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Effect of thermal treatment on impurity ion ([Mn.sup.2+]) in dolomite through ESR study.

INTRODUCTION

ESR is a branch of absorption spectroscopy in which radiation having frequency in the microwave region is absorbed by paramagnetic substances to induce transitions between magnetic energy levels of electrons with unpaired spins. The magnetic energy splitting is done by applying a static magnetic field. The electron spin resonance phenomenon is shown by atoms having an odd number of electrons, ions having partly filled inner electron shells and other molecules that carry angular momentum of electronic origin. One of the most important uses of ESR technique lies in the detection of extremely short-lived (transient) free radical intermediates in chemical reactions. The ESR spectroscopy has been most widely employed in the study of chemical, photochemical and electrochemical reactions which proceed via free radical mechanisms, because of the occurrence of hyperfine structure, which is the result of interaction between the unpaired electrons and magnetic nuclei in the paramagnetic species. In many cases, hyperfine structure enables unambiguous identification of the species.

The main interest of electron spin resonance lies in the study of free radicals having unpaired electrons. ESR spectra of [Mn.sub.2+] in heat-treated aragonite were analysed by Low and Zeira (1972). Electron spin resonance (ESR) study in deposited carbonates was investigated by Li Hu Hou (1988). K band ESR spectra of calcite stalagmites collected from southeast and south Brazil were studied by Angela Kinoshita et al. (2005). The present paper reports a study of impurity ion in the dolomite samples and effect of thermal treatment through ESR study.

MATERIALS AND METHODS

For this investigation, ten dolomitic rock specimens were collected from Salem and Namakkal districts of Tamil Nadu, India. All these crystals were crushed and ground carefully with a mortar and pestle, and washed for 2min with 1% HCl solution and finally with distilled water to remove any organic material, and then dried in an oven.

The powdered dolomite sample was sieved to select grains between 150 and 200[micro]m. The sample mass of 150 mg was used to record the ESR spectra at room temperature and at various temperatures range of 200 to 900[degrees]C in steps of 50[degrees]C, below saturation, by means of an X-band (8.8-9.67GHz) spectrometer (Varian model E109) with a 100kHz magnetic field modulation at a [TE.sub.102] mode cavity. The modulation amplitude was 0.1mT peak to cavity. Effects of operating conditions for the spectrometer on the ESR spectra of natural calcites were studied in literature (Yokoyama et al., 1985; Lyons et al., 1988; Renyou et al., 1989). The standard DPPH was used to calibrate the applied magnetic field settings.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The ESR spectra were recorded for all the dolomite samples which show a same impurity presence as [Mn.sub.2+] ion. However, there is a change in intensity of the peaks from sample to sample. Therefore, one of the representative samples (no.5) is taken for further analysis which shows highly intense compared to all other samples. The room temperature ESR spectrum of sample no.5 is shown in Figure 1. From this, six doublet line form of [Mn.sup.2+] is observed at room temperature from ESR spectra. For normal decomposition process, the samples were subjected to the thermal treatment in the range of 200[degrees] to 900[degrees]C in steps of 50[degrees]C and are shown in Figure 2. It is observed that the six doublet line form of [Mn.sup.2+] ESR signal changes, as the temperature increases and it becomes six singlet line forms at 800[degrees]C. The magnetic field separation between these singlet lines being approximately 8.0 mT. The six doublet line ESR signal of [Mn.sup.2+] is due to the characteristics of dolomite structure [(CaMg([Co.sub.3]).sub.2] , that has a rhombohedral symmetry. The structure gives rise to two distinct calcium sites (Kikuchi and Matarrese, 1960; Low and Zeira, 1972). The local symmetry of each of the sites is due to a distorted oxygen octahedron of nearest neighbours. On the other hand, the hyperfine splitting and g-lines being approximately 8.0 mT. From this study, it is observed that the change in six doublet line form of [Mn.sup.2+] signal into six singlet line form at 800[degrees]C indicates the formation of CaO and the lattice disturbance ([Mn.sup.2+] ions are dislocated or disordered). However, the same trend was observed by Ponnusamy et al. (2004) for calcites. But the changes took place only at 700[degrees]C.

CONCLUSION

From the above observations the following are the conclusive remarks.

1. The ESR spectra of all the sample shows six doublet line form ([Mn.sup.2+]) at room temperature.

2. From thermal treatment, the six doublet lines ([Mn.sup.2+]) are changed into six singlet lines at 800[degrees]C which shows the formation of CaO and lattice disturbance.

REFERENCES

1. Low, W. and Zeira, S. 1972. ESR spectra of [Mn.sup.2+] in heat treated aragonite. "The American Mineralogist", 57: 1115-1124.

2. Li Hu Hou, 1988. Properties of thermoluminescence (TL) and electron spin resonance (ESR) in deposited carbonates. "Nuclear Tracks Radiation Measurements", 14(1/2): 259-265.

3. Angela Kinoshita, Ivo Karmann, Francisco William da Cruz Jr., Carlos F.O. Graeff, and Oswaldo Baffa, 2005. K-band ESR spectra of calcite stalagmites from south-east and south Brazil. "Applied Radiation and Isotopes", 62: 247-250.

4. Yokoyama, Y. Bibron, R. Leger, C. 1985. ESR dating of paleolithic calcite: Fundamental studies. "Nuclear Tracks", 10: 929-936.

5. Lyons, R.G. Bowmaker, G.A. O'Connor, C.J. 1988. Dependence of accumulated dose in ESR dating on microwave power: a contra-indication to the routine use of low power levels. "Nuclear Tracks Radiation Measurements", 14: 243-251.

6. Renyou, L. Zicheng, P. Sizhao, J. Peihua, H. 1989. Estimation of the influence of experimental conditions on ESR dating results. "Applied Radiation Isotopes", 40: 1071-1075.

7. Kikuchi, C. and Matarrese, L. M. 1960. Paramagnetic resonance absorption of ions with spin 5/2 : [Mn.sup.2+] in calcite. "Journal of Chemical Physics", 33: 601-606.

8. Ponnusamy, V. Ramasamy, V. Dheenathayalu, M. and Hemalatha, J. 2004. Effect of annealing in thermostimulated luminescence (TSL) on natural blue colour calcite crystals. "Nucl. Instru. And Meth. In Phys. Res.", B217: 611-620.

V. Ramasamy (1), V. Ponnusamy (1), S. S. Gomathi (2) and M. T. Jose (3) (1) Department of Physics, Annamalai University, Annamalainagar--608 002, (Tamilnadu), India (2) Department of Physics, Sri Saradha College for Women, Salem, (Tamilnadu), India. (3) Health and Safety division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, (Tamilnadu), India
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Author:Ramasamy, V.; Ponnusamy, V.; Gomathi, S. S.; Jose, M. T.
Publication:Bulletin of Pure & Applied Sciences-Geology
Date:Jan 1, 2008
Words:1091
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