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Reddy, B. Jagannadha & Frost, Ray L. (2004) Electronic and vibrational spectra of gaspeite. Neues Jahrbuch fuer Mineralogie. Monatshefte, 2004(11), pp. 525-536.

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Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reﬂected in this document. For a deﬁnitive version of this work, please refer to the published source: http://dx.doi.org/10.1127/0028-3649/2004/2004-0525 ELECTRONIC AND VIBRATIONAL SPECTRA OF GASPEITE

B. J. Reddy and R. L. Frost •••

Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, Brisbane, GPO Box 2434, Queensland 4001, Australia

Abstract: Visible, nearinfrared, IR and Raman spectra of magnesian gaspeite are presented along with explanations of spectral features. Of the cations, nickel ion is main source for the electronic features as a principal constituent in the mineral where as the bands in IR and Raman spectra are due to the vibrational processes in the carbonate ion as an entity. The combination of electronic absorption spectra and vibrational spectra (including nearinfrared, FTIR and Raman) of magnesian gaspeite

2 are presented and explained the cation coordination and the behaviour of CO 3 anion in the NiMg carbonate. The electronic absorption spectrum consists of three broad and intense bands at 8130, 13160 and 22730 cm1 due to spinallowed transitions and two weak bands at 20410 and 30300 cm 1 are assigned to spin forbidden transitions of Ni 2+ in an octahedral symmetry. The splitting of the spin allowed bands around 14000 and 8000 cm 1 may be attributed to distortion of the cation from idealised symmetry. The crystal field parameters evaluated from the observed bands are Dq = 810; B = 800 and C = 3200 cm 1.

The two bands in the nearinfrared spectrum at 4330 and 5130 cm 1 are overtone and combination of CO 3 vibrational modes. For the carbonate group, infrared bands

1 1 1 1 are observed at 1020 cm (ν1), 870 cm (ν2), 1418 cm (ν3) and 750 and 673 cm

(ν4); of which ν3, the asymmetric stretching mode is most intense. Three well

1 resolved Raman bands at 1571, 1088 and 331 cm are assigned to ν3, ν1 and ν4

• Author to whom correspondence should be addressed ( [email protected] ) P: +61 7 3864 2407 F: +61 7 3864 1804

1 vibrational modes respectively. The weak band at 739 cm is due to ν2 mode. While

ν3 and ν4 modes showed components. This complexity of Raman bands results from the loss of degeneracy of the CO stretching modes. The two Raman bands 1571 and

1088 cm 1 are in increasing order due to increase of magnesium concentration when compared to calcite. The substitution of Mg by Ni is reflected by Raman shifts especially ν2 and ν4 modes to lower frequencies. The splitting of the spin allowed bands and lowering of crystal field parameters observed in the electronic spectrum confirm the distorted octahedral symmetry for Ni 2+ ion in the magnesian gaspeite.

2 Introduction

The structure of calcite is descriptively fairly simple, and it is same as that taken by several other anhydrous, single carbonates including magnesite (MgCO 3), siderite

(FeCO 3), rhodochrosite (MnCO 3), otavite (CdCO 3), smithsonite (ZnCO 3), sphaerocobaltite (CoCO 3), and gaspeite (NiCO 3) (REEDER 1983). The calcitestructure carbonates (space group R3c) represent a mineral group that is structurally simple and different from oxides and silicates; the slightly distorted octahedra are exclusively cornerlinked through shared oxygen anions of CO 3 groups, which are extremely incompressible structural units.

Calcitestructure carbonates exist for divalent cations with ionic radii ranging from

0.69 to 1.0 Å. Since ionic radii for Ni 2+ (0.69 Å) and Mg 2+ (0.72 Å) are very similar, there is a strong indication that other factors associated with electronic configuration play a major role.

GAINS & GOLDSMITH 1971 suggested that substitution of Ni 2+ for Mg 2+ (or vice versa) might distort the octahedral coordination sufficiently so as to reduce the

large crystal field stabilization energy (CFSE) for Ni 2+ . In octahedral coordination,

Ni 2+ has a maximum CFSE in a regular octahedral environment; site distortion does not result in addition stabilization (BURNS 1970).

