Long-term thermal stability of clinoptilolite; the development of a "B" phase

The U.S. Department of Energy is assessing the suitability of Yucca Mountain, Nevada, to act as a repository for high-level radioactive waste. The rocks below the candidate repository horizon contain significant quantities of clinoptilolite. In order to assess the long-term thermal stability of clinoptilolite under the thermal conditions imposed by the candidate repository, a series of experiments has been conducted at temperatures of 100 degrees and 200 degrees C for times up to five years. Heating experiments were conducted in a dry (room) atmosphere using five powdered natural clinoptilolites, two of which were also Na-exchanged. The structural effects of heating were monitored using X-ray powder diffraction. The results of these experiments contrast with conventional short-term heating to >250 degrees C which has often been used to differentiate between clinoptilolite and heulandite. Only one clinoptilolite sample and its Na-exchanged form were partially destroyed by overnight heating to 480 degrees C, and the other samples were unaffected. Long-term heating at 100 degrees C had little effect on all samples. However, one natural and the two Na-exchanged clinoptilolites heated at 200 degrees C for at least one year experienced a significant decrease in the b axis in a portion of the sample, as reflected in movement of the 020 reflection to higher angles in the diffraction pattern. Moreover, the collapsed clinoptilolites did not re-expand after five years in the room atmosphere, and the reaction appears to be irreversible or at most sluggishly reversible under room conditions. The conversion of clinoptilolite to a collapsed phase in many ways resembles the transformation of heulandite to heulandite B. Samples undergoing this transformation are characterized by a combination of low Si/Al ratio, high Na, and low K contents. Clinoptilolites that were unaffected by long-term heating generally have the largest K and the smallest Na contents of the materials studied. The clinoptilolite that was largely destroyed by overnight heating at 480 degrees C was unaffected by long-term heating at 200 degrees C. Likewise, two of the three samples experiencing transformation during long-term heating at 200 degrees C were unaffected by overnight heating to 480 degrees C. Thus, these results suggest that the factors controlling the structural destruction commonly observed with heulandite upon short-term heating are distinct from those contributing to the irreversible contraction of clinoptilolite. The irreversible contraction of clinoptilolites observed here appears to be similar to that observed during in-situ high-temperature X-ray diffraction experiments, in which Na-exchanged clinoptilolites underwent significant volume reduction below 100 degrees C. The factors governing the collapse of clinoptilolite during long-term, low-temperature heating include the size, charge, hydration energy, and number of exchangeable cations and, to a lesser extent, the Si/Al ratio of the zeolite. These results demonstrate that significant volume changes can potentially occur in heated zeolitic tuffs underlying the candidate repository at Yucca Mountain.