a similar st ability to smectites that require ESP almost-equal-to 15 to disaggrega te quasi-crystals. The EM of the illite particles increased drasticall y when the SAR increased from 2 to 10 (mmol liter-1)0.5. This increase in EM could not be explained exclusively by the change in the particl e size. Cation demixing is required to explain the increase of the zet a potential at the shear plane. The EM of the Silver Hill illite was d oubled when the pH increased from 5 to 9 at SAR > 15, but no pH effect was found when SAR < 15. The effect of pH on the EM at SAR values > 1 5 can be understood if we consider that at SAR > 15 most of the partic les are single platelets. The relative importance of variable charge o n single platelets or crystals is apparently greater than on domains b ecause the pH affected the mobility of the individual crystals but not the mobility of the domains. The combination of particle size distrib ution and EM data gives additional information about the zero point of charge of the variable charge, also called point of zero net proton c harge (PZNPC) of the clay. For Silver Hill illite, we estimate a PZNPC value between 5 and 7.