Abstract
The crystallization of magmas makes rock-forming minerals including more or less uranium and thorium isotopes other than major elements. This means that all terrestrial rocks become naturally radioactive. Since uranium and thorium are lithophile elements that are easy to be bond to silicates, radioactive concentrations of acid rock such as granite tend to be high. In this study, the natural radioactivity, radon (222Rn) emanation fraction and specific surface area of minerals separated from raw rocks were measured for comparing among rock-forming minerals. One granitic gneiss and weathered granite soil were prepared and sieved into not more than 500 μm. Minerals included in the two were first identified roughly by X-ray diffraction and then mineral separation from the raw samples was carried out using a heavy liquid, SPT (sodium polytungstate). Subsequently, the radioactive concentrations (238U, 232Th, 226Ra, 40K), the radon emanation fractions of the separated minerals were measured using a high-purity germanium detector by γ-ray spectrometry. The specific surface areas were also determined according to the BET method. As a result of the mineral separation, two different minerals of quartz (SiO2) and muscovite (KAl2(Si3Al)O10(OH)2) and three ones of quartz, microcline (KAlSi3O8) and goethite (FeO(OH)) were obtained from the gneiss and weathered granite samples, respectively. For the gneiss sample, the radioactivities of muscovite were 10 times higher than those of quartz, although the radon emanation fraction of quartz was much higher. For the weathered granite sample, the order of the higher radioactivities of decay series nuclides and 40K was goethite > microcline > quartz and microcline > quartz ≈ goethite, respectively. In contrast, the radon emanation fraction of microcline was significantly lower than quartz and goethite. The inhomogeneous distribution of 40K can be reasonably understood from the viewpoint of mineral compositions. On the other hand, we can explain for the other nuclides, taking into account the order of the crystallization of object minerals (quartz, muscovite and microcline). Because the latest crystallized mineral is quartz, more uranium and thorium may have been already contained in muscovite and microcline of the earlier silicates. Here, grains of goethite, which is a clay mineral with large surface areas, may adsorb natural radioactive nuclides. We will attempt to discuss why the radon emanation fractions are inhomogeneous between rock-forming minerals.
