Buried waste storage tanks at the USDOE Hanford Reservation in Washington State have released solutions containing high concentrations of Na, OH, NO3, and Al into the vadose zone. When such solutions contact vadose zone sediments, mineral transformations will change the sediment matrix. We hypothesized that Si, dissolved from primary and secondary minerals, will combine with Al from the tank waste to form crystalline or poorly crystalline network silicates such as zeolites and feldspathoids. In this study, we characterized the colloidal (<2 μm equivalent diam.) minerals formed when simulated tank solutions reacted with vadose zone Hanford sediments. Variables studied included simulated tank waste (STW) composition, reaction time, and temperature. Hanford sediments were reacted with a series of simulated tank solutions in batch experiments at 25 and 50°C for 1, 10, 25, 40, and 50 d. The mineralogical, structural, and chemical properties of the resulting colloidal fractions and bulk solutions were determined by x-ray diffraction (XRD), Fourier transform infrared (FTIR), 27Al- and 29Si-magic angle spinning-nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDAX), colorimetry, atomic absorption spectroscopy, and inductively coupled plasma–atomic emission spectroscopy (ICP–AES). Upon contact with STW, Si was released from the sediments and a portion was incorporated into poorly crystalline solids. The amount of poorly crystalline solids increased initially and reached maximum quantities between 0 and 25 d. Lability of minerals in the presence of NaOH followed the order quartz → kaolinite → illite. New secondary minerals, NO3–cancrinite, NO3–sodalite, and zeolite A, were formed at the expense of the original clay minerals. Zeolite A was labile and disappeared after about 25 d of reaction time. Cancrinite and sodalite, however, appeared to be stable and increased in abundance with time.