Radiation damage metamictization

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

In this section, we summarize radiation damage data for minerals however, we focus only on those minerals for which substantial amounts of data are available or which are of considerable geologic interest. We do not include a discussion of phases that contain actinides but are not metamict (i.e. AO2 A = Zr baddeleyite Th thorianite U uraninite). 

The rarity of metamict monazite may be attributed to the low critical temperatures ( 300°C) at which the radiation-damaged monazite completely recovered its atomic-scale periodicity (Ehlert et al. 1983, Karioris et al. 1982). 

Ewing RC, Haaker RF (1980) The metamict state Implications for radiation damage in crystalline waste forms. Nuclear Chem Waste Management 1 51-57... 

Ewing RC, Chakoumakos BC, Lumpkin GR, Murakami T, Greegor RB, Lytle FW (1988) Metamict minerals natural analogues for radiation damage effects in ceramic nuclear waste forms. Nucl Instr Meth B32 487-497... 

Farges F, Ewing RC, Brown GE Jr. (1993) The stracture of aperiodic, metamict (Ca,Th)ZrTi207 (zirconolite) An EXAFS study of the Zr, Th, and U sites. J Mater Res 8 1983-1995 Fleet ME, Henderson GS (1985) Radiation damage in natural titanite by crystal structure analysis. Mater Res Soc Symp Proc 50 363-370... 

Robinson MT (1994) Basic physics of radiation damage production. J Nucl Mater 216 1-28 Salje EKH, Chrosch J, Ewing RC (1999) Is metamictization of zircon a phase transition Am Mineral 84 1107-1116... 

As previously noted, monazite usually contains significant amounts of thorium as well as uranium, and xenotime also contains uranium. Naturally radioactive minerals are, therefore, exposed to displacive radiation damage events over geological time scales. Accordingly, such minerals are frequently found in the metamict state, i.e. they can... 

Radiation effects. Radiation damage in apatite has been studied mainly for purposes of understanding the thermal behavior of fission tracks. Apatite is never found metamict, but this is surely in part due to its limited U-content. In rare cases, apatite can contain as... 

Radiation effects. The radioactive element content of xenotime is lower than in monazite (e.g., Bea and Montero 1999) but still high enough to produce significant radiation damage. However, as with monazite, no metamict xenotime has ever been described. External irradiation experiments (Meldrum et al. 1997b) have shown that radiation damage in synthetic phosphates with xenotime structure are also healed easily at a temperature about 100°C higher than monazite. Thus the conclusions drawn above for monazite also likely apply to xenotime. 

The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are (1) a high solubility for actinides and rare earths (10 to 20 wt %) (2) evidence from natural occurrences of good chemical durability (3) an apparent resistance to radiation damage, as natural monazites are seldom found in the metamict state, despite very high alpha-decay event doses (Boatner and Sales 1988). There have been extensive studies of monazite and apatite as potential waste form phases, and a considerable amount of work on a number of synthetic phosphate phases has been completed. 

Crystallographic studies are impeded by the fact that some rare earth minerals always occur in the metamict state, which is an amorphous state mainly caused by radiation damage to the crystal structure caused by radioactive decay of elements such as uranium and thorium. Crystallographic data and crystal structures of metamict minerals were therefore determined using samples recrystallized by annealing. In such cases, we must note that there is some doubt about the identity of the crystal structure of the recrystallized phase and the original structure of pre-metamict minerals. 

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). 

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