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

Authors: S. Lakshmi Reddy; R. L. Frost; G. Sowjanya; N. C. G. Reddy; G. Siva Reddy; B. J. Reddy
Journal: Advances in Condensed Matter Physics
 (2008)

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## Abstract

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 ofg for Fe(III) and g,A, and D 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.


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## Body

## 1. Introduction

Carbonate
minerals in various forms such as limestone, dolomite, and calcite constitute
the earth’s largestCO2 resource. The dolomite group of minerals have the
general formula A++B++(CO3)2. When
large amount of iron is present, the mineral ankerite forms and when excess
manganese is present the mineral kuntnohorite forms instead of dolomite. All these minerals have the same internal
structure but differ chemically from each other [1]. This
group of minerals is classified [2] into several types as given below:
Ankerite: Ca(Fe,Mg,Mn)(CO3)2Benstonite:(Ba,Sr)6(Ca,Mn,Mg)6(CO3)12Dolomite: CaMg(CO3)2Huntite:CaMg3(CO3)4Kuntnohorite: Ca(Mn,Mg,Fe)(CO3)2Minrecordite: (Ca,Zn)(CO3)2Norsethite: BaMg(CO3)2.
In the above group of minerals, Mg(II) can be replaced by
ferrous iron and manganese. In dolomites the Mn(II) ion is a natural
substitutional impurity at the sites of
both Ca(II) and Mg(II) ions. Composition of low-Mn ankerite generally approached
the formula CaMg(CO3)2-CaFe(CO3)2
[3]. This group of minerals has the wide useful applications especially as
fillers some of which are given below:
(i)
in cement paints, exterior paints, primers, putties, powder coatings, and industrial finishes;(ii)
in PVC footwear, PVC pipes, cables, and others;(iii)
in paper to give smoothness and gloss;(iv)
in adhesives and carpet backing;(v)
in toothpaste, cosmetic, and soap industry.Dolomites are isostructural having space groupR3. Dolomite structure reveals
that there are two distinct cation sites denoted by A and B [4]. These both sites form nearly regular
octahedra in which each corner of an octahedron is oxygen from a different CO3 group. The site A is occupied by Ca and
the B site is occupied by Mg in the ideally ordered case with layers of Ca
octahedron alternating with layers of (Mg,Fe,Mn) octahedron. The octahedra are linked by sharing corners
simultaneously with octahedra of the opposite kind with CO3 groups [4]. The two octahedra in dolomite are trigonally
distorted by elongation along the 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 not on dolomite of Indian origin [3, 6–8]. Several studies have been made on natural dolomites from different
origins using EPR studies [9–11]. However, few EPR studies have been done which
could identify paramagnetic radicals in dolomites around g=2.0023 [11]. Nevertheless, complete interpretation of the EPR spectra of
dolomites is not available. Hence, optical absorption and EPR studies on some
natural carbonate minerals have been made [12].Transition
metal ions such as Fe(III), Mn(II), and V(IV) display a rich coordination in
minerals. For the identification and
characterization of these metal ions in natural systems, electron paramagnetic resonance
has been found to be the most powerful analytical technique. In India,
Kadapa Basin is endowed with rich mineral resources. In this
paper, we report the EPR and optical absorption spectral studies of dolomites obtained
from Vempalli Mandal of Kadapa district, Andhra Pradesh, India.


## 2. Theory

Several EPR parameters (i.e.,g,A,D,
and E) are used while interpreting EPR data. The g parameter is a measure of the coupling between the unpaired electron's spin
angular momentum (S) and its orbital angular momentum (L)
[13].The unpaired electron interacts (couples) with the nuclear
spin (I) to form a (2I+1) line hyperfine structure
centered on g and spaced with the distance quantified by the
hyperfine coupling parameter A. The coupling between the
nuclear and electron spins becomes stronger as the A parameter becomes larger. The combinationof g and A parameters can be utilized to differentiate between electron
environments of Fe3+ and Mn2+ ions. The EPR
zero field splitting (ZFS) parameters D and E measure the
deviation of the ion crystal field from ideal tetrahedralor
octahedral symmetries and they apply to ions with more than one
unpaired electron (e.g., low field Fe3+ and Mn2+).
However, the broad nature of EPR spectra of Fe3+ makes the
determination of D and E difficult [14].Mn(II),
being ad5 ion, has total spin S=5/2. This state splits into three Kramers’
doublets, ±5/2>, ±3/2>,
and ±1/2> separated
by 4D and 2D, respectively, where D is
the zero-field splitting parameter. The
deviation from axial symmetry leads to a term known as E in the spin
Hamiltonian. The value of E can be
easily calculated from single crystal measurements. A nonzero value of E results in making the
spectrum unsymmetrical about the central sextet.