However, EFFENBERGER et al. 1981 have estimated both the magnitude of

octahedral distortion and the MO bond length for NiCO 3 based on lattice parameters

given by GRAF 1961. Their results suggest that both these parameters are somewhat

less than in magnitude, although not greatly different; the octahedral site in magnesite is only marginally distorted.

3 IR and Raman studies on aragonite group minerals extensively reported (GRIFFITH 1970, FRECH et al. 1980, MARTENS et al. 2002). KOHLS & RODDA (1966) reported the infrared spectra of gaspeite. Correlations between the strength of the cation bond with the radicals and the position of the maximum in the absorption bands were observed. The absorption bands shifted towards the longer wavelength region with increase of bond strength. In this work we study the effect of cation on the carbonate bands of gaspeite.

The crystal structure, chemical, infrared, thermal and pertrograph analysss of the sample, (magnesian gaspeite (Ni,Mg,Fe)(CO 3) originated from Gaspe Peninsula, Quebec, Canada) were reported by KOHLS & RODDA 1966. The new carbonate mineral belongs to rhombohedral system with cell constants, a = 4.62 Å and c =14.93 Å. The chemical analysis of this magnesian gaspeite sample reprted as

35.0% NiO, 16.4% MgO, 5.7%FeO, and 42.0% CO 2. Three infrared absorption bands for this mineral were reported by KOHLS & RODDA 1966.

The present experimental results demonstrate that crystalfield effects can account for the electronic structure of the magnesian gaspeite. We present here for first time

the combination of electronic absorption spectra and vibrational spectra (near infrared,

FTIR and Raman) on the same sample of magnesian gaspeite and explain the cation coordination and

2 the behaviour of CO 3 anion in the NiMg carbonate.

Experimental

Sample preparation

A light green coloured mineral magnesion gaspeiete procured from the Museum ,

Department of Mineralogy which originated from the Gaspe Peninsula, Quebec

Canada, designated as magnesian gaspeite was employed in the present investigations.

As the sample was massive, it was made into fine powder and used for spectroscopic characterization. This sample is the same sample studied by previous workers.

4 Electronic spectrum

The electronic spectrum of the sample was recorded in the region 2002000 nm on Varian Cary 5E UVVISNIR spectrophotometer at a scanning speed of 600 nm /min. It has a spectral width of 2 nm and the data interval is 1 nm.This technique allows the recording the electronic reflectance spectrum. The sample is in a powder form. Only a few grams are required. The spectrum was transformed according to the Kubleka Munk algorithm.

Infrared and near-infrared spectroscopy (NIR)

Infrared spectrum was obtained using a Nicolet Nexus 870 FTIR spectrometer with a

Smart Endurance single bounce diamond ATR cell. The spectrum was obtained over the 4000500 cm 1 region by the coaddition of 64 scans with a resolution of 4 cm 1 and a mirror velocity of 0.6329 cm/s. NIR spectrum was collected on a Nicolet Nexus

FTIR spectrometer with a Nicolet NearIR Fibreport accessory. A white light source was employed with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 11000 to 4000 cm 1 by the coaddition of 64 scans at the resolution of 8 cm 1. A mirror velocity of 1.2659 was used. The spectrum was transformed using the KubelkaMunk algorithm to provide spectrum for comparison with that of absorption spectra.

Spectral manipulations such as baseline adjustment, smoothening and normalisation were performed using the Spectracalc software package GRAMS

(Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel ‘peakfit’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a LorentzGauss crossproduct function with the minimum number of component bands used for the fitting process. The Gauss

Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than

0.995.

Raman microprobe spectroscopy

The powdered crystals of gaspeite were placed and oriented on the stage of an

Olympus BHSM microsope, equipped with 10x and 50x objectives and part of a

Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a Charge coupled Device (CCD). Raman spectra were excited by a

HeNe laser (633 nm) at a resolution of 2 cm 1 in the range between 100 and 4000 cm 1.

Repeated acquisition using the highest magnification was accumulated to improve the signal to noise ratio. Spectra were calibrated using the 520.5 cm 1 line of silicon wafer.

In order to ensure that the correct spectra are obtained, the incident excitation light was scrambled. Details of the experimental technique have already been reported

(FROST &WEIER 2003, FROST et al. 2003, FROST & WEIER 2003, FROST &

WEIER 2004).

