HYDROUS MINERAL PHASE TRANSFORMATIONS AT ELEVATED TEMPERATURES

Bedded salt units contain water associated with hydrated minerals contained in the salt. For example, at the WIPP site, the dominant clay mineral is corrensite, and the sulfate minerals are bassanite, anhydrite and polyhalite. Clay / sulfate minerals present as minor mineral phases in salt rocks may play a role in the performance of the repository. These hydrous phases can undergo dehydration processes and change physical structure and chemical composition under heat and pressure loads. These changes in turn may influence their capacity to retain water, and their sorption/desorption capacity and concomitant retention of radionuclides. Such water losses and volume changes may have play a role on salt repository performance at high heat loads. Therefore, we undertook this study to characterize the hydrous minerals found at WIPP.

The two experimental studies were: (1) XRD mineral characterization using a heated stage, with temperatures ranging from 25 to 275 ^oC and 20% RH, and, (2) experimental mineral runs of 6 weeks at 300^oC and 150 bar using Dickson rocking autoclaves. Experiments used WIPP corrensite, Naica Mexico gypsum, and WIPP bassanite, anhydrite.

The clay run on the heated XRD stage showed water dehydration at between 65 and 75 ^oC. This involved release of the interlayer water. The autoclave experiments at 300 ^oC, 150 bar indicated no further water release occurred.

Determination of WIPP sulfates by powder XRD indicate bassanite, anhydrite, and polyhalite. However, low overall sulfate abundances at WIPP necessitated the use of gypsum samples from Naica, Mexico for the phase stability studies. The phase transition from gypsum to anhydrite reveals that bassanite forms at approximately 75 ^oC, 1 bar. At higher temperatures, bassanite exists at 275 ^oC, 1 bar, with slow in-growth of anhydrite. Anhydrite abundance increases with time.

Experiments run at 300 ^oC and 150 bar (both “dry” ie, gypsum powder starting material, and saturated with DI) show that anhydrite is stable, bassanite no longer occurs, and remnant gypsum is still present. The remnant gypsum may be a kinetic artifact. Time, temperature, water, and pressure all seem to be controlling factors on the “stability” of metastable bassanite. Determining the metastable bassanite field would be critical to understanding the sulfate dehydration stability fields.