A radiological study of some ornamental stones: the bluish granites from Extremadura (Spain)

Abstract

In the area of Trujillo (Extremadura, Spain) we have found a variety of striking bluish and secondary yellow granites outcropping within the Plasenzuela pluton. They are all quarried under different names and are characterized by leucocratic minerals in which a bluish phosphate is dispersed throughout the rock. Their physical and mechanical properties make these granites a perfect option for most applications as ornamental rocks. Within the pluton the radiological background is fairly homogeneous, with no significant differences between the gamma ray fluxes of the different facies. U (6.4 ± 0.51 ppm), Th (2.9 ± 0.47 ppm) and K₂O (4.32 ± 0.26%) contents determined in the laboratory by gamma-ray spectrometry from representative samples are in good accordance with fast in situ measurements carried out with the same technique and also with conventional chemical analysis. Estimated activities for K-40, Ra-226 and Th-232 were, on average, 1022 ± 36 Bq kg⁻¹, 84 ± 9 Bq kg⁻¹ and 6.8 ± 1.6 Bq kg⁻¹. On the basis of these results, the <i>I</i> index of EU technical document 112 was determined, with an average result of 0.66 ± 0.03. This suggests that the rock can be used with no restrictions for all types of construction purposes. Striking differences were observed between the blue facies and the yellow facies in radon exhalation tests from crushed material (0.03 Bq kg^&minus;1 vs. 0.20 Bq kg^&minus;1). Since both granites have similar uranium contents (6.3 ppm and 6.5 ppm, respectively), these results can probably be explained in terms of important differences in the mineralogical distribution of this chemical element, radon emanation being enhanced in the yellow facies. However, due to the ordinary use as ornamental stone, the yellow facies does not present a radiological risk in terms of excess effective dose due to internal exposure to radon.