**5. Effect of thermal annealing on metamict zircon and titanite**

Zircon has a high melting temperature (above 2700–2800 K), and crystalline ZrO2

~2670 K [66]. It is difficult to quench zircon melts without decomposition into ZrO2

treated zircon in a study with laser melting, near the boundaries between the unmolten and molten regions (where a relatively large temperature gradient could exist) [67]. The large tem‐ perature gradient is expected to increase the quench rate and facilitate a "freeze" of the local

region of 600 and 1100 cm–1 [Ref, [40], modified). This indicates the spectral dispersion related to Si‐O and Ti‐O vibrations in the two types of disordered materials. However, the two materials exhibit some local maximums with similar or close

glasses produced by quenching titanite melts (thermal glass‐1 and thermal glass‐2 are prepared by

melts) show overall spectra patterns different from those of metamict titanite, especially in the

results indicate that the metamict state is different from the glassy state obtained by quench‐ ing melts. Apparently, the processes and structural states associated with metamictization and irradiation amorphization are more complex than those in common thermal glasses. The formation of the metamict state involves not only amorphization, but also defect accumula‐ tion caused by alpha‐particle damage and further radiation or irradiation may lead to damage

So far, although the issue remains under debates, there have been substantial evidences indicating characteristic discrepancies between two types of amorphous states (metamict and glass states) [68], for example: (i) the two types of materials commonly have spectral and structural discrepancies [40, 43, 69, 70]; (ii) high‐energy heavy ion irradiation may lead to significant modifications in local structures of glasses [71–74]; (iii) upon heating, radiation‐damaged minerals tend to recrystallize epitaxially and recover to their original

may coexist for a composition of ZrSiO4

peak positions (indicated by dash lines) in the far infrared region (100 and 500 cm–1).

uid of SiO2

**Figure 4.** CaTiSiO<sup>5</sup>

quenching CaTiSiO<sup>5</sup>

112 Raman Spectroscopy and Applications

however, glass‐like zircon (ZrSiO4

configuration of the ZrSiO4

as well as recrystallization.

and a liq‐

and SiO2

;

at a temperature region of ~1960 K and

) has been recorded with Raman spectroscopy in laser‐

melts before the decomposition takes place. These experimental

Thermal annealing of metamict minerals at high temperatures is commonly used in studies of radiation‐damaged minerals [30]. Its aims are to restore the original crystal structure for the purpose of studying recrystallization temperature, activation energy, types of radiation‐ induced local defects and phase identification, and to obtain a good comprehension of the recrystallization process and mechanism. Extensive studies were carried out to investigate changes at the atomic level in metamict materials during high‐temperature annealing [10, 15, 36, 40, 44, 47, 48, 50, 52, 54, 55, 65, 75, 78–81]. However, controversies remain regarding the recrystallization path and activation energy.

Thermal annealing results in recrystallization of metamict zircon (**Figure 5**). The effect of annealing temperature on the structural recovery of damaged zircon can be clearly seen in the frequency and FWHM of the ν3 Si**‐**O stretching (B1g) as a function of temperature (**Figure 6**). With increasing annealing temperature, the frequency of this mode shows, systematically, a large increase in the region between 800 and 1050 K and a weaker increase with temperature above 1050 K. This is due to the healing of the defective lattice and the recrystallization of remaining crystalline domains. Highly damaged zircon tends to decompose into tetragonal ZrO2 and SiO2 near 1100 K, and the transformation of tetragonal ZrO2 into monoclinic ZrO2 is reported at higher temperatures [47]. The findings may explain the cause of some previously reported ZrO2 and SiO2 in natural zircon, which likely experienced natural heating processes.

