Introduction

Solid aerosol particles, including mineral dust, are important sources of air pollution and are known to exert profound impacts on global climate changes and biological processes (Lohmann and Feichter 2005; Andreae and Rosenfeld 2008; Buseck and Pósfai 2010). Quantitative understanding of the effects of solid aerosol particles on global climate changes require detailed data about their sources and physicochemical properties. However, knowledge about solid aerosol particles such as mineral dust, unlike their gaseous and liquid counterparts, remains limited (Buseck and Pósfai 2010). For example, solid aerosol particles in the atmosphere are known to experience prolonged exposure to and interactions with various cosmic radiations, including high-energy particles such as electrons, muons, and protons. However, dedicated studies on the effects of cosmic radiation on the bulk and surface properties of mineral dust are rare. In this context, the effects of radiation on surface species on quartz are particularly interesting, because it is one of the most common constituents in mineral dust (Claquin et al. 1999; Tatarov and Sugimoto 2005).

Our previous study of amethyst pulverized in air showed that its electron paramagnetic resonance (EPR) spectra are characterized by a broad peak at the effective g = ~10.8 (Fig. 1; SivaRamaiah et al. 2011). This peak does not correspond to any known lattice-bound Fe3+ centers in quartz (Barry and Moore 1964; Matarrese et al. 1969; Mombourquette et al. 1986; Halliburton et al. 1989; Minge et al. 1989, 1990; Weil 1994; Balitsky et al. 2000; Di Benedetto et al. 2010) and was interpreted to represent surface Fe3+ species formed during sample preparation (SivaRamaiah et al. 2011). It is well known that Fe2+ is the dominant iron species in natural quartz and is expected to oxidize, migrate, and cluster to form surface Fe3+ species during pulverization in air (Cressey et al. 1993; Di Benedetto et al. 2010). Interestingly, the EPR spectrum of the same amethyst sample pulverized in acetone does not contain any signal in the magnetic field below g = 4.28 (Fig. 1), further supporting the interpretation that the surface Fe3+ species on the amethyst pulverized in air formed largely from the oxidation of Fe2+. Therefore, the powder sample of amethyst prepared by pulverization in air represents an excellent analogue of mineral dust containing surface Fe3+ species.

Fig. 1
figure 1

Comparison of powder EPR spectra of a natural amethyst quartz pulverized in air (from SivaRamaiah et al. 2011) and in acetone, measured at 294 K and microwave frequencies of ~9.38 GHz. Note that the substitutional Fe3+ center at g = ~4.28 in the sample pulverized in acetone is marked

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SivaRamaiah et al. (2011) performed detailed powder EPR measurements of this amethyst sample to obtain various thermodynamic properties and magnetic susceptibility of the surface Fe3+ species on quartz. In this contribution, we have conducted detailed EPR measurements of this same amethyst sample after a series of gamma-ray-irradiations (0.14–70 kGy). Specifically, new EPR spectra of the gamma-ray-irradiated sample are used to obtain thermodynamic properties (Gibbs energy, enthalpy, entropy, and activation energy), magnetic susceptibilities, relaxation times (i.e., spin–lattice relaxation time T 1 and phase memory relaxation time T m), and the geometrical radius of the surface Fe3+ species on quartz. These data are compared with those from as-is amethyst (SivaRamaiah et al. 2011) to investigate the effects of gamma-ray-radiation on the surface Fe3+ species on quartz, with relevance to aerosol–radiation interactions.

Sample and experimental procedure

The same sample of natural amethyst investigated in SivaRamaiah et al. (2011) was used in this study to facilitate direct comparison. Powders of this sample, obtained from pulverization in air (SivaRamaiah et al. 2011), were irradiated at room temperature in a 60Co cell with a dose rate of ~460 Gy/h for total doses ranging from 0.14 to 70 kGy.

All EPR spectra of the gamma-ray-irradiated amethyst and a polycrystalline CuSO4·5H2O standard were recorded using a Bruker EMX spectrometer equipped with a high-sensitivity ER 4119 cavity, an automatic frequency controller, and an Oxford liquid-helium cryostat, at the Saskatchewan Structural Sciences Centre, University of Saskatchewan. An empty amorphous silica tube was first recorded at room temperature (~294 K) to ensure free of any paramagnetic species. This tube was then used for recording the powder EPR spectra of the gamma-ray-irradiated amethyst and the polycrystalline CuSO4·5H2O standard. All experimental conditions were kept to be similar to those described in SivaRamaiah et al. (2011).

