Natural zircon (ZrSiO4) has a particular importance as a basic source of Zr and Hf. Zircon is used as a thermal stable and chemically resistant material. In Earth science zircon is a very important mineral for isotope geochronology. Usually natural zircon contains radioactive elements at high concentration which may lead to the formation of structural defects and electron-hole centers and as a consequence to the change of physical properties. In this work the radiation sensitivity, time stability and optical sensitivity of [SiO4 5-] paramagnetic centers in natural zircon crystals have been studied. The [SiO4 5-] center is formed under natural or laboratory γ-irradiation, is observed at room temperature, and is almost isotropic (g = 2.0009). In this study the rather high radiation sensitivity of [SiO4 5-] centers was recorded. The concentration of the centers increased approximately by an order of magnitude at a radiation dose of ~ 100 krad. Meanwhile, the natural concentration of centers appeared suddenly low. Also during the measurements changes in the concentration of [SiO4 5-] centers in zircon when the samples were kept at room temperature were observed. Due to this long-time (6 months) study on the changes in the concentration of [SiO4 5-] centers in zircons at room temperature has been done. Three series of measurements have been carried out; in the first series sample annealed at 500°C and irradiated with 200 krad dose was kept at room temperature and diffuse natural illumination. In the second series an identical sample was kept under conditions that excluded light impact. In the third series a sample annealed at 500°C and not irradiated was stored in the dark without diffuse illumination. Figure 1 shows the dependence of the concentration of [SiO4 5-] centers in samples, irradiated with 200 kRad dose. Under impact of diffuse solar illumination over 180 days, the concentration of the centers decreased to approximately 39% of the initial value (a). In the absence of illumination, the concentration of [SiO4 5-] centers also decreases but with a lower rate, and after 6 months it was approximately 78% of the initial value (b). In the sample from the third series, the initial concentration of centers after annealing was lower than the sensitivity of the EPR spectrometer. After 6 months the concentration of the centers increased and became sufficient to be recorded by the EPR spectrometer. Obtained results show that [SiO4 5-] centers in zircon have low thermal stability, which is attested by a decrease in their concentration at room temperature over 180 days. At the same time, [SiO4 5-] centers are light sensitive; natural illumination over 20–30 days resulted in a drastic decrease in their concentration. Thereafter, the concentration of [SiO4 5-] centers continued to decrease; however, it happened at a considerably slower rate. The increase in the concentration of [SiO4 5-] centers in zircons annealed at high temperature and kept without illumination is explained by the fact that the concentration of radioactive elements is enough to form a measurable amount of radiation [SiO4 5-] centers. Thus, concentration of [SiO4 5-] centers will be determined by the dynamic equilibrium between the formation rate of the centers and the rate of their decomposition, both light induced and thermal.figureFig.1. Decreasing concentration of [SiO4 5-] paramagnetic centers in zircon after the annealing and γ-irradiation with 200 krad dose: (a) with illumination by diffuse natural light, (b) without illumination of the sample.