## 3. Results and Discussion

### 3.1. Total Chemical Analyses

Dolomite minerals examined in this
study are ankerite and dolomite. Elemental compositions of metal are listed in Table1 [15–17].Table 1
Chemical composition of dolomite.
ComponentPercentage123(green)(white)(pale yellow)CaCO353.656.6MgCO342.043.0CaO31.731.731.27MgO20.520.521.12Fe2O30.300.30SiO25.501.500.12Al2O30.500.50FeO0.221: The Good Earth Udaipur, Rajasthan, India; 2: Famous Minerals & Chemicals Private Limited; 3: Haley, Ontario, Canada, Mineral data publishing version I, 2001–2005.


### 3.2. EPR Spectral Analysis

EPR spectrum of the dolomite mineral samples
recorded at room temperature is given in Figures1(a) and 1(b). The overall EPR
spectral features are similar in both
dolomites. It is well known that the EPR studies of transition metal ion
Mn(II) is easily observed at room temperature, even if present in minute
levels, compared to other ions. Both spectra
(Figures 1(a) and 1(b)) clearly indicate the presence of Mn(II) in the
sample. An expanded version of central
sextet is shown in Figures 2(a) and 2(b).(a) EPR spectrum of green dolomite at RT (v=9.39219GHz). (b) EPR spectrum of white dolomite at RT (v=9.39208GHz).
(a)(b)(a) Expanded form of EPR spectrum of green dolomite at RT (v=9.39128GHz). (b) Expanded form of EPR spectrum of white dolomite at RT (v=9.39183GHz).
(a)(b)Figures1(a) and 1(b) contain a strong sextet at the centre of the spectrum
corresponding to the electron spin transition +1/2> to −1/2>. Generally, in most of the cases, the powder
spectrum is characterized by a sextet, corresponding to this transition. The other four transitions, corresponding to ±5/2>↔±3/2> and ±3/2>↔±1/2>
are not
seen due to their high anisotropy in D.
If E≠0, the EPR spectrum will not be symmetrical about the central
sextet. The expanded version of the powder
spectrum (Figures 2(a) and 2(b)) of both dolomite mineral samples indicate the
presence of at least four types of Mn(II) impurities in the mineral. The sixth manganese hyperfine resonance
clearly contains two lines (marked by a and b). This is also followed by two
more sets of weak lines (marked by ∗ and ∗∗) on either side of the intense Mn(II)
signals as shown in Figures 2(a) and 2(b)). This is the same in both samples.
This is due to the presence of Mn(II)
ions in a low symmetry environment [18]. The EPR line intensity of Mn(II) hyperfine lines observed in green dolomite
is more intense, when compared to that in the white dolomite mineral. This indicates clearly that the concentration
of Mn(II) ions in green dolomite sample is more than that in the white dolomite. Even when viewed with a naked eye, the sample
is pale greenish pink in color indicating the substitution of manganese ion in
larger concentration in this mineral.The
observed EPR spectrum can be explained by the spin-Hamiltonian of the form as shown
in the following equation (see
[19]):
(1)H=βBgS+D{SZ2−13S(S+I)}+SAI.
Here the first term represents the electron-Zeeman
interaction. The second term represents the zero field contribution, and the third
term represents the nuclear-Zeeman interaction.
The extra set of resonances in between the main sextet is due to the
forbidden transitions. From the forbidden doublet lines, the Zero field
splitting parameter, D, has been calculated
using the formula (see
[20, 21])
(2)ΔH=(2D2Hm)[1+16(Hm−8Am)29HiHm−64Am].
Here
(3)Hm=Ho−Am−[I(I+1)−m2]A22Ho.
From the powder spectrum of the mineral, the following
