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Weathering behavior of kaolinite was studied in batch systems under geochemical conditions characteristic of tank waste released to the vadose zone at the Hanford Site, WA (0.05 M Al(T), 2 M Na+, 1 M N03-, pH approximately 14, Cs+ and Sr2+ present as co-contaminants). Time series experiments were conducted from 0 to 369 d, with initial Cs+ and Sr2+ concentrations ranging from 10(-5) to 10(-3) M. Dissolution of kaolinite increased soluble Si and Al to maximum levels at 7 d (Cs and Sr concentrations of 10(-5) and 10(-4) M) or 33 d (Cs and Sr concentrations of 10(-3) M). Subsequent precipitation of Si and Al was coupled to the formation of oxalate-extractable solids that incorporated Cs and Sr. Strontium sorption was nearly complete within 24 h for initial Sr concentrations (Sr0) < or = 10(-4) whereas Cs uptake increased over the full year of the experiment for all initial Cs concentrations. Spectroscopic analyses revealed neoformed solids including the zeolite Na-Al silicate (Al-chabazite), and feldspathoids sodium aluminum nitrate silicate (NO3-sodalite), and sodium aluminum nitrate silicate hydrate (NO3-cancrinite), which can incorporate Cs. Single-pulse 27Al solid-state nuclear magnetic resonance (NMR) spectroscopyyielded first-order rate constants (k)for mineral transformation that decreased from 3.5 x 10(-3) to 2 x 10(-3) d(-1) as Cs and Sr concentrations were increased from 10(-5) to 10(-3) M. Discrete strontium silicate solids were also observed. The incongruent dissolution of kaolinite promoted the sequestration of contaminants into increasingly recalcitrant solid phases over the 1-yr time period.