Results and discussion

The powder sample of amethyst changed from white to smoky color after gamma-ray-irradiation. The smoky coloration intensifies with increase in the radiation dose. Previous experimental and theoretical studies have shown that the smoky color of quartz is attributable to the formation of the [AlO4]0 center during irradiation (Meyer et al. 1984; Walsby et al. 2003; To et al. 2005).

EPR spectra of gamma-ray-irradiated amethyst at 294 K

Figure 2a shows that the 294 K powder EPR spectrum of 10-kGy gamma-ray-irradiated amethyst consists of three resonance signals at g = ~2.00, ~4.28, and ~15. The signal at g = ~4.28 is attributable to the substitutional Fe3+ ions at the Si site (Weil 1994; SivaRamaiah et al. 2011) and remains essentially constant in intensity after gamma-ray-irradiation. The signal at g = ~2.00, the well-known oxygen-vacancy electron center E1′, on the other hand, shows a minor but noticeable increase in intensity with increase in the radiation dose. Similarly, the broad peak at g = 10.8 (i.e., surface Fe3+ species; SivaRamaiah et al. 2011) shows a marked increase in intensity after gamma-ray-irradiation and shifts to a higher effective g value of ~15 (Fig. 2a). It is also interesting to note that the surface Fe3+ species appears to shift in the effective g value from ~14 to ~15 with the increase in the gamma-ray dose from 0.14 to 10 kGy. This shift in the effective g value toward higher values after gamma-ray-irradiation is probably attributed to interactions between the surface Fe3+ species and mobile electrons. Another notable feature in the EPR spectra of the surface Fe3+ species with increasing radiation doses is that its line shape changes from Gaussian to Lorentzian (Fig. 2a).

Fig. 2
figure 2

Representative powder EPR spectra of gamma-ray-irradiated amethyst quartz: a effects of gamma-ray radiation (0.14 and 10 kGy) at 294 K, in comparison with that without irradiation (SivaRamaiah et al. 2011); and b at a radiation dose of 10 kGy and temperatures from 90 to 270 K

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Free Fe3+ ions belong to the d5 configuration with 6S5/2 as the ground state, without any spin–orbit interaction. The g value of free Fe3+ ions is expected to lie very close to the free-electron value of 2.0023. However, the experimental g values of Fe3+ species in crystalline solids often deviate significantly from 2.0023, with those at ~4.28 and ~15 related to specific symmetry environments being particularly common. When Fe3+ ions are placed in a ligand field environment, the 6S5/2 ground state splits into three Kramers doublets |±1/2>, |±3/2>, and |±5/2>. The resonance signal at g = ~4.28 arises from the middle Kramers doublet |±3/2> for Fe3+ ions in almost completely rhombic environments with the ratio of the zero-field splitting parameters |E|/|D| ≈ 1/3 (Golding et al. 1978; Pan et al. 2009). Similarly, the g = ~15 signal has been linked to Fe3+ ions in distorted octahedral environments as well (Sreekanth Chakradhar et al. 2005). The contrasting dependence on the radiation dose also demonstrates that the g = ~4.28 and ~15 signals in our EPR spectra (Fig. 2) belong to separate Fe3+ species.

EPR spectra of gamma-ray-irradiated amethyst at 90–294 K

Figure 2b shows the powder EPR spectra of gamma-ray-irradiated amethyst (10 kGy) recorded from 90 to 294 K. The broad signal at g = ~15 observed at 294 K is shifted to g = ~17 at 270 K (not shown in the Fig. 2b). This shift in the effective g value from 294 to 270 K is similar to that observed for the surface Fe3+ species on as-is amethyst and has been interpreted to represent a reduction in the overall paramagnetism (SivaRamaiah et al. 2011; Zhang et al. 2009). However, the effective g value at ~17 remains constant in the temperature range from 270 to 90 K (Fig. 2b).

The intensity of the g = ~4.28 signal, on the other hand, appears to be constant in the temperature range from 294 to 90 K. Therefore, this g = ~4.28 signal also differs that at g = ~15 in thermal property, further supporting our interpretation that they belong to separate Fe3+ species. The intensity of E1′ at g = ~2.00 decreases with increase in temperature from 90 to 294 K, which is attributable to decrease in mobile or conduction electrons when temperature is increased and is consistent with the Boltzmann law.

