  We study the diffusion of water in weakly-hydrated samples of the smectite
clay Na-fluorohectorite. The quasi one-dimensional samples are dry compounds of
nano-layered particles consisting of ~ 80 silicate platelets. Water diffuses
into a sample through the mesoporosity in between the particles, and can
subsequently intercalate into the adjacent particles. The samples are placed
under controlled temperature. They are initially under low humidity conditions,
with all particles in a 1WL intercalation state. We then impose a high humidity
at one sample end, triggering water penetration along the sample length. We
monitor the progression of the humidity front by monitoring the intercalation
state of the particles in space and time. This is done by determining the
characteristic spacing of the nano-layered particles in situ, from synchrotron
wide-angle X-ray scattering measurements. The spatial width of the
intercalation front is observed to be smaller than 2mm, while its velocity
decreases with time, as expected from a diffusion process.
