Metamictization

Many other silicate minerals contain zirconium as a constituent (164,165). These minerals may be altered or metamict zircons such as cyrtoHte or malacon. 

Uraninite is associated with the following minerals zircon, monazite, carbonaceous material (thucolite ), mica, feldspar, and with rare-earth-bearing minerals. It occurs in small distinct crystals or may be massive. Some grains are discoloured, especially in the centre, metamict, and fractured. 

Hoffmann, R. The Metamict State University of Central Florida Press Orlando, 1987. 

Finally, we thank Dr. Roald Hoffmann for permission to reproduce his poem Next Slide Please from The Metamict State (1987 Orlando University of Central Florida Press, pp 51-52). 

Ch akoumakos, B. C., Murakami, T., Lumpkin, G. R. Ewing, R. C. 1987. Alpha-decay-induced fracturing in zircon the transition from the crystalline to the metamict state. Science, 236, 1556-1559. 

A variety of alteration effects have been observed in completely metamict Ti- and U-rich pyrochlores. In the samples from Adamello, Italy, the pyrochlore occurs as overgrowths on zoned zirconolite grains and contains 29-34 wt% U02. Electron microscopy and microanalytical work has revealed that these pyrochlore samples have... 

Bellatreccia, F., Della Ventura, G., Williams, C. T., Lumpkin, G. R., Smith, K. L. Colella, M. 2002. Non-metamict zirconolite polytypes from the feldspathoid-bearing alkali-syeni-tic ejecta of the Vico volcanic Complex (Ladum, Italy). European Journal of Mineralogy, 14, 809-820. 

Bulakh, A. G Nesterov, A. R Williams, C. T. Anisimov, I. S. 1998. Zirkelite from the Sebl yavr carbonatite complex, Kola peninsula, Russia and x-ray and electron microprobe study of a partially metamict mineral. Mineralogical Magazine, 62, 837-846. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

There are three significant possible effects when radiation interacts with matter (5,6). First, the radiation can interact with the nucleus and induce radioactivity as in the case of neutrons. Second, displacement of atoms can occur. This has happened in a number of uranium- and thorium-containing minerals over geological periods. The outstanding example is zircon, which can contain over 10% Th and 2% U. The internal bombardment from these materials and their decay products over geological periods produces low or metamict zircon, where the disorder gives an amorphous state having a low density. 

The second provenance criterion is based on the identification of inclusions in gemstones. Micro-Raman spectrometry was used for this task in almandine garnets. Various inclusions were observed like apatite, zircon, monazite, calcite, and quartz and two of them, curved needles of sillimanite (Al2Si05) and 10-pm metamict radioactive crystals, were specifically found in archaeological garnets. Fig. 6 shows the Raman spectra of a sillimanite needle, which is a mineral formed under a high temperature and high pressure metamorphism. 

Figure 5.9 O spikelet-echo MAS NMR spectra of a partially metamict ZrSiOj. Adapted from ref [53] reprinted with permission from Elsevier.

Fission-fragment tracks in neutron-irradiated zircon and in natural metamict zircon have been observed by Vance and Boland (1975) and by Yada, Tanji, and Sunagawa (1981), respectively, and will be considered in the following section. 

Metamictization of zircon (ZrSi04). Zircons from different localities show a considerable variation in physical properties, and it is generally accepted that these variations are the result of structural damage due to the radiations emitted by U and/or Th impurities over geological periods of time. The extensive literature on zircon has been reviewed by Speer (1982). 

Optical microscope observations have shown that zircons often exhibit extremely complex microstructures (on the scale of 1-100 /tm), in which changes of birefringence correlate with the distribution of U and Th (Chakoumakos et al. 1987). However, no attempts appear to have been made to relate these microstructures to other impurities (such as water-related species) and crystal defects, both of which may significantly influence the processes of metamictization and recrystallization. Such a study involving TEM might also provide important information about the diffusion and leaching of radioactive impurities (and the products of their decay), processes that have important implications for ceramic nuclear-waste disposal and for techniques of age determination based on measurements of Pb/U isotopic ratios. 

Ellsworth et al. (1994) measured the enthalpy of amorphous (metamict) zircon, ZrSi04, produced by radiation damage. Assuming that this dense amorphous material has... 

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