Results and discussion

Electronic absorption spectrum

Nickel is an infrequent contributor to spectra of minerals. Usually Ni 2+ present in an octahedral coordination and gives rise to a green colour to the mineral (FAYE 1974). Some of the minerals were studied to elucidate the relationship between crystal field splitting of Ni 2+ and site distortion (REDDY et al. 1987, REDDY et al. 2000, ROSSMANN et al. 1981, SARMA et al. 1983, VEDANAND et al. 1994, WOOD 1974). In an octahedral environment, Ni 2+ ion gives rise to eight transitions of which, three spinallowed (∆ s = 0) and five spinforbidden ((∆ s ≠ 0). Spinallowed bands are much more intense than spinforbidden bands since transitions are between the states of same multiplicity in the former one where as in the later case transitions are between the states of different multiplicity. So, it is expected that three intense bands and five weak bands for Ni 2+ in octahedral symmetry. The energies of these bands are represented by Tanabe and Sugano diagram drawn for d 8 configuration (TANABE & SUGANO 1954). Accordingly, three broad and intense bands at 8130, 13160 and 22730 cm 1 identified as spin allowed bands from the electronic absorption spectrum 3 3 of gaspeite depicted in Fig. 1 are assigned to the transitions, A2g (F) → T2g (F),

6 3 3 3 3 A2g (F) → T1g (F) and A2g (F) → T1g (P). The other two week band observed at 1 3 1 3 1 20410 and 30300 cm are assigned to A2g (F) → T2g (D) and A2g (F) → Eg(D) spin forbidden transitions. The values of Dq and B are determined from the observed energies of the spin allowed bands (WOOD 1974).

3 3 A2g (F) → T2g (F) = 10Dq = υ1

3 3 1/2 A2g (F) → T1g (F) = 15Dq + 7.5B6B(1 + ) = υ2

3 3 1/2 A2g (F) → T1g (P) = 15Dq + 7.5B+6B(1 + ) = υ3 where = [(10Dq – 9B) / 12B] 2

When Dq and B are of similar magnitude, is obtained as of the order of 0.1 and may be neglected. The value of B is calculated from υ3 υ2 = 12B. Thus the approximate values of the two parameters are evaluated, Dq =813 and B = 797 cm 1. Using these values, the energy matrices of d 8 configuration are solved for different values of Dq, B and C. There is no significant contribution by iron present as an impurity. Thus the all the bands are due to nickel ion. The assignments of the bands are given in Table 1. There is a close agreement between observed and calculated energies of the bands with the crystal field parameter, Dq = 810, Racah parameter B = 800 and C = 3200 cm 1. If one studies the spectrum of gaspeite, one can find the split component at 1 3 3 14705 for the ν2 broad band at13160 cm , assigned to A2g (F) → T1g (F). Thus it seems reasonable to consider the structure to be distorted octahedral symmetry. This type of splitting was observed in several nickel complexes (ELBINDARY & EL SONBATI 2000, LEVER 1968, SARMA et al.1983). Considering the splitting of the octahedral states due to lowering of symmetry to tetragonal field (D 4h ), the two bands 3 3 1 3 3 1 are attributed to B1g → A2g (13160 cm ), B1g → Eg (14705 cm ). The crystal field parameters for different nickel minerals are presented along with the present study. There is a marked difference of Dq value in the case of carbonate minerals when compared with other minerals. This supports further that crystal field stabilisation energy (CFSE) which depends on Dq value which is lowered in MgNi complexes due to distorted octahedral coordination of Ni 2+ ion (GAINS & GOLDSMITH 1971).

Near-infrared (NIR Spectroscopy

Features in the visible and nearinfrared regions where bands due to overtone and or combination tones of the anion and bands due to electronic transitions in the cation also occur (HUNT & SALISBURY 1971). From the nature and position of the bands it can readily be distinguished between the electronic and vibrational spectra.

