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EPR, UV-Visible, and Near-Infrared Spectroscopic Characterization of Dolomite

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EPR, UV-Visible, and Near-Infrared Spectroscopic Characterization of Dolomite Hindawi Publishing Corporation Advances in Condensed Matter Physics Volume 2008, Article ID 175862, 8 pages doi:10.1155/2008/175862 Research Article EPR, UV-Visible, and Near-Infrared Spectroscopic Characterization of Dolomite S. Lakshmi Reddy,1 R. L. Frost,2 G. Sowjanya,1 N. C. G. Reddy,3 G. Siva Reddy,3 andB.J.Reddy2 1 Department of Physics, S.V.D. College, Kadapa 516003, India 2 Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, Brisbane, GPO Box 2434, Queensland 4001, Australia 3 Department of Chemistry, Yogi Vemana University, Kadapa 516003, India Correspondence should be addressed to R. L. Frost, [email protected] Received 17 June 2008; Accepted 21 October 2008 Recommended by David Huber Dolomite mineral samples having white and light green colors of Indian origin have been characterized by EPR, optical, and NIR spectroscopy. The optical spectrum exhibits a number of electronic bands due to presence of Fe(III) ions in the mineral. From EPR studies, the parameters of g for Fe(III) and g, A,andD for Mn(II) are evaluated and the data confirm that the ions are in distorted octahedron. Optical absorption studies reveal that Fe(III) is in distorted octahedron. The bands in NIR spectra are due to the overtones and combinations of water molecules. Thus EPR and optical absorption spectral studies have proven useful for the study of the solid state chemistry of dolomite. Copyright © 2008 S. Lakshmi Reddy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction a natural substitutional impurity at the sites of both Ca(II) and Mg(II) ions. Composition of low-Mn ankerite generally Carbonate minerals in various forms such as limestone, approached the formula CaMg(CO3)2-CaFe(CO3)2 [3]. This dolomite, and calcite constitute the earth’s largest CO2 group of minerals has the wide useful applications especially resource. The dolomite group of minerals have the general ++ ++ as fillers some of which are given below: formula A B (CO3)2. When large amount of iron is present, the mineral ankerite forms and when excess man- (i) in cement paints, exterior paints, primers, putties, ganese is present the mineral kuntnohorite forms instead of powder coatings, and industrial finishes; dolomite. All these minerals have the same internal structure (ii) in PVC footwear, PVC pipes, cables, and others; but differ chemically from each other [1]. This group of minerals is classified [2] into several types as given below: (iii) in paper to give smoothness and gloss; Ankerite: Ca(Fe,Mg,Mn)(CO3)2 (iv) in adhesives and carpet backing; Benstonite: (Ba, Sr)6(Ca, Mn, Mg)6(CO3)12 (v) in toothpaste, cosmetic, and soap industry. Dolomite: CaMg(CO3)2 Dolomites are isostructural having space group R3. Huntite: CaMg3(CO3)4 Dolomite structure reveals that there are two distinct cation Kuntnohorite: Ca(Mn,Mg,Fe)(CO3)2 sitesdenotedbyAandB[4]. These both sites form nearly regular octahedra in which each corner of an octahedron is Minrecordite: (Ca,Zn)(CO3)2 oxygen from a different CO3 group. The site A is occupied Norsethite: BaMg(CO3)2. by Ca and the B site is occupied by Mg in the ideally In the above group of minerals, Mg(II) can be replaced by ordered case with layers of Ca octahedron alternating with ferrous iron and manganese. In dolomites the Mn(II) ion is layers of (Mg,Fe,Mn) octahedron. The octahedra are linked 2 Advances in Condensed Matter Physics Table 1: Chemical composition of dolomite. octahedral symmetries and they apply to ions with more than one unpaired electron (e.g., low field Fe3+ and Mn2+). Percentage However, the broad nature of EPR spectra of Fe3+ makes the Component 12 3determination of D and E difficult [14]. (green) (white) (pale yellow) Mn(II), being a d5 ion, has total spin S = 5/2. This CaCO3 53.6 56.6 state splits into three Kramers’ doublets, ±5/2>, ±3/2>,and MgCO3 42.0 43.0 ±1/2> separated by 4D and 2D,respectively,whereD is the CaO 31.7 31.7 31.27 zero-field splitting parameter. The deviation from axial sym- MgO 20.5 20.5 21.12 metry leads to a term known as E in the spin Hamiltonian. The value of E can be easily calculated from single crystal Fe2O3 0.30 0.30 SiO 5.50 1.50 0.12 measurements. A nonzero value of E results in making the 2 spectrum unsymmetrical about the central sextet. Al2O3 0.50 0.50 FeO 0.22 1: The Good Earth Udaipur, Rajasthan, India; 2: Famous Minerals & 3. Results and Discussion Chemicals Private Limited; 3: Haley, Ontario, Canada, Mineral data publishing version I, 2001–2005. 