parameters have been calculated:White dolomite(4)g=1.992±0.015,A=9.15±0.43mT,D=34.45±0.36mT.Green dolomite(5)g=1.993±0.017,A=9.35±0.43mT,D=34.25±0.35mTThe observedg,A, and D values are
comparable with other similar systems and agree well within the experimental
error [8, 22–25]. The large value of D indicates a considerable amount of
distortion around the central metal ion. As EPR is highly sensitive to Mn(II)
impurity, four such sites have been noticed in the EPR spectrum. A close look at the EPR spectrum indicates a
nonzero value for E, which is very difficult to estimate from powder spectrum.
A close look at the centre of the EPR spectrum indicates a broad line
underneath the Mn(II) sextet. This can
be attributed to the iron impurity in the sample. Generally, Fe(III) and Fe(II) impurities will
give rise to a broad line around with a g value of 2. No extra information can be obtained from the
powder spectrum recorded at room temperature and low temperature except that
iron is present as an impurity. Even
though the percentage of iron is more than that of manganese, its intensity is
relatively smaller compared to the signals obtained from the manganese impurity. The EPR spectrum of white dolomite exhibits a
broad resonance centered when g is 4.20; similarly a weak resonance is observed
when g is 4.34 in the case of green dolomite.
Those are attributed to the presence of isolated Fe(III) ions
incorporated as an impurity in the mineral.The hyperfine constant ‘‘A’’ value
provides a qualitative measure of the ionic nature of bonding with Mn(II) ion.
The percentage of covalency of Mn-ligand bond in green dolomite has been
calculated using ‘‘A’’ (9.35 mT) value obtained from the EPR spectrum and Matumura’s
plot [26]. It corresponds to an ionicity of 94%, where, as in the case of white
dolomite, ‘‘A’’ with a value of 9.15 mT gives an ionicity of 91%. Also the approximate value of hyperfine
constant (A) is calculated for both dolomites by using covalency (C)
using the following equation (see [27, 28]):
(6)Aiso=(2.04C−104.5)10−4cm−1.The value obtained is92×10−4cm−1 for green dolomite, whereas for the white dolomite it is 91×10−4cm−1.
This calculated value agrees well with
the observed hyperfine constant for both samples indicating ionic character for
Mn–O bond in the mineral under study. The
number of ligands around Mn(II) ion is estimated using the covalency [27] equation for C(7)C=1n[1−0.16(Xp−Xq)−0.035(Xp−Xq)2].
Here
Xp
and Xq represent
electronagativities of metal and ligand.
Assuming Xp=XMn=1.4 and Xq=XO=3.5, the number of ligands (n) obtained are 20. This suggests that Mn(II) may
be surrounded by four CO32− and two oxygens. Further the g value for the
hyperfine splitting is indicative of the nature of bonding. If the g value shows negative shift with
respect to free electron g value of 2.0023, the bonding is ionic and
conversely, if the shift is positive, the bonding is more covalent in nature [29].
In the present work, from the observed negative values of 0.0103 for white dolomite
and that of −0.0093 for green dolomite, it is apparent that the Mn(II) is in an
ionic environment. EPR spectrum of white dolomite (Figure 2(b)) reveals that in
between first and second manganese hyperfine resonance lines a very weak sharp
peak at (319.2 mT) having a g value 2.1019 is observed. This can be ascribed to a radical such as CO32− or NO32−. The absence of any super hyperfine structure
in the radical suggests that the radical
may be CO32− [11].


