A three-component system consisting of butylammonium vermiculite, butylammo
nium chloride, and water was studied by X-ray and neutron diffraction exper
iments in freezing cycles between -5 and +5 degrees C. In liquid water, all
the samples studied were in a colloidally swollen gel phase, with interlay
er separations between the clay platelets of the order of hundreds of angst
roms. Upon freezing gf the water, the colloidal state collapsed into a tact
oid phase with an interlayer separation of d = 19.4 Angstrom. This phenomen
on was observed throughout a wide range of clay concentrations r and salt c
oncentrations c. The phase transition was observed to be reversible, the ge
l phase always being recovered upon warming through the freezing point of w
ater. Such a reversible phase transition between swollen and collapsed clay
mineral phases may be important in the weathering of rocks in freezing cyc
les.