Irradiation-Induced Amorphization Of Titanite
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Irradiation-Induced Amorphization Of Titanite A. Meldrum Dept. of Physics, University of Alberta, Edmonton, AB T6G 2J1 CANADA ABSTRACT Titanite (CaTiSiO5) is a widely occurring accessory mineral that contains ppm-level concentrations of U and Th. Radiation effects in titanite are important because this phase is commonly-used for U-Pb age dating of rock formations, and it is also the main crystalline constituent of certain glass-ceramic nuclear waste forms. Previous work suggested that titanite is highly susceptible to natural alpha-decay-induced amorphization, but ion irradiation experiments have so far been reported only at room temperature. In this work, the first temperature-dependent amorphization data for titanite are reported. High-purity single-crystal specimens from the Khan Mine, Namibia were characterized by analytical electron microscopy and powder XRD. Suitable specimens were then irradiated in-situ at the IVEM Facility using 800 keV Kr ions at temperatures ranging from 30 to 1100 K. Conventional imaging and diffraction techniques were used to monitor the transformation to the amorphous state. Titanite was amorphized at a relatively low dose over the entire temperature range investigated. Limited crystallization of ionbeam-amorphized titanite was observed at temperatures above 1100 K. INTRODUCTION Titanite is a naturally occurring mineral phase that is used in the earth sciences for U-Pb geochronology [1]. Titanite is monoclinic (space group P21/a) and is composed of kinked chains of corner-sharing TiO6 octahedra running parallel to the a-axis. The chains of octahedra are cross-linked by SiO4 tetrahedra. Large cavities in the network of octahedra are occupied by 7- or 9-fold coordinated Ca ions [2,3]. Within the TiO6 octahedra, the Ti cations are displaced toward one of the six surrounding oxygens. When titanite is heated above 493 K, the Ti cations are shifted to the geometric center of the TiO6 octahedra, causing a non-quenchable phase transition between the low-temperature P21/a and the high-temperature A2/a structures [4]. Titanite can accommodate several types of impurities on the various cation sites. Al, Ta, and Nb can substitute for up to 30 mol % of the Ti in the octahedral site [5]. Ca may also be replaced by minor quantities of Na, Y, Th, U, and the lanthanides. The concentration of U and Th rarely exceeds a few thousand parts-per-million (ppm), but titanite has long been known to be susceptible to metamictization (i.e., amorphization due to natural α-decay processes: see Refs. 6,7). These early compositional studies laid the groundwork for future investigations of the effects of α-decay damage on titanite [8,9,10]. Titanite is a constituent crystalline phase in titanium-based “glass ceramic” waste forms proposed in the 1980s to incorporate actinide waste [8,9]. Several investigations were undertaken in order to obtain a better understanding of the crystalline-to-amorphous transition in titanite. Xray diffraction studies on a suite of titanite specimens from various localities reported damage
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