The resonance signal at g = ~ 15–17 decreases with decrease in temperature from 294 to 90 K, suggesting that change in the magnetic ordering is expected to be in the antiferromagnetic conditions at low temperatures. As temperature is lowered, the relaxation time increases, and the resonance peak shapes become more prominent. There is a gradual decrease in the spin moment and a loss of coherence (consistency) of spin coupling from 294 to 90 K. Also, the signal at g = ~15–17 appears to become sharper from 294 to 90 K, which is attributable to decrease in spin–spin and spin–orbit interactions and an overall decrease in the paramagnetic character (Zhang et al. 2009).

The progressive decrease in the spins, to build an antiferromagnetic (AF) structure with lowering temperature, involves the rise of the AF contribution to the EPR line, and the monotonic decrease in the number of spins is visible at low temperature. The occurrence of AF implies a long-range order between Fe3+ species. The linewidth of this signal at g = ~15–17 also decreases from 294 to 90 K (Fig. 3), which is attributable to increase in the spin–lattice relaxation time (see below). This temperature dependence of the EPR linewidth observed in the irradiated amethyst is attributable to interaction between conduction electrons and phonons (cf. Augustyniak-Jabłokow et al. 2010). This interaction leads to a temperature-dependent spin-relaxation time on the order of ~10−8 s. The observed line broadening from 90 to 294 K is attributable to dipolar interactions (cf. Gaite et al. 1993).

Fig. 3
figure 3

Linewidth (ΔB) of the surface Fe3+ center on gamma-ray-irradiated (10 kGy) amethyst as a function of absolute temperature from 90 to 294 K

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Calculation of absolute number of spins and thermodynamic properties

The absolute number of spins (N) participating in resonance can be calculated by comparing the area under the absorption curve with that of a standard of known concentration. Weil and Bolton (2007) gave the following formula for the sample (x) and standard (std):

$$ N_{x} = \frac{{A_{x} ({\text{scan}}_{x} )^{2} G_{\text{std}} ({\text{Bm}})_{\text{std}} (g_{\text{std}} )^{2} [S(S + 1)]_{\text{std}} N_{\text{std}} }}{{A_{\text{std}} ({\text{scan}}_{\text{std}} )^{2} G_{x} ({\text{Bm}})_{x} (g_{x} )^{2} [S(S + 1)]_{x} }} $$
(1)

where A is the area under the absorption curve, which is obtained from double integration of the first-derivative spectrum; scan is the magnetic field corresponding to unit length of the spectrum; G is the receiver gain; Bm is the modulation amplitude; g is the g factor; S the spin of the system in its ground state (S = 5/2 and 1/2 for Fe3+ and Cu2+, respectively). N std denotes the number of spins in the 100 mg CuSO4·5H2O standard. Repeated analyses of the amethyst sample and the standard at 294 K show that the uncertainty of our calculated absolute spin number is ~2 %.

The absolute number of spins for the surface Fe3+ signal at g = ~15 in gamma-irradiated amethyst at 294 K has been found to be on the order of 1018 spins/g (Fig. 4a), which is comparable to those reported in the literature. SivaRamaiah et al. (2011) calculated a spin number of ~4.6 × 1018 spins/g for Fe3+ on the as-is quartz. Figure 4a shows that the number of spins in the gamma-ray-irradiated amethyst increases from 0.15 × 1018 to 1.21 × 1018 spins/g, when temperature increases from 90 to 294 K.

Fig. 4
figure 4

a The number of spins (N) of the surface Fe3+ center on the gamma-ray-irradiated (10 kGy) amethyst as a function of absolute temperature from 90 to 294 K; b plots of log10 (N/T) and log10(N) versus reciprocal temperature. Note that the data point at 294 K was excluded from calculations

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Figure 4b shows a linear relationship between log10(N/T) and reciprocal of absolute temperature. This relationship allows the calculation of the entropy ΔS and the enthalpy ΔH on the basis of the intercept on the Y axis and the slope, respectively. The calculated values of entropy and enthalpy of the Fe3+ center in 10 kGy gamma-ray-irradiated amethyst are 0.8 (meV/K) (12.8 × 10−23 J/K) and 8.9 (meV) (14.24 × 10−22 J), respectively. These ΔS and ΔH values are notably different from their respective values of 1 meV/K and 6.29 meV for the Fe3+ center on the as-is quartz (SivaRamaiah et al. 2011). This decrease in ΔS and increase in ΔH are attributable to both decrease in the density of mobile/conduction electrons with radiation and high exchange interaction and high strain at the field boundaries of the domains in the irradiated amethyst.