Features in the visible and nearinfrared spectra of the carbonate mineral are due to the result of the electronic process in the cation and or vibrational process in the carbonate anion. The bands occur at longer wavelengths beyond ~ 1300 nm and are

7 of vibrational origin and are due to overtone and combination tones of water or carbonate ion. The nearIR spectrum of the sample shown in Fig. 2 consists of a typical broad band around 8000 cm 1 split up into two bands with band maxima at

7714 and 8685 cm 1 have halfwidths 1355 and 1841 cm 1 respectively. This type of splitting indicates distortion of the regular octahedral symmetry of Ni 2+ in a compelx. This feature is more significantly appeared here in NIR spectrum than in the

UVVISNIR region of the spectrum (Fig. 1) since the NIR spectrophotometer is much more sensitive in this region and hence it is identified as 10 Dq band for Ni 2+

3 3 assigned to spin allowed transition, A2g (F) → T2g (F). The other two weak bands at

4330 and 5130 cm 1 are of vibrational origin. The carbonates usually display a series of five distinctly characteristic bands in the nearinfrared (HUNT & SALISBURY

1971). These bands can be assigned in terms of overtone and combinations of fundamental modes of carbonate ion. In the present case the two bands appeared on shorter wavelength side of the spectrum. Using the Raman data for ν1, observed (1088

1 1 cm ) and ν3 from the IR spectrum observed at 1418 cm , the bands at 4330 and 5130

1 cm are assigned to 3υ3 and combinational modes of 2ν1 +2ν3 respectively and agree

1 well with the calculated values; 3ν3 = 3 x 1418 (4254 cm ) and (2ν1 +2ν3) = 2 x 1088

+ 2 x 1418) = (5012 cm 1).

Infrared (IR) and Raman Spectroscopy

2 The free ion, CO 3 with D 4h symmetry exhibits four normal vibrational modes; a symmetric stretching vibration (ν1), an outofplane bend (ν2), a doubly degenerate asymmetric stretch (ν3) and another doubly degenerate bending mode (ν4). The symmetries of these modes are A1´ (R) + A 2´´ (IR) + E´ (R, IR) + E´´ (R, IR) and occur at 1063, 879, 1415 and 680 cm 1 respectively (HERZBERG 1945).

Generally, strong Raman modes appear around 1100 cm1 due to the symmetric stretching vibration (ν1), of the carbonate groups, while intense IR and weak Raman

1 peaks near 1400 cm are due to the asymmetric stretch (ν3). Infrared modes near 800

1 cm are derived from the outofplane bend (ν2). Infrared and Raman modes around

1 700 cm region are due to the inplane bending mode (ν4). This mode is doubly

2 degenerate for undistorted CO 3 groups. As the carbonate groups become distorted from regular planar symmetry, this mode splits into two components.. Infrared and

2 Raman spectroscopy provide sensitive test for structural distortion of CO 3

(SCHEETZ & WHITE 1977, BISCHOFF et al. 1985).

For the analyses of vibrational spectra of gaspeite mineral, as a convenience both, infrared absorption and Raman spectra are shown in the Fig. 3 one over the other in the region of 200 1700 cm 1 where all vibrational modes could be indexed and

2 1 characterised for CO 3 groups. The most intense band (1088 cm ) in the Raman spectrum is identified as ν1, the symmetric stretching mode, where as the most intense

1 band at 1418 cm with maximum halfwidth in IR is due to ν3, the antisymmetric stretching mode. The second intense band at 870 cm 1 in the IR spectrum is attributed to outofplane bending mode ν2 and the sharp line like band on the lower frequency

1 side, 750 cm is due to inplane bending mode (ν4). The CO symmetric stretching mode is also activated in the IR spectrum at 1020 cm 1. In the case of inplane bending region of the Raman spectrum, one sharp band that appear at 331 cm 1 with

1 two split components at 223 and 193 cm due to υ4 while less intense band at 739 cm 1 with two broad bands on its either side at 850 and 500 cm 1 and on the higher frequency side, the band at 1571 cm 1 with two components ( 1467 & 1422 cm 1) are

1 assigned to ν2 and ν3 modes respectively. The band at 1734 cm is assigned to the

1 overtone of the vibrational mode, 2ν2 ( 2x 870 cm ). This complexity of Raman bands results from the loss of degeneracy of the CO stretching modes. Such type of spectral features was observed in azurite (FROST et al. 2002).