### 3.3. Optical Absorption Spectral Analysis

Ferric
iron, Fe(III), has the electronic configuration Ar (3d5) with a half
filled d-shell having one unpaired electron in each of the orbitals. Hence ground state configuration is t2g3eg2.
The free ion levels of Fe(III), in the order of increase in energy, are S6,G4,P4,D4,F4. The energy
levels for Fe(III) in an octahedral environment are A1g6(S6), T1g4(G4), T2g4(G4), A1g4(G)-Eg4(G4), T2g4(D4),
and Eg4(D4). The A1g4(G)-Eg4(G4)
and Eg4(D4) levels have relatively less
influence compared to other levels by crystal field. It means that the relative sharp levels can
be expected in the absorption spectrum which is the criterion for assignment of
levels for Fe(III) ion. Since all the
excited states of Fe(III) ion will be either quartets or doublets, the optical
absorption spectra of Fe(III) ions will have only spin forbidden d-d
transitions. These occur from the ground state A1g6(S) to
the excited states T1g4(G), T2g4(G), A1g4(G), Eg4(G), T1g2(D), Eg4(D), and T1g4(P) of regular
octahedron [30].Optical absorption spectrum of green dolomite recorded in
the mull form at RT in the range of 200–1200 nm is shown
in Figure3 and that of white dolomite is shown in Figure 4. The green dolomite spectrum shows energies at
13160 cm−1 (760 nm), 13515 cm−1 (740 nm). 14285 cm−1 (700 nm), 14995 cm−1 (665 nm), and 19800 cm−1 (505 nm) cm−1 in the UV-Vis region whereas
white dolomite spectrum shows energies at 12660 cm−1 (790 nm), 13070 cm−1 (765 nm), 13425 cm−1 (745 nm), 22220 cm−1 (450 nm) cm−1 in the UV-Vis region. In green dolomite, the band observed at 13515 cm−1 with split component on left side at 13160 cm−1 is assigned to the transition A1g6(S)→T1g4(G) whereas the band at 14995 cm−1 is
assigned to T2g4(G) transition. The third band visible around 19800 cm−1 is assigned to A1g4(G),Eg4(G)
transition. These bands are characteristic
of Fe(III) ion in octahedral symmetry in the mineral. This is also further supported by EPR
studies. Using Tree’s polarization term α=90cm−1 [26], the energy matrices of the d5 configuration are solved for
various B,C, and Dq values. The
evaluated parameters are presented in Table 2. A comparison is also made between the
calculated and observed energies of the bands and these are presented in Table 2. The sharp band observed in green dolomite at 14285 cm−1 may be due
to Mn(II). The very sharp band observed
at 13070 cm−1 in white dolomite with split component on either side
(at 12660 and 13425 cm−1 with average of 13043 cm−1) is
assigned to the transition A1g6(S)→T1g4(G).
Similar assignment is also made in green dolomite. The intensity of the band in
white dolomite is large when compared to green dolomite. Further in white dolomite
the band visible around 16000 cm−1 due to the transition of A1g6(S)→T2g4(G) is not seen.
This may be due to low concentration of iron in the mineral. The third band at 22220 cm−1 is assigned
to A1g4(G),Eg4(G) transition. These bands are characteristic of Fe(III) ion
in octahedral symmetry in the mineral.
This is also further supported by EPR studies. Using Tree’s polarization
term α=90cm−1 [31], the energy matrices
of the d5 configuration are solved for various B,C, and Dq values. A comparison is also made between the
calculated and observed energies of the bands and these are presented in Table 2. These results agree well with the reported values
of different dolomite minerals
of Indian origin [12].Table 2
Band headed data with assignments for Fe(III) in natural dolomite mineral.
Dolomite greenDolomite whiteTransition fromA1g6Dq=900, B=605C=2600cm−1Dq=900, B=600C=2600cm−1α=90cm−1α=90cm−1Wave length (nm)Wavenumber (cm−1)Wave length (nm)Wavenumber (cm−1)ObservedCalculatedObservedCalculated79012660760131601316176513070130877401351574513425T1g4(G)70014285——6651499516951——16852T2g4(G)50519800208134502222020739Eg4(G)A1g4(G)Figure 3
Optical absorption spectrum of green dolomite at room temperature.Figure 4
Optical absorption spectrum of white dolomite at room temperature.


### 3.4. Near Infrared Spectroscopy

Water has three fundamental modes. They are symmetric OH stretch (ν1), H–O–H bending mode (ν2),
and asymmetric OH stretch (ν3). In solid phase, these modes appear at 3220 (ν3),
1620 (ν2),
and 3400 (ν1)
cm−1 [32]. The shifting of ν1 and ν3 towards lower frequency and ν2 towards higher frequency is due to hydrogen bonding [33].


#### 3.4.1. The 11500 to 5500cm−1 Spectral Region

Figure5 shows near infrared spectrum of dolomite mineral
from 11500–5500 cm−1 region. The near-IR spectral regions may
be conveniently divided as follows: the high wave number region >7500cm−1: in this region, electronic bands due to the characteristic of Fe2+ and Fe3+ ions are observed. The two bands observed in green dolomite
at 9286 and 8581 cm−1 are broad and are of very low intensity
which are assigned to the two components of the transition T22g→E5g. This may be
due to trace of Fe(II) present in the sample. The average of these bands 8934 cm−1 is taken as 10Dq. But in white dolomite no Fe(II) bands are noticed.NIR spectrum of dolomite at room temperature from 11000–5500cm−1.The high wave number region between 7200cm−1 and 6300 cm−1 is attributed to the first overtone of the fundamental
hydroxyl stretching mode. The very sharp group of bands at 7127 and split
components on either side at 7237 and 7036 cm−1 with high intensity
are due to 2ν3, the asymmetric OH stretch in the green dolomite. Exactly the similar very
sharp group of bands observed at 7154 cm−1 with components at 7232, 7173,
and 6920 cm−1 with maximum intensity are assigned to the same 2ν3 asymmetric OH stretch in the white dolomite.