Spectroscopic data [47, 48, 61] revealed different recrystallization processes between partially and heavily damaged zircons, that is, the recrystallization process depends on the cumula‐ tive radiation dose (**Figure 5**, **6a** and **6b**). Being similar to metamict zircon [47], the thermal response of the damaged titanite (CaTiSiO<sup>5</sup> ) is affected by their initial degrees of damage, that is, at the same treatment conditions, weakly or partially damaged samples are more likely to recover to crystalline titanite as compared with highly metamict samples [40]. Intermediately and heavily damaged titanite samples show a recovery of Ti‐O and Si‐O bands after annealing at 1300–1400 K, and these recovered crystals are consistent with the *P*21 */a* symmetry, although in terms of band widths, they are far from a fully recovering [40]. Similar results were reported by the work of another group [44] who thermally treated a metamict titanite sample, which has an accumulated radiation dose of 1.2 × 1018 alpha‐event/g by multistep annealing up to 1173 K, and found it was insufficient to recover the crystalline structure of the studied sample.

**Figure 5.** Raman spectra recorded from weakly metamict (dose = 1.8 × 1018 alpha‐events g−1 ), intermediately metamict (dose = 3.5) and highly metamict zircon (dose = 13.1) thermally treated in N2 up to 1700 K and then punched (Ref. [47], modified).

The failure of a full recovery from the damage in thermally treated metamict titanite is also revealed by infrared spectroscopy [43]; however, the physics behind this remain unclear.

The thermally induced structural recovery and recrystallization of metamict zircon and titanite is also characterized a recovery of the anisotropy of the sample, which is restored Raman Study of the Crystalline-to-Amorphous State in Alpha-Decay–Damaged Materials http://dx.doi.org/10.5772/65910 115

**Figure 6.** Phonon frequency and FWHM of the Raman ν3 Si‐O stretching (B1g) in zircon (ZrSiO4 ) with different degrees of damage as a function of annealing temperature (Ref. [47], modified with unpublished data).

during annealing, as evidenced by the recovery of orientational dependence of IR (as well as Raman) spectra along with the original crystallographic orientations as shown in **Figure 7**, which indicates an epitaxial recrystallization. This behavior indicates that in highly metamict zircon and titanite, crystalline nanodomains with original crystallographic orientations might still exist.

In conclusion, Raman spectroscopy, as shown above, is a very powerful tool for study of radia‐ tion damage in actinide‐bearing phases and for estimation of their long‐term durability of their physical properties and chemical stability. This type of Raman applications can provide a better understanding of the mechanism of radiation damage and thermal recrystallization processes. It has a wide usage in condensed mater physics, material science, nuclear material sciences, mineralogist, and geochemistry. It has also been used analysis other radiation‐damaged min‐ erals, such as fergusonite [82, 83], actinide‐bearing monazite [32], titanoaeschynite (Nd) and

The failure of a full recovery from the damage in thermally treated metamict titanite is also revealed by infrared spectroscopy [43]; however, the physics behind this remain unclear.

), intermediately metamict

up to 1700 K and then punched (Ref. [47],

**Figure 5.** Raman spectra recorded from weakly metamict (dose = 1.8 × 1018 alpha‐events g−1

(dose = 3.5) and highly metamict zircon (dose = 13.1) thermally treated in N2

modified).

114 Raman Spectroscopy and Applications

The thermally induced structural recovery and recrystallization of metamict zircon and titanite is also characterized a recovery of the anisotropy of the sample, which is restored nioboaeschynite (Ce) [84], uranyl titanate mineral davidite‐(La) [85], aeschynite‐(Y) and poly‐ crase‐(Y) [86], and steenstrupine [87], and pyrochlore (Zietlow et al., personal communication).

**Figure 7.** Orientational dependence of polarized infrared spectra of radiation‐damaged zircon crystal (dose = 3.8 × 1018 alpha‐events g−1) annealed between 292 and 1700 K (Ref. [48], modified). The interval of the angle between the c‐axis and the incident radiation E is 10°. The arrows indicate the appearance of an extra signal near 790 cm−1, suggesting a possible intermediate phase. Thermal annealing results in a recovery of the anisotropy of the damage zircon crystal to its original crystallographic orientations.
