Amorphization in zircon: evidence for direct impact damage

Abstract

Abstract. X-ray diffraction has been used to characterize the amorphous phase present in a series of radiation-damaged natural zircons with radiation doses ranging from 0.06 to 16 × 10 18 α-decay events g −1 . The fraction of amorphous material present in each of the samples studied has been determined, and its dependence on the radiation dose has been calibrated. Direct determination of the amorphous fraction confirms that amorphization in natural zircon occurs as a consequence of the direct impact within cascades caused by α-recoil nuclei. These results are not consistent with the commonly accepted double-overlap model of damage accumulation.The volume swelling of amorphous regions changes as a function of dose. Thus, the density of amorphous regions depends on the degree of damage up to a certain point (i.e. 8 × 10 18 α-decay events g −1 ), unlike in previous models for which a constant value independent of the radiation dose was assumed.

Structural Characterization of Natural and Processed Zircons with X-Rays and Nuclear Techniques

“…The values show a reasonable agreement with those expected from the content of radioactive elements determined in the sands (see Table 1) and consequently with radiation dose [9]. According to Farges and modeling the Zr-rich region as the core of a sphere surrounded by the bridge region as shown in Figure 5, it is possible to estimate an order of a magnitude for the size of the metamict regions based on the relative abundance of the 2 and 4 interactions.…”

“…Cumulative radioactivity of zircon sands ranges between 5000 and 12000 Bq/kg depending on ore age, weathering, and deposit location [5,6]. Due to radioactive decays of U and Th families, in particular in the form of alpha radiation with energy ≥4,6 MeV whose emission is accompanied by relevant recoil interactions, the structure of zircon can be heavily damaged over geological times, resulting in a partially aperiodic state, the so-called metamict state [7][8][9]. The metamict state can be described as a mixture of crystalline and amorphous domains whose extent depends on the dose of radiation absorbed by zircon [8,9].…”

“…Due to radioactive decays of U and Th families, in particular in the form of alpha radiation with energy ≥4,6 MeV whose emission is accompanied by relevant recoil interactions, the structure of zircon can be heavily damaged over geological times, resulting in a partially aperiodic state, the so-called metamict state [7][8][9]. The metamict state can be described as a mixture of crystalline and amorphous domains whose extent depends on the dose of radiation absorbed by zircon [8,9]. For this reason exposure time, or, in other words, the age of zircons together with the enrichment in uranium and thorium, plays a relevant role in the degree of metamictization of each deposit.…”

29Si and 17O (Q)CPMG-MAS solid-state NMR experiments as an optimum approach for half-integer nuclei having long T1 relaxation times

“…This sample has been damaged by naturally occurring uranium which substitutes in the Zr site. Along with other radiation damaged Zircons it has been analyzed by X-ray diffraction [14] and singlepulse 29 Si MAS NMR [15] with respect to amorphization induced by radiation. Some very long longitudinal relaxation times have been reported for these samples [15] and they therefore serve as excellent test materials for the CPMG-MAS experiment.…”

(An)elastic softening from static grain boundaries and possible effects on seismic wave propagation

“…While the actual swelling process is complex with percolation of interfaces (Trachenko et al 2000(Trachenko et al , 2003Zhang et al 2000;Rios et al 2000;Salje et al 1999), one can estimate the effect of interfaces towards the elastic softening by a simple scaling argument (Salje 2007). In terms of a simple mixing model the volume scales as…”