The coexistence of two phases in concentrated dispersions of monodispe
rsed platelike particles is reported. Small-angle neutron diffraction
has been used to identify a highly ordered phase which coexists at equ
ilibrium with a less ordered, less dense phase. The interparticle inte
ractions were Varied by adjusting the salt concentration, and this inf
luenced the concentration at which the phase transition occurred. Colu
mnar or layer structures which could account for the observed diffract
ion pattern from the ordered phase are discussed.