#### 3.4.2. The 5500 to 4000cm−1 Spectral Region

The spectral region from 5500cm−1 to 4000 cm−1 is made up of two parts: the 4000–4500 cm−1 region in which OH combination bands are found and 4500–5500 cm−1 region in which water combination bands are observed. Figure 6 shows a profile of bands at 4300 cm−1 with high intensity and with components on either side at 4397 and 4123 cm−1. These bands in green dolomite are assigned
to the water OH combination. In white dolomite, similar bands are observed with
less intensity at 4293 cm−1 with a shoulder at 4388 cm−1 which
may be attributed to the water OH combination.
The intense band observed at 5083 cm−1 in green dolomite is
attributed to the water OH overtone. The
same band with very low intensity observed at 5078 cm−1 in white dolomite
is assigned to the water OH overtone. In
general, the NIR bands observed for white dolomite are less intense when
compared to green dolomite which indicates that water is more in green dolomite
than that present in white dolomite.NIR spectrum of dolomite at room temperature from 5500–4000cm−1.


## 3.1. Total Chemical Analyses

Dolomite minerals examined in this
study are ankerite and dolomite. Elemental compositions of metal are listed in Table1 [15–17].Table 1
Chemical composition of dolomite.
ComponentPercentage123(green)(white)(pale yellow)CaCO353.656.6MgCO342.043.0CaO31.731.731.27MgO20.520.521.12Fe2O30.300.30SiO25.501.500.12Al2O30.500.50FeO0.221: The Good Earth Udaipur, Rajasthan, India; 2: Famous Minerals & Chemicals Private Limited; 3: Haley, Ontario, Canada, Mineral data publishing version I, 2001–2005.


## 3.2. EPR Spectral Analysis

EPR spectrum of the dolomite mineral samples
recorded at room temperature is given in Figures1(a) and 1(b). The overall EPR
spectral features are similar in both
dolomites. It is well known that the EPR studies of transition metal ion
Mn(II) is easily observed at room temperature, even if present in minute
levels, compared to other ions. Both spectra
(Figures 1(a) and 1(b)) clearly indicate the presence of Mn(II) in the
sample. An expanded version of central
sextet is shown in Figures 2(a) and 2(b).(a) EPR spectrum of green dolomite at RT (v=9.39219GHz). (b) EPR spectrum of white dolomite at RT (v=9.39208GHz).
(a)(b)(a) Expanded form of EPR spectrum of green dolomite at RT (v=9.39128GHz). (b) Expanded form of EPR spectrum of white dolomite at RT (v=9.39183GHz).
(a)(b)Figures1(a) and 1(b) contain a strong sextet at the centre of the spectrum
corresponding to the electron spin transition +1/2> to −1/2>. Generally, in most of the cases, the powder
spectrum is characterized by a sextet, corresponding to this transition. The other four transitions, corresponding to ±5/2>↔±3/2> and ±3/2>↔±1/2>
are not
seen due to their high anisotropy in D.
If E≠0, the EPR spectrum will not be symmetrical about the central
sextet. The expanded version of the powder
spectrum (Figures 2(a) and 2(b)) of both dolomite mineral samples indicate the
presence of at least four types of Mn(II) impurities in the mineral. The sixth manganese hyperfine resonance
clearly contains two lines (marked by a and b). This is also followed by two
more sets of weak lines (marked by ∗ and ∗∗) on either side of the intense Mn(II)
signals as shown in Figures 2(a) and 2(b)). This is the same in both samples.
This is due to the presence of Mn(II)
ions in a low symmetry environment [18]. The EPR line intensity of Mn(II) hyperfine lines observed in green dolomite
is more intense, when compared to that in the white dolomite mineral. This indicates clearly that the concentration
of Mn(II) ions in green dolomite sample is more than that in the white dolomite. Even when viewed with a naked eye, the sample
is pale greenish pink in color indicating the substitution of manganese ion in
larger concentration in this mineral.The
observed EPR spectrum can be explained by the spin-Hamiltonian of the form as shown
in the following equation (see
[19]):
(1)H=βBgS+D{SZ2−13S(S+I)}+SAI.