3.1. Total Chemical Analyses by sharing corners simultaneously with octahedra of the Dolomite minerals examined in this study are ankerite and opposite kind with CO3 groups [4]. The two octahedra in dolomite. Elemental compositions of metal are listed in dolomite are trigonally distorted by elongation along the Table 1 [15–17]. three-fold axis. The larger A octahedron is always slightly more distorted than that in B [4, 5]. Detailed spectral studies have been reported on dolomite-ankerite series but 3.2. EPR Spectral Analysis not on dolomite of Indian origin [3, 6–8]. Several studies EPR spectrum of the dolomite mineral samples recorded at have been made on natural dolomites from different origins room temperature is given in Figures 1(a) and 1(b). The using EPR studies [9–11]. However, few EPR studies have overall EPR spectral features are similar in both dolomites. been done which could identify paramagnetic radicals in It is well known that the EPR studies of transition metal dolomites around g = 2.0023 [11]. Nevertheless, complete ion Mn(II) is easily observed at room temperature, even interpretation of the EPR spectra of dolomites is not if present in minute levels, compared to other ions. Both available. Hence, optical absorption and EPR studies on spectra (Figures 1(a) and 1(b)) clearly indicate the presence some natural carbonate minerals have been made [12]. of Mn(II) in the sample. An expanded version of central Transition metal ions such as Fe(III), Mn(II), and V(IV) sextet is shown in Figures 2(a) and 2(b). display a rich coordination in minerals. For the identification Figures 1(a) and 1(b) contain a strong sextet at the centre and characterization of these metal ions in natural systems, of the spectrum corresponding to the electron spin transition electron paramagnetic resonance has been found to be the +1/2> to −1/2>. Generally, in most of the cases, the powder most powerful analytical technique. In India, Kadapa Basin is spectrum is characterized by a sextet, corresponding to endowed with rich mineral resources. In this paper, we report this transition. The other four transitions, corresponding the EPR and optical absorption spectral studies of dolomites to ±5/2> ↔±3/2> and ±3/2> ↔±1/2> arenotseendueto obtained from Vempalli Mandal of Kadapa district, Andhra their high anisotropy in D.IfE = 0, the EPR spectrum will Pradesh, India. / not be symmetrical about the central sextet. The expanded version of the powder spectrum (Figures 2(a) and 2(b)) of 2. Theory both dolomite mineral samples indicate the presence of at least four types of Mn(II) impurities in the mineral. The Several EPR parameters (i.e., g, A, D,andE)areusedwhile sixth manganese hyperfine resonance clearly contains two interpreting EPR data. The g parameter is a measure of lines (marked by a and b). This is also followed by two more the coupling between the unpaired electron’s spin angular sets of weak lines (marked by ∗ and ∗∗) on either side of momentum (S) and its orbital angular momentum (L)[13]. the intense Mn(II) signals as shown in Figures 2(a) and 2(b). The unpaired electron interacts (couples) with the This is the same in both samples. This is due to the presence nuclear spin (I)toforma(2I + 1) line hyperfine structure of Mn(II) ions in a low symmetry environment [18]. The centered on g and spaced with the distance quantified EPR line intensity of Mn(II) hyperfine lines observed in by the hyperfine coupling parameter A. The coupling green dolomite is more intense, when compared to that in between the nuclear and electron spins becomes stronger the white dolomite mineral. This indicates clearly that the as the A parameter becomes larger. The combinationof g concentration of Mn(II) ions in green dolomite sample is and A parameters can be utilized to differentiate between more than that in the white dolomite. Even when viewed electron environments of Fe3+ and Mn2+ ions. The EPR with a naked eye, the sample is pale greenish pink in zero field splitting (ZFS) parameters D and E measure the color indicating the substitution of manganese ion in larger deviation of the ion crystal field from ideal tetrahedralor concentration in this mineral. Advances in Condensed Matter Physics 3 Green dolomite Green dolomite a b ∗∗∗ ∗∗∗ 235 435 Magnetic field (mT) 0 500 (a) Magnetic field (mT) (a) White dolomite White dolomite a ∗ ∗∗ b ∗ ∗∗ 235 435 0 500 Magnetic field (mT) Magnetic field (mT) (b) (b) Figure 1: (a) EPR spectrum of green dolomite at RT (v = Figure 2: (a) Expanded form of EPR spectrum of green dolomite 9.39219 GHz). (b) EPR spectrum of white dolomite at RT (v = at RT (v = 9.39128 GHz). (b) Expanded form of EPR spectrum of 9.39208 GHz).
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