Gamma Irradiation Effect on Rock Crystals

Abstract

Gamma irradiation effect on rock crystals from various geologic origins was studied. Gamma irradiation produced smoky color and sometimes citrine color in rock crystals, but a few of rock crystals were not affected. The pattern of smoky coloration is classified into three types which are related to the geologic origin; 1. normal type, 2. lamellar type, 3. irregular type. The degree of blackening of rock crystals from pegmatite is roughly related to the temperature at which the crystal was formed, and the thermal history of the crystal can be well presumed. On absorption spectra of rock crystals, A2 peak position is variable in 440-500 mμ range. Rock crystals with A2 peak at about 440-460 mμ have high intensity in smoky color and mostly occur in pegmatite, whereas crystals with A2 peak at about 500 mμ have low intensity in smoky color and mostly occur in hydrothermal vein. The wide variation (440-500 mμ) of A2 peak position is thought to be caused by the difference of interstitial ion in Al defect. Citrine color has absorption peak at 390-410 mμ and is often coexistent with smoky color. Rock crystals with citrine color occur in hydrothermal vein or skarn.