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

David L. Bish
Long-term thermal stability of clinoptilolite; the development of a "B" phase
European Journal of Mineralogy (December 1990) 2 (6): 771-777

Abstract

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.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 2
Serial Issue: 6
Title: Long-term thermal stability of clinoptilolite; the development of a "B" phase
Author(s): Bish, David L.
Affiliation: Los Alamos National Laboratory, Los Alamos, NM, United States
Pages: 771-777
Published: 199012
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 12
Accession Number: 1994-008250
Categories: Mineralogy of silicatesEngineering geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, 7 anals.
N36°00'00" - N39°07'60", W118°15'00" - W115°00'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe, Hanover, Germany
Update Code: 199404
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