Here the first term represents the electron-Zeeman
interaction. The second term represents the zero field contribution, and the third
term represents the nuclear-Zeeman interaction.
The extra set of resonances in between the main sextet is due to the
forbidden transitions. From the forbidden doublet lines, the Zero field
splitting parameter, D, has been calculated
using the formula (see
[20, 21])
(2)ΔH=(2D2Hm)[1+16(Hm−8Am)29HiHm−64Am].
Here
(3)Hm=Ho−Am−[I(I+1)−m2]A22Ho.
From the powder spectrum of the mineral, the following
parameters have been calculated:White dolomite(4)g=1.992±0.015,A=9.15±0.43mT,D=34.45±0.36mT.Green dolomite(5)g=1.993±0.017,A=9.35±0.43mT,D=34.25±0.35mTThe observedg,A, and D values are
comparable with other similar systems and agree well within the experimental
error [8, 22–25]. The large value of D indicates a considerable amount of
distortion around the central metal ion. As EPR is highly sensitive to Mn(II)
impurity, four such sites have been noticed in the EPR spectrum. A close look at the EPR spectrum indicates a
nonzero value for E, which is very difficult to estimate from powder spectrum.
A close look at the centre of the EPR spectrum indicates a broad line
underneath the Mn(II) sextet. This can
be attributed to the iron impurity in the sample. Generally, Fe(III) and Fe(II) impurities will
give rise to a broad line around with a g value of 2. No extra information can be obtained from the
powder spectrum recorded at room temperature and low temperature except that
iron is present as an impurity. Even
though the percentage of iron is more than that of manganese, its intensity is
relatively smaller compared to the signals obtained from the manganese impurity. The EPR spectrum of white dolomite exhibits a
broad resonance centered when g is 4.20; similarly a weak resonance is observed
when g is 4.34 in the case of green dolomite.
Those are attributed to the presence of isolated Fe(III) ions
incorporated as an impurity in the mineral.The hyperfine constant ‘‘A’’ value
provides a qualitative measure of the ionic nature of bonding with Mn(II) ion.
The percentage of covalency of Mn-ligand bond in green dolomite has been
calculated using ‘‘A’’ (9.35 mT) value obtained from the EPR spectrum and Matumura’s
plot [26]. It corresponds to an ionicity of 94%, where, as in the case of white
dolomite, ‘‘A’’ with a value of 9.15 mT gives an ionicity of 91%. Also the approximate value of hyperfine
constant (A) is calculated for both dolomites by using covalency (C)
using the following equation (see [27, 28]):
(6)Aiso=(2.04C−104.5)10−4cm−1.The value obtained is92×10−4cm−1 for green dolomite, whereas for the white dolomite it is 91×10−4cm−1.
This calculated value agrees well with
the observed hyperfine constant for both samples indicating ionic character for
Mn–O bond in the mineral under study. The
number of ligands around Mn(II) ion is estimated using the covalency [27] equation for C(7)C=1n[1−0.16(Xp−Xq)−0.035(Xp−Xq)2].
Here
Xp
and Xq represent
electronagativities of metal and ligand.
Assuming Xp=XMn=1.4 and Xq=XO=3.5, the number of ligands (n) obtained are 20. This suggests that Mn(II) may
be surrounded by four CO32− and two oxygens. Further the g value for the
hyperfine splitting is indicative of the nature of bonding. If the g value shows negative shift with
respect to free electron g value of 2.0023, the bonding is ionic and
conversely, if the shift is positive, the bonding is more covalent in nature [29].
In the present work, from the observed negative values of 0.0103 for white dolomite
and that of −0.0093 for green dolomite, it is apparent that the Mn(II) is in an
ionic environment. EPR spectrum of white dolomite (Figure 2(b)) reveals that in
between first and second manganese hyperfine resonance lines a very weak sharp
peak at (319.2 mT) having a g value 2.1019 is observed. This can be ascribed to a radical such as CO32− or NO32−. The absence of any super hyperfine structure
in the radical suggests that the radical
may be CO32− [11].