Figure 4b also shows a graph between the logarithmic number of spins and the reciprocal of absolute temperature. The slope of this graph gives the activation energy E a  = 19.7 meV (31.5 × 10−22 J) for the Fe3+ species on the gamma-ray-irradiated amethyst. The pre-exponential coefficient thus determined is 39 s−1. The latter value from the gamma-ray-irradiated amethyst is similar to that for the as-is quartz (40; SivaRamaiah et al. 2011). The former value, on the other hand, is significantly larger than that (7 meV or 11.2 × 10−22 J) obtained for the Fe3+ species without irradiation (SivaRamaiah et al. 2011). The pre-exponential coefficient is useful for determining the spin orientation, spin concentration, and spin dynamics (SivaRamaiah and Lakshmana Rao 2012). The increase in activation energy is attributable to a decrease in the density of mobile/conduction electrons with irradiation. These results demonstrate that energy required to liberate an electron from the surface Fe3+ species on the gamma-ray-irradiated amethyst is greater than its counterpart without irradiation.

The Gibbs energy (ΔG) can be calculated using the equation

$$ \Updelta G \, = \, 2.303 RT\log_{10} (k_{\text{B}} T/\lambda h), $$
(2)

where R is the universal gas constant (8.31 J/K/mol); k B is the Boltzmann constant (1.38 × 10−23 J/K); T is absolute temperature; λ is the rate constant and is equal to the absolute number of spins per 100 mg; and h is Planks constant (6.63 × 10−34 Js). The Gibbs energy of −24.2 (kJ/mol) for the Fe3+ center on irradiated amethyst at 294 K is higher than the value (−27.4 kJ/mol) reported for the as-is quartz (SivaRamaiah et al. 2011). This increase in Gibbs energy by 3.2 kJ/mol after gamma-ray irradiation is an excellent line of evidence for chemical reactions involving the surface Fe3+ species during radiation. Such radiation-induced chemical reactions may include conversions of Fe3+ to Fe2+ and/or Fe4+ (Cox 1976; Dedushenko et al. 2004; Di Benedetto et al. 2010). Unfortunately, direct EPR detection of Fe2+ and Fe4+, both paramagnetic, is not possible under the experimental condition utilized in this study.

Figure 5 shows that the Gibbs energies of the Fe3+ species on gamma-ray-irradiated amethyst increase with temperature, similar to that observed on the as-is quartz (SivaRamaiah et al. 2011). This temperature dependence of Gibbs energy supports strong dipolar interactions among the surface Fe3+ ions.

Fig. 5
figure 5

Calculated Gibbs free energies (ΔG) of the surface Fe3+ center on the gamma-ray-irradiated (10 kGy) amethyst as a function of absolute temperature

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Magnetic susceptibility

The magnetic susceptibility χ for the g = ~15–17 signal has been calculated using the equation

$$ \chi = \frac{{Ng^{2} \beta^{2} S(S + 1)}}{{3k_{B} T}} $$
(3)

where N is the number of spins per m3; g is the g factor and is equal to 15 or 17; β is the Bohr magneton (9.27 × 10−24 J/T); S is the spin quantum number of unpaired electrons = 5/2; kB is Boltzmann constant (1.38 × 10−23 J/K), and T is absolute temperature. The magnetic susceptibilities have been calculated from 294 to 90 K. The calculated magnetic susceptibility of 1.69 × 10−2 m3/kg at 294 K is three orders of magnitude larger than that reported by Hrouda (1986) and is also an order larger than that (3.44 × 10−3 m3/kg) obtained from the as-is sample (SivaRamaiah et al. 2011).

