A Vibrational Spectroscopic Study of the Silicate Mineral Kornerupine

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Frost, Ray, Lopez Toro, Andres, Xi, Yunfei, & Scholz, Ricardo (2015) A vibrational spectroscopic study of the silicate mineral kornerupine. Spectroscopy Letters, 48(7), pp. 487-491.

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Ray L. Frosta•, Andrés Lópes, a Yunfei Xia, Ricardo Scholzb

a School of Chemistry, Physics and Mechanical Engineering, Science and Engineering

Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001,

Australia.

b Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do

Cruzeiro, Ouro Preto, MG, 35,400-00, Brazil.

Abstract:

We have studied the mineral kornerupine a boro-silicate mineral by using a combination of scanning electron microscopy with energy dispersive analysis and Raman and infrared spectroscopy. Qualitative chemical analysis of kornerupine shows a magnesium-aluminium silicate. Strong Raman bands at 925, 995 and 1051 cm-1 with bands of lesser intensity at

1035 and 1084 cm-1 are assigned to the silicon-oxygen stretching vibrations of the siloxane units. Raman bands at 923 and 947 cm-1 are attributed to the symmetrical stretching vibrations of trigonal boron. Infrared spectra show greater complexity and the infrared bands are more difficult to assign. Two intense Raman bands at 3547 and 3612 cm-1 are assigned to the stretching vibrations of hydroxyl units. In the infrared bands are observed at 3544 and

3610 cm-1. Water is also identified in the spectra of kornerupine.

• Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1

Keywords: kornerupine, borate, silicate, Raman spectroscopy, infrared spectroscopy

Introduction

Kornerupine is a boro-silicate mineral of formulaMg3Al6(Si,Al,B)5O21(OH) [1]. The mineral is orthorhombic –dipyramidal [2-6] and is colourless to yellow, green and even brown; no doubt the colour is a function of the composition of the mineral sample [6, 7]. Kornerupine occurs in boron rich volcanic and sedimentary rocks which have undergone high grade metamorphism. It is also found in metamorphosed anorthosite complexes [8, 9]. The mineral is known from many parts of the world including Australia [8, 10-12]. The mineral is of importance because of its relatively rare indicator of deep crustal processes. The mineral is often associated with other high temperature phases including sillimanite, corundum, sapphirine and kyanite [13].

The mineral is cherished for his gem-like qualities and is sought after in the gem market [14,

15]. Kornerupine is noted for its wonderful emerald green color. The mineral shows pleochroic properties in other words the colour of the mineral is dependent upon the direction of the viewing angle. Kornerupine will never replace emerald as a jewel because of its rarity; mot only this but the green colour of kornerupine is very rare. The mineral kornerupine is more of a collectors mineral and gemstone. Techniques have been developed for the rapid identification of the mineral [16].

According to the original work of Moore [7] the mineral belongs to the space group Cmcm, with a = 16.100(2) Å, b = 13.767(2) Å, c = 6.735(2) Å with Z = 4. The unusual crystal structure includes walls of Al-O edge- and corner-sharing octahedra, and chains of alternating

Mg-O and Al-O octahedra fused to the walls by further edge-sharing to form dense slabs.

These slabs are held together by [Si2O7] corner-sharing tetrahedral pairs and [(Al,Si)2SiO10] corner-sharing tetrahedral triplets. The structure has refined by Hawthorne et al. [6].

The aim of this paper is to report the Raman spectra of well-defined natural kornerupine minerals, and to relate the spectra of this molecule to the crystal structure. This paper follows the systematic research of the large group of oxyanion containing minerals, and especially their molecular structure using vibrational spectroscopy.

Experimental

Samples description and preparation

The Kornerupine sample studied in this work forms part of the collection of the Geology

Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code

SAC-143. The studied sample is from the gem deposits of Mautia Hill, Tanzania. The kornerupine studied in this work occurs as a single crystal. Scanning electron microscopy

(SEM) in the energy dispersive spectroscopy (EDS) mode was applied to support the mineral characterization.