## 3.3. Optical Absorption Spectral Analysis

Ferric
iron, Fe(III), has the electronic configuration Ar (3d5) with a half
filled d-shell having one unpaired electron in each of the orbitals. Hence ground state configuration is t2g3eg2.
The free ion levels of Fe(III), in the order of increase in energy, are S6,G4,P4,D4,F4. The energy
levels for Fe(III) in an octahedral environment are A1g6(S6), T1g4(G4), T2g4(G4), A1g4(G)-Eg4(G4), T2g4(D4),
and Eg4(D4). The A1g4(G)-Eg4(G4)
and Eg4(D4) levels have relatively less
influence compared to other levels by crystal field. It means that the relative sharp levels can
be expected in the absorption spectrum which is the criterion for assignment of
levels for Fe(III) ion. Since all the
excited states of Fe(III) ion will be either quartets or doublets, the optical
absorption spectra of Fe(III) ions will have only spin forbidden d-d
transitions. These occur from the ground state A1g6(S) to
the excited states T1g4(G), T2g4(G), A1g4(G), Eg4(G), T1g2(D), Eg4(D), and T1g4(P) of regular
octahedron [30].Optical absorption spectrum of green dolomite recorded in
the mull form at RT in the range of 200–1200 nm is shown
in Figure3 and that of white dolomite is shown in Figure 4. The green dolomite spectrum shows energies at
13160 cm−1 (760 nm), 13515 cm−1 (740 nm). 14285 cm−1 (700 nm), 14995 cm−1 (665 nm), and 19800 cm−1 (505 nm) cm−1 in the UV-Vis region whereas
white dolomite spectrum shows energies at 12660 cm−1 (790 nm), 13070 cm−1 (765 nm), 13425 cm−1 (745 nm), 22220 cm−1 (450 nm) cm−1 in the UV-Vis region. In green dolomite, the band observed at 13515 cm−1 with split component on left side at 13160 cm−1 is assigned to the transition A1g6(S)→T1g4(G) whereas the band at 14995 cm−1 is
assigned to T2g4(G) transition. The third band visible around 19800 cm−1 is assigned to A1g4(G),Eg4(G)
transition. These bands are characteristic
of Fe(III) ion in octahedral symmetry in the mineral. This is also further supported by EPR
studies. Using Tree’s polarization term α=90cm−1 [26], the energy matrices of the d5 configuration are solved for
various B,C, and Dq values. The
evaluated parameters are presented in Table 2. A comparison is also made between the
calculated and observed energies of the bands and these are presented in Table 2. The sharp band observed in green dolomite at 14285 cm−1 may be due
to Mn(II). The very sharp band observed
at 13070 cm−1 in white dolomite with split component on either side
(at 12660 and 13425 cm−1 with average of 13043 cm−1) is
assigned to the transition A1g6(S)→T1g4(G).
Similar assignment is also made in green dolomite. The intensity of the band in
white dolomite is large when compared to green dolomite. Further in white dolomite
the band visible around 16000 cm−1 due to the transition of A1g6(S)→T2g4(G) is not seen.
This may be due to low concentration of iron in the mineral. The third band at 22220 cm−1 is assigned
to A1g4(G),Eg4(G) transition. These bands are characteristic of Fe(III) ion
in octahedral symmetry in the mineral.
This is also further supported by EPR studies. Using Tree’s polarization
term α=90cm−1 [31], the energy matrices
of the d5 configuration are solved for various B,C, and Dq values. A comparison is also made between the
calculated and observed energies of the bands and these are presented in Table 2. These results agree well with the reported values
of different dolomite minerals
of Indian origin [12].Table 2
Band headed data with assignments for Fe(III) in natural dolomite mineral.
Dolomite greenDolomite whiteTransition fromA1g6Dq=900, B=605C=2600cm−1Dq=900, B=600C=2600cm−1α=90cm−1α=90cm−1Wave length (nm)Wavenumber (cm−1)Wave length (nm)Wavenumber (cm−1)ObservedCalculatedObservedCalculated79012660760131601316176513070130877401351574513425T1g4(G)70014285——6651499516951——16852T2g4(G)50519800208134502222020739Eg4(G)A1g4(G)Figure 3
Optical absorption spectrum of green dolomite at room temperature.Figure 4
Optical absorption spectrum of white dolomite at room temperature.


## 3.4. Near Infrared Spectroscopy

Water has three fundamental modes. They are symmetric OH stretch (ν1), H–O–H bending mode (ν2),
and asymmetric OH stretch (ν3). In solid phase, these modes appear at 3220 (ν3),
1620 (ν2),
and 3400 (ν1)
cm−1 [32]. The shifting of ν1 and ν3 towards lower frequency and ν2 towards higher frequency is due to hydrogen bonding [33].


### 3.4.1. The 11500 to 5500cm−1 Spectral Region

Figure5 shows near infrared spectrum of dolomite mineral
from 11500–5500 cm−1 region. The near-IR spectral regions may
be conveniently divided as follows: the high wave number region >7500cm−1: in this region, electronic bands due to the characteristic of Fe2+ and Fe3+ ions are observed. The two bands observed in green dolomite
at 9286 and 8581 cm−1 are broad and are of very low intensity
which are assigned to the two components of the transition T22g→E5g. This may be
due to trace of Fe(II) present in the sample. The average of these bands 8934 cm−1 is taken as 10Dq. But in white dolomite no Fe(II) bands are noticed.NIR spectrum of dolomite at room temperature from 11000–5500cm−1.The high wave number region between 7200cm−1 and 6300 cm−1 is attributed to the first overtone of the fundamental
hydroxyl stretching mode. The very sharp group of bands at 7127 and split
components on either side at 7237 and 7036 cm−1 with high intensity
are due to 2ν3, the asymmetric OH stretch in the green dolomite. Exactly the similar very
sharp group of bands observed at 7154 cm−1 with components at 7232, 7173,
and 6920 cm−1 with maximum intensity are assigned to the same 2ν3 asymmetric OH stretch in the white dolomite.