Figure 6 also shows that the reciprocal magnetic susceptibility correlates linearly with absolute temperature. The intercept of this plot gives the Curie temperature of +475 K, while the reciprocal of the slope yields the Curie constant of 19.2 emu/mol. The large positive value of Curie temperature demonstrates that strong ferromagnetic interactions are present in the gamma-ray-irradiated amethyst. In comparison, the Curie temperature and the Curie constant in the as-is amethyst are only +83 K and 11.6 emu/mol, respectively (SivaRamaiah et al. 2011). These differences indicate that the surface Fe3+ species on the gamma-ray-irradiated amethyst has stronger ferromagnetic but weaker antiferromagnetic interactions than its counterpart on the as-is sample.

Fig. 6
figure 6

Reciprocal Curie magnetic susceptibility of the surface Fe3+ center on gamma-ray-irradiated (10 kGy) amethyst as a function of absolute temperature. Note that the data point at 90 K was excluded from calculation

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Relaxation times (T m and T 1)

The phase memory time T m can be calculated using the equation (SivaRamaiah and Lakshmana Rao 2012)

$$ T_{\text{m}} = 9\surd 3\hbar /4\pi^{2} g^{2} \beta^{2} N $$
(4)

where ħ = h/2π; h is Planks constant; g is the g factor value of ~15–17; β is Bohr magneton; and N is the number of spins in 100 mg of 10 kGy gamma-ray-irradiated amethyst. This formula deals with the absolute concentration and, therefore, is applicable for determining T 2 or T m from both pulse EPR and CW EPR spectra. This formula yields T m at 17.9 ns at 294 K, which is in agreement with values reported in literature (Yamanaka et al. 1996; Gubaidullin et al. 2007). For example, Yamanaka et al. (1996) reported T m values of 18–22 ns for the E′ center in the gamma-ray-irradiated quartz. Gubaidullin et al. (2007) reported a T m value of 200 ns at the temperature range 1.6–4.2 K and noted that T m is independent of temperature, suggesting strong dipolar interactions between paramagnetic centers in their sample. Figure 7 shows that T m increases from 17.9 to 140 ns when the temperature decreases from 294 to 90 K. Interestingly, the calculated T m values in the as-is amethyst are from 91 to 225 μs. It is expected that strong dipole coupling between Fe3+ ions results in fast relaxing with short T m. Therefore, the calculated T m values indicate stronger dipolar interactions among the surface Fe3+ ions in the gamma-ray-irradiated amethyst than those without irradiation.

Fig. 7
figure 7

Calculated phase memory time (T m) and spin–lattice relaxation time (T 1) of the Fe3+ center on gamma-ray-irradiated (10 kGy) amethyst as a function of absolute temperature

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The spin–lattice relaxation time T 1 can be calculated using the equation

$$ T_{1} \approx 2\hbar /\surd 3g\beta \Updelta B $$
(5)

where ΔB is the linewidth, ħ = h/2π and h is Planck’s constant, g = ~15–17, and β is Bohr magneton. The spin–lattice relaxation time for the Fe3+ species is estimated to be ~10−8 s from 90 to 294 K, which is of the same order of magnitude reported by Augustyniak-Jabłokow et al. (2010). This T 1 value is four orders of magnitude larger than those (21–25 ps) for its counterpart in as-is amethyst. Figure 7 shows that T 1 decreases slightly with increasing temperature, which is attributable to spin-phonon interaction. The calculated T 1 values indicate stronger spin–lattice interactions for surface Fe3+ ions in gamma-ray-irradiated amethyst than its counterpart without irradiation.

Geometrical radius of surface Fe3+ species

The ionic radius of a transition metal ion can be calculated using the equation

$$ r_{t} = 1/2(\pi /6N)^{1/3} $$
(6)

where r t is the geometrical radius, and N is the number of spins per 100 mg. The geometrical ionic radius of the surface Fe3+ ions in the gamma-ray-irradiated amethyst is calculated as 9.67 μm. The geometrical ionic radius of Fe3+ ions in un-irradiated amethyst, on the other hand, is calculated to be 4.99 nm only. Therefore, the geometrical ionic radius of the surface Fe3+ species on the gamma-ray-irradiated amethyst is nearly three orders of magnitude larger than those on the as-is sample. The geometrical ionic radius can be calculated for any material where the absolute number of spins is known.

