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.