[en] An investigation of the dehydration of smectite clay as a function of pressure and temperature was completed for the Basalt Waste Isolation Project as part of the Waste Package Development Program. The clay sample was obtained from fracture mineralization that occurs in Hanford basalt. This work was completed to assess the chemical stability of smectite clays in the backfill component of a waste package which would be emplaced in a deep underground repository located in Hanford basalt. The range of experimental temperatures and pressures was chosen to represent those that would be imposed on the waste package in the repository. 5 refs., 11 figs., 3 tabs