### 3.4.2. The 5500 to 4000cm−1 Spectral Region

The spectral region from 5500cm−1 to 4000 cm−1 is made up of two parts: the 4000–4500 cm−1 region in which OH combination bands are found and 4500–5500 cm−1 region in which water combination bands are observed. Figure 6 shows a profile of bands at 4300 cm−1 with high intensity and with components on either side at 4397 and 4123 cm−1. These bands in green dolomite are assigned
to the water OH combination. In white dolomite, similar bands are observed with
less intensity at 4293 cm−1 with a shoulder at 4388 cm−1 which
may be attributed to the water OH combination.
The intense band observed at 5083 cm−1 in green dolomite is
attributed to the water OH overtone. The
same band with very low intensity observed at 5078 cm−1 in white dolomite
is assigned to the water OH overtone. In
general, the NIR bands observed for white dolomite are less intense when
compared to green dolomite which indicates that water is more in green dolomite
than that present in white dolomite.NIR spectrum of dolomite at room temperature from 5500–4000cm−1.


## 3.4.1. The 11500 to 5500cm−1 Spectral Region

Figure5 shows near infrared spectrum of dolomite mineral
from 11500–5500 cm−1 region. The near-IR spectral regions may
be conveniently divided as follows: the high wave number region >7500cm−1: in this region, electronic bands due to the characteristic of Fe2+ and Fe3+ ions are observed. The two bands observed in green dolomite
at 9286 and 8581 cm−1 are broad and are of very low intensity
which are assigned to the two components of the transition T22g→E5g. This may be
due to trace of Fe(II) present in the sample. The average of these bands 8934 cm−1 is taken as 10Dq. But in white dolomite no Fe(II) bands are noticed.NIR spectrum of dolomite at room temperature from 11000–5500cm−1.The high wave number region between 7200cm−1 and 6300 cm−1 is attributed to the first overtone of the fundamental
hydroxyl stretching mode. The very sharp group of bands at 7127 and split
components on either side at 7237 and 7036 cm−1 with high intensity
are due to 2ν3, the asymmetric OH stretch in the green dolomite. Exactly the similar very
sharp group of bands observed at 7154 cm−1 with components at 7232, 7173,
and 6920 cm−1 with maximum intensity are assigned to the same 2ν3 asymmetric OH stretch in the white dolomite.


## 3.4.2. The 5500 to 4000cm−1 Spectral Region

The spectral region from 5500cm−1 to 4000 cm−1 is made up of two parts: the 4000–4500 cm−1 region in which OH combination bands are found and 4500–5500 cm−1 region in which water combination bands are observed. Figure 6 shows a profile of bands at 4300 cm−1 with high intensity and with components on either side at 4397 and 4123 cm−1. These bands in green dolomite are assigned
to the water OH combination. In white dolomite, similar bands are observed with
less intensity at 4293 cm−1 with a shoulder at 4388 cm−1 which
may be attributed to the water OH combination.
The intense band observed at 5083 cm−1 in green dolomite is
attributed to the water OH overtone. The
same band with very low intensity observed at 5078 cm−1 in white dolomite
is assigned to the water OH overtone. In
general, the NIR bands observed for white dolomite are less intense when
compared to green dolomite which indicates that water is more in green dolomite
than that present in white dolomite.NIR spectrum of dolomite at room temperature from 5500–4000cm−1.


## 4. Conclusion

EPR results indicate that Fe(III) and Mn(II) are present in the mineral in distorted octahedral environment. Theg value of 4.34 in green dolomite and that of 4.20 in white dolomite are ascribed to Fe(III). This supports that the sample contains isolated Fe(III) ions in the lattice of the dolomite mineral In both dolomites, the values of g,A, and D in the spectra are g=1.992±0.015, A=9.15±0.43mT, and D=34.45±0.36mT (white dolomite) and g=1.993±0.017, A=9.35±0.43mT, and D=34.25±0.35mT, respectively, and they are due to Mn(II) indicating distorted octahedral environment in the mineral. These results suggest that Mn(II) is replaced either by Ca or Mg site. Also white dolomite shows a free radical at g=2.1019 which is ascribed to CO32− which may be a geological indicator. Thus EPR is a very sensitive technique for the magnetic analysis of paramagnetic components in diamagnetic (carbonate) minerals.The optical absorption studies indicate that Fe(III) ion is present in major quantity in both dolomites whereas Fe(II) is in traces in the green dolomite mineral. These ions are also present in distorted octahedral environment. Green color of the mineral is due to Fe(II) which is evident from the optical absorption spectrum.Near-infrared spectrum is due to overtones and combinations of water molecules. Also green dolomite sample contains more lattice water than the white dolomite mineral.



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*Source: 175862-2009-01-28.xml*