Implications for aerosol–radiation interactions

EPR studies of carbonaceous radicals in soot from incomplete combustion of fossil fuels and biomass have been shown to provide important information about the source, speciation, concentration, formation condition, and chemisorption properties of aerosols in the atmosphere (e.g., Dzuba et al. 1988; Yordanov et al. 1996; Yordanov and Najdenova 2004; Ledoux et al. 2002; Saathoff et al. 2003; Yamanaka et al. 2005; Xie et al. 2007). Similarly, other paramagnetic species (e.g., transition metal ions such as Mn2+ and Fe3+) in natural and anthroprogenic aerosols have been investigated by the EPR technique as well (Ledoux et al. 2002, 2004). For example, Ledoux et al. (2002) investigated the evolution of Mn2+ and Fe3+ ions in the atmospheric particulate aerosols emitted from a ferromanganese metallurgy plant near Wimereux, France.

Interestingly, the EPR spectra of their fine-grained aerosols (<1 μm; see Fig. 7 in Ledoux et al. 2002) contain a broad peak in the low magnetic field, which was interpreted to arise from a ferromagnetic phase containing Mn (cf. Petrakovskii et al. 1983). Our results from amethyst quartz offer an alternative explanation for this broad peak in the low magnetic field (i.e., ferromagnetic surface Fe3+ species on fine-grained aerosol particulates). One possible way for distinguishing Fe3+ from Mn is measuring the temperature dependence of linewidth. Figure 3 shows that the Fe3+ center has the observed linewidth reduced from ~45 mT at 294 K to <25 mT at 90 K. On the other hand, the linewidth of a Mn signal, with contributions from its 55Mn hyperfine structure, is unlikely to go below 40 mT. Moreover, our results suggest that surface Fe3+ species on aerosol particulates are affected significantly by interactions with cosmic radiations and potentially exert important effects on the atmospheric cloud formation and ultimately the global climate changes.

First of all, the experimental studies (Svensmark et al. 2007; Enghoff et al. 2008, 2011) have shown that cosmic rays are active in producing small thermodynamically stable clusters, which are important for aerosol nucleation processes and cloud formation in the atmosphere. For example, Enghoff et al. (2011) demonstrated sulfuric acid aerosol nucleation in an atmospheric pressure reaction chamber by using both a 580-MeV electron beam and a 33.5-MBq Na-22 gamma source and showed that the nature of ionizing particles is not important in the radiation-induced aerosol nucleation processes. Our calculated relaxation times show that surface Fe3+ ions on quartz have stronger dipolar interactions after gamma-ray irradiation, which are strong evidence for radiation-induced clustering of the surface Fe3+ ions. Moreover, our calculated Gibbs and activation energies for the surface Fe3+ ions suggest that heterogeneous nucleation (i.e., those promoted by surface defects) is potentially important in radiation-induced aerosol nucleation.

Secondly, most inorganic aerosols do not absorb solar radiation but produce a negative radiative forcing by scattering incoming solar radiation back into space (Saathoff et al. 2003). Also, the radiative properties, including scattering and extinction coefficients, of aerosols are known to be sensitive to their chemical compositions and grain sizes (Tang 1996; Yu and Zhang 2011). For example, Yu and Zhang (2011) showed that the scattering and extinction coefficients increase with the geometrical radius of aerosols. Our data show that the geometric radius of the surface Fe3+ species increases dramatically with radiation. Therefore, interactions between aerosols and cosmic radiations may be a significant contributor to the radiative forcing of climate change.

Finally, the Fe3+ catalyzed oxidation of SO2 to SO4 2− in the tropospheric clouds has long been suggested to be a major contributor to acid rain (Conklin and Hoffmann 1988; Martin et al. 1991). Our thermodynamic data for the surface Fe3+ species on quartz before and after gamma-ray irradiation show that energy required to liberate an electron from the former is greater than the latter. This result suggests that cosmic radiation may have a negative impact on the catalytic property of Fe3+ in the oxidation processes in the tropospheric clouds.

Conclusions

This presentation of EPR spectra provides evidence for significant effects of gamma-ray-radiation on the surface Fe3+ species on quartz. In particular, the EPR spectra of the surface Fe3+ species on quartz before and after gamma-ray-irradiation not only differ in the effective g value but change in the observed line shape from Gaussian to Lorentzian. Also, the thermodynamic properties, the magnetic susceptibilities, the relaxation times, and the geometric radius of the surface Fe3+ species on quartz are affected by gamma-ray-radiation as well. These results provide compelling evidence for radiation-induced clustering of the surface Fe3+ ions, with relevance to the atmospheric cloud formation and precipitation.