9 The behaviour of carbonate ion both in synthetic and biogenic magnesian calcites was studied by means of Raman spectra (BISCHOFF et al. 1985) and showed that half widths of the Raman bands increase smoothly with magnesium concentration in synthetic phases. Bands in the spectra of biogenic magnesian calcites generally show an increase in frequency with increasing magnesium concentration. Accordingly, the Raman bands observed in magnesian gaspeite are in increasing order when compared to calcite. Raman shifts are mostly controlled by crystal field: significant changes in CO bond lengths are not seen in rhombohedral carbonates (EFFENBERGER ET AL. 1981, REEDER 1983), where as significant differences in Raman frequencies are observed as a function of cation size (WHITE 1974, RUTT & NICOLA 1974). When compared with calcites, the Raman spectrum of magnesian gaspeite shows anomaly for ν4, inplane bending mode observed in low frequency side here at 331

1 1 cm with components at 223 and 193 cm and also the ν3 mode showed a significant band at 1571 cm 1 with two components at 1467 and 1422 cm 1. This complexity behaviour is accounted for the symmetry of cation in the gaspeite mineral (FROST et al. 2002). Thus the effect of crystal field showed significantly in the electronic spectrum due to distorted octahedral site for Ni 2+ ion. Thus the effect of crystal field is significant in the electronic spectrum.

Conclusions

The application of NIR spectroscopy along with the combination of FTIR and Raman spectroscopy leads to a precise characterisation of minerals. The study of carbonate minerals is of interest because of its relevance to the existence of carbonate minerals in planetary materials or on Planets. The shifting of inplane bending mode, the ν4 group of bands to low frequency side and splitting of ν3 vibrational mode into a number of components at ~ 1500 cm 1 in the Raman spectrum confirms the loss of symmetry of the carbonate ion. The low values of crystal field parameters, Dq and B and the splitting of the two spinallowed bands around 14000 and 8000 cm 1 in the electronic absorption spectrum confirm distortion of octahedral coordination for Ni 2+ ion in the complex.

Acknowledgements

The financial and infrastructure support of the Queensland University of Technology (QUT), Inorganic Materials Research Program is gratefully acknowledged. We thank the Australian Research council for funding the research work. One of the authors (B. J. Reddy) is grateful to the QUT for the award of Visiting Fellowship. We thank Fernando Nieto and Antonio Sanchez for providing the sample and UVVISNIR recordings of the sample at the Scientific Instrumentation Centre, University of

10 Granada, Granada, Spain. The keen interest shown in this work by Theo Kloprogge is very much appreciated.

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Table1. Observed and calculated energies of the bands with their assignments for

Ni 2+ in gaspeite.

______

Observed Calculated Transition

(Cm 1) (Cm 1) ______

3 3 8130 8100 A2g (F) → T2g (F)

3 3 13160 13329 A2g (F) → T1g (F) 14705 3 1 20410 20108 A2g (F) → T2g (D) 3 3 22730 22971 A2g (F) → T1g (P) 3 1 30300 30592 A2g (F) → T2(G) ______

Table 2. Crystal field parameters for Ni 2+ in octahedral environment of different complexes.

______

Complex Dq B Reference (Cm1) (Cm1)

______

Olivine 870 900 WOOD 1974 [(MgNi)SiO 4]

Garnierite 910 947 FAYE 1974 [(Mg 3(OH) 4Si 2O5)

Ullmannite 860 840 REDDY et al. 2000 [(Ni 4Sb 4S4)]

Polydymite 900 840 VEDANAND et al. 1994 [(Ni 3S4)]

Zaratite 820 899 SARMA et al. 1983 [(Ni 3CO 3(OH)4.4H2O)]

Gaspeite 810 800 PRESENT WORK [(Ni, Mg,Fe)(CO 3)] ______

15 Figure 1

16 Figure 2

0.0005

0.00045 Gaspeite

0.0004

0.00035 8685

0.0003 7714

0.00025 Intensity 0.0002

0.00015

0.0001 5130 4330

0.00005

0 11000 10000 9000 8000 7000 6000 5000 4000 Wavenumber/cm -1

17 Figure 3

0.01

Gaspeite ννν1 0.009 Raman 0.008

0.007 ννν3

0.006

ννν4 ννν2 20.005ννν2

Intensity 0.004

ννν3 0.003 ννν4 Infrared

0.002 ννν1 ννν2

0.001

0 1700 1200 700 200 Wavenumber/cm -1

18 List of Figures

Figure 1 Electronic absorption spectrum of Ni 2+ in gaspeite.

Figure 2 NIR reflectance spectrum of gaspeite.

Figure 3 Raman and infrared spectra of gaspeite.