Abstract
Instrumental neutron activation analysis (INAA) is a powerful technique for trace element determination in rocks. Nine alabaster samples were collected from Wadi El-Nakhil located at the intersection of lat. 26°10′50′′N and long. 34°03′40′′E, central Eastern Desert, Egypt, for investigation by INAA and Energy Depressive X-Ray Fluorescence (EDXRF). The samples were irradiated by thermal neutrons at the TRIGA Mainz research reactor at a neutron flux of 7 × 1011 n/cm2·s. Twenty-two elements were determined, namely, As, Ba, Ca, Co, Cr, Sc, Fe, Hf, K, Mg, Mn, Na, Rb, U, Zn, Zr, Lu, Ce, Sm, La, Yb, and Eu. The chemical analysis of alabaster indicated having high contents of CaO and MgO and LOI and low contents of SiO2, Al2O3, Na2O, K2O, MnO, and Fe2O3.
1. Introduction
Neutron activation analysis (NAA) is based on the irradiation of a sample with neutrons from a neutron source like a nuclear reactor and the subsequent measurement of the induced radioactivity. The NAA has become a very capable multielement analysis technique with improved sensitivity. In many cases, more than 30 elements can be determined from a single experiment [1–3]. The technique is highly sensitive and highly specific. The demand for multielement analysis enhanced the need for the standardization of an absolute method. It is not practiced because of the following reasons. It needs nuclear data set such as atomic mass (), isotopic abundance, () capture cross section (), absolute -ray abundance, and absolute neutron flux (). It is well known that measurement of the absolute values of and for a reactor neutron spectrum is difficult [4–6].
The Egyptian alabaster is one of the ornamental stones used by ancient Egyptian for art and decorative purposes. The term Egyptian alabaster is applied to a beautifully banded rock composed of crystalline calcium carbonate. It occurs in several locations in the Eastern Desert near the Nile Valley, at Wadi El Assuity, at Wadi Sannur, opposite Beni Suef and Sohag area and near El-Minya El-Gedida, and East Wadi El Shurafa [7]. Given that, there is no specialized database for elemental analysis of alabaster. In the present work, more light will be shed on the elemental content of alabaster. The obtained result can be used as a reference database to those interested in the elemental content of alabaster and those interested in geochemistry and industry of alabaster and other fields.
2. Geological Setting
Wadi El-Nakhil alabaster is located at the intersection of lat. 26°10′50′′N and long. 34°03′40′′E, central Eastern Desert, Egypt. The studied alabaster deposits at Wadi El-Nakhil occur in the form of veins of lensoidal shaped bodies along fault plane trending N59W enclosed in the Eocene limestones. The contacts between alabaster and the enclosing limestones are sharp. The extension of the vein reaches up to 100 m and its thickness ranges from 30 cm to 1 m. The alabaster usually displays some xenoliths of limestones up to 30 cm in diameter. El-Nakhil alabaster is hard, compact body with banded textures and is characterized by attractive colors and beautiful shapes. The banding is the result of the change in the physicochemical environment of mineralization with time [8]. The banded textures extend laterally and exhibit colloform and botryoidal textures. The thickness of alternating bands varies from 0.2 to 1.2 cm for the milky white bands and is commonly in the range of 0.3 to 2.8 cm for the translucent bands. The botryoidal textures usually consist of spherical aggregates ranging from 1 cm to 12 cm. Some bands are stained reddish brown or black by iron and manganese oxides [9]. Wadi El-Nakhil alabaster deposits occur as hard compact bodies with banded textures. The deposits occur in the form of alternating bands that extend laterally and exhibit colloform and botryoidal textures. Banded texture is common in this alabaster and occurs in the form of successive regular bands in contact with the enclosing limestones. The alabaster has successive milky white and varicolored translucent bands altering with each other. Colloform texture occurs as lining of larger vugs; the colloform banding consists of successive bands with curved surfaces. Botryoidal texture is arranged in the form of spherical aggregates. Quartz grains range in size from fine to medium usually rounded to subangular and nonfractured. Quartz usually occurs as individual grains scattered in the carbonate groundmass. Calcitization of the dolomite crystals predominates during the formation of alabaster. Lee and Harwood [10] stated that this process took place under the effect of meteoric conditions. The replacement of the dolomite by calcite is usually near-surface process and commonly associated with the dissolution of sulfide [11]. Figure 1 shows location map of Wadi El-Nakhil. The full points in the map indicate the sampling sites.
3. Experimental Technique
3.1. Samples Preparation and Irradiation
Nine alabaster rock samples were collected from Wadi El-Nakhil located at the intersection of lat. 26°10′50′′N and long. 34°03′40′′E, central Eastern Desert, Egypt, for investigation by instrumental neutron activation analysis. The samples, each about 1 kg in weight, were dried in an oven at about 105°C to ensure that moisture is completely removed. The powdered samples were sieved using a standard set of sieves to a diameter range of less than 125 μm and greater than 63 μm. Each powdered sample was homogenized using an electric shaker [12–15]. 100 mg from powder alabaster samples was filled in polyethylene capsules and then irradiated with a Dolerite WSE and Microgabbro PMS standard reference material with a flux of 7 × 1011 n/cm2 ·s at the University of Mainz TRIGA research reactor. The concentrations of the elements in the irradiated samples were quantitatively determined by comparison with the activities of the reference materials [16–18]. After appropriate cooling times, the data were collected for various measurements [19]. Table 1 shows the irradiation conditions for the elements determined.
