In-Situ Observation of the Alpha/beta Cristobalite Transition Using High Voltage Electron Microscopy
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IN-SITU OBSERVATION OF THE ALPHA/BETA CRISTOBALITE TRANSITION USING HIGH VOLTAGE ELECTRON MICROSCOPY A. MEIKE* and W. E. GLASSLEY** *Material and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 CA 94550
"*EarthSciences Division, Lawrence Livermore National Laboratory, Livermore, ABSTRACT
A high temperature water vapor phase is expected to persist in the vicinity of high level radioactive waste packages for several hundreds of years. We have begun an investigation of the structural and chemical effects of water on cristobalite because of its abundance in the near field environment. A high voltage transmission electron microscope (HVEM) investigation of bulk synthesized a-cristobalite to be used in single phase dissolution and precipitation kinetics experiments revealed the presence f6cristobalite, quartz and amorphous silica, in addition to a-cristobalite. Consequently, this apparent metastable persistence of #-cristobalite and amorphous silica during the synthesis of a-cristobalite was investigated using a heating stage and an environmental cell installed in the HVEM that allowed the introduction of either dry CO 2 or a CO 2 + H20 vapor. Preliminary electron diffraction evidence suggests that the presence of water vapor affected the a-fl transition temperature. Water vapor may also be responsible for the development of an amorphous silica phase at the transition that may persist over an interval of several tens of degrees. The amorphous phase was not documented during the dry heating experiments. INTRODUCTION Prediction of chemical behavior in the near-field environment of high-level radioactive waste containers requires single phase dissolution and precipitation kinetics data. Cristobalite is a common secondary mineral in the rock matrix and along fracture surfaces in the Topopah Spring Tuff, which is the host rock for the proposed high level radioactive waste repository at Yucca Mountain, Nevada. Cristobalite may also provide an analogue for certain phases in concrete that will experience a similar thermal history in the vicinity of the waste containers. In the course of obtaining dissolution rate data for acristobalite [1], it was noted that synthetic cristobalite dissolved at rates slightly higher than natural cristobalite. Detailed microstructural characterization of the starting and etched natural and synthetic acristobalite was undertaken because of this unpredicted behavior. Cristobalite has been the subject of repeated investigations [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]. Its physical and chemical properties are of considerable interest because it begins a structural transition from the primitive tetragonal (P412 12) a phase to the face centered cubic (Fd3m) '6 phase at some temperature below 240 C, which is the maximum temperature expected to be attained by the host rock in the vicinity of spent fuel waste packages. The structural transition is thought to involve a slight displacive adjustment [12], but a consistently reproducible transition temperature has not been iden
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