The measuring of activity concentration for radionuclide in studied samples was defined using gamma-ray spectrometer system by HPGe detector with its electronic circuit. The HPGe detector was equipped with specifications as follows: energy resolution (FWHM) is 1.70 keV at 1.33 MeV Co-60, peak-to-Compton ratio Co-60 is 65.2, and relative efficiency is 29.2 at 1.33 MeV Co-60. The measurements were analyzed and performed by using the Inter-Gamma Software. In all measurements, the electronic dead time is less than 10% and the Inter-Gamma Software performed the correction automatically [19, 20]. Table 1 shows the most appropriate irradiation, cooling, and measuring time and the elements determined.
The chemical analyses of major oxide contents in this study were conducted on a Thermo Scientific QUANT’X EDXRF spectrometer, located in the department of physics; all powder samples were dried at a temperature of 105°C and stored in desiccators, until they were analyzed. Fill up 3/4 parts of the cup with the sample analyzed. Then, cover the cup with the film. Fix the slim ring by sliding it down and fix the thick ring at the top of the cup. When the cups are prepared, place them on the sample stage inside the spectrometer chamber, with the side to be analyzed facing down toward the X-ray tube (QUANT’X Technical Manual, Thermo Fisher Scientific, 2008).
4. Results and Discussion
Table 2 shows the average concentration values of nine alabaster samples in four irradiation cycles. Twenty-two elements were identified. The elements determined are Na, Mg, Ca, Mn, Fe, K, Sc, Cr, Co, Zn, As, Rb, Zr, Ba, La, Ce, Eu, Sm, Yb, Lu, Hf, and U. The concentration for all elements was expressed in ppm except for Na, Mg, Ca, Mn, Fe, and K which were given in %. The concentration of determined elements was specified by means of multiple activities produced by () reactions, since some of the radionuclides determined exhibit more than one obvious and distinct gamma line. In all other situations, the elements were measured by their most distinctive peaks, with lowest statistical error, free of interference. The measurements accuracy has been estimated using the PMS and WSE analysis, for the standard reference materials. From the obtained results, we can say that INAA is an effective and successful method to supply valuable data for clay samples with a satisfying precision. The accuracy for most elements in present results is in the range of 10% of the reference values, and a good precision has been shown in most results [21].
The statistical counting errors were for Na, Mn, Sc, Co, Hf, and Sm, 2–5% for Eu, La, Cr, Fe, Ce, Yb, and Lu, 5–10% for Zn, As, K, Mg, and Ca, 10–15% for Rb, Ba, and U.Quantitative analysis was carried out for each isotope by comparing the activities from the most favorable peaks in the gamma spectra of samples with those of the standard reference material. In this analysis the highest-energy peaks were usually used, as in the case of , , and , since these peaks normally had less interference than lower-energy peaks due to the Compton effect. In some cases, the use of low-energy gamma lines permitted the determination of some elements, since these radionuclides have no high-energy peak, as in the case of and . Scandium is the most favorable element to be determined by INAA, due to the 100% abundance of its single stable nuclide and its 100% branching ratio of the measured gamma lines at 889.4 keV and 1120.5 keV and its convenient half-life of 83.8 days [22].
The rare earth elements determined are La, Ce, Sm, Eu, Yb, and Lu. The selection of photopeaks for the analysis is briefly discussed below for each element. For lanthanum, the high abundance photopeak of at 1596 keV was used, which is free of interference. The other peak at 487 keV cannot be used due to interferences from 47Ca 489 keV and 192Ir 488 keV. For cerium, the photopeak of 141Ce at 145 keV was used. For samarium, the isotope 153Sm is used. As far as europium is concerned, 152Eu has a number of photopeaks where the high abundance peaks at 1408 and 799 keV are free from interference. Both peaks were used for the determination of this element. Another peak at 122 keV cannot be resolved from the 124 keV line of 154Eu. However, the combined peaks can be used as these are obtained from two isotopes of the same element and have similar half-lives. Ytterbium can be determined using the 198 keV peak of 169Yb. The 396 keV peak cannot be resolved from nearby peaks of 152Eu 383 keV and 233Pa 381 keV. For lutetium, the high abundance peak at 208 keV of was used [23–25].
The concentration of uranium via in alabaster samples was measured according to the following equation:The feature -rays can be detected using -spectroscopy [23].
The isotopes 140La, 141Ce, and 147Nd commonly used in the activation analysis of the corresponding elements are also produced by fission of 235U. There is a possibility of interference of the 320 keV line with the 319.4 keV line, but neodymium has not been identified in any of the -ray spectra. Scandium was determined from the 889 keV line intensity, which is not interfered with by any of the activation products [24–26]. Table 3 shows interferences due to energy and reactions.
The results of chemical analysis of alabaster samples are given in Table 4. In the studied alabaster, CaO ranges from 50.45 to 70.75% with an average of 64.9%. MgO ranges from 1.21 to 21.5% with an average of 8.28%. SiO2 content ranges from 0.4 to 1.8% with an average of 0.79%. Small quartz grains in the silicified carbonate bands in some alabaster samples represent silica. Fe2O3 ranges from 1.07 to 1.58% with an average of 1.18%. Na2O contents range from 0.18 to 0.54% with an average of 0.28%. P2O5 ranges from 0.01 to 0.1% with an average of 0.03%. LOI (H2O+) ranges from 24.6 to 25.5% with an average of 25.2%.
5. Conclusion
Twenty-one elements were determined including major and trace elements in addition to some rare earth elements. The chemical analyses of alabaster reveal the enrichment of CaO and MgO and LOI and the depletion of SiO2, Al2O3, Na2O, K2O, MnO, and Fe2O3. The source of calcium is dissolution of Eocene limestone due to the action of meteoric water.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant no. 363-651-D1435. The authors, therefore, acknowledge with thanks DSR technical and financial support.