Scanning electron microscopy (SEM)

Experiments and analyses involving electron microscopy were performed in the Center of

Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais,

Brazil (http://www.microscopia.ufmg.br).

Kornerupine crystals were coated with a 5nm layer of evaporated carbon. Secondary Electron and Backscattering Electron images were obtained using a JEOL JSM-6360LV equipment.

Qualitative and semi-quantitative chemical analyses in the EDS mode were performed with a

ThermoNORAN spectrometer model Quest and was applied to support the mineral characterization.

Raman microprobe spectroscopy

Crystals of kornerupine were placed on a polished metal surface on the stage of an Olympus

BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a

Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range between

200 and 4000 cm-1. Repeated acquisitions on the crystals using the highest magnification

(50x) were accumulated to improve the signal to noise ratio of the spectra. Raman Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. The Raman spectrum of at least

10 crystals was collected to ensure the consistency of the spectra.

Clearly the crystals of Kornerupine are readily observed, with gem quality, making the

Raman spectroscopic measurements readily obtainable.

Infrared spectroscopy

Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000−525 cm-1 range were obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of

0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. The infrared spectra are given in the supplementary information.

Spectral manipulation such as baseline correction/adjustment and smoothing were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH,

USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package that 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 Lorentzian-Gaussian cross-product function with the minimum number of component bands used for the fitting process. The

Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995.

Results and discussion

Chemical characterization

The SEM/BSI image of the kornerupine crystal fragment studied in this work is shown in

Figure 1. A perfect cleavage can be observed. Qualitative chemical analysis of kornerupine shows a Mg/Al silicate (Figure 2). Boron was not detected as its measurement is below the limits of the instrumentation. Iron is known to substitute for Al in the kornerupine minerals

[9, 17]. Fe was not observed in the studied sample.

Vibrational Spectroscopy

The Raman spectrum of kornerupine in the 4000 to 100 cm-1 spectral range is illustrated in

Figure 3a. This spectrum displays the position and relative intensity of the Raman bands. It is noteworthy that there are large parts of the spectrum where no Raman intensity is observed.

Therefore, the spectrum is subdivided into sections based upon the type of vibration being analyzed. In a similar fashion, the infrared spectrum of kornerupine over the 4000 to 500 cm-

1 spectral range is reported in Figure 3b. This spectrum shows the position and relative

6 intensity of the infrared bands of kornerupine. A comparison between the Raman and infrared spectra may be made. Because of the lack of intensity in the infrared spectrum between for example 3000 to 2000 cm-1 spectral range, the spectrum is subdivided into sections based upon the type of vibration being studied.

The Raman spectrum of kornerupine in the 850 to 1100 cm-1 spectral range is reported in

Figure 4a. The infrared spectrum over the 650 to 1200 cm-1 spectral range is provided in

Figure 4b. What is readily observed is the complexity of the infrared spectrum when compared with the Raman spectrum, The infrared spectrum shows complexity with many overlapping bands when compared with the Raman spectrum which shows simplicity in this spectral range. Three Raman bands of significant intensity are observed at around 925, 995 and 1051 cm-1. The band at around 925 cm-1 may be resolved into two components at 923 and

947 cm-1. Bands of lesser intensity are noted at 1035 and 1084 cm-1. The band at 1051 cm-1 is assigned to the SiO stretching vibration of Si2O7 units. Dowty showed that the -SiO3 units had a unique band position of 980 cm-1 [18] (see Figures 2 and 4 of this reference). Dowty

-1 also showed that Si2O5 units had a Raman peak at around 1100 cm . The Raman band at

-1 995 cm is attributed to the SiO symmetric stretching mode of SiO5 units. There has been a

Raman study of kornerupine in which the spectra of a range of kornerupine minerals with various compositions were used [19]. In particular the mineral samples were selected for the variation in the boron composition [19]. Raman bands were observed at 803 and 884 cm-1.

The authors stated that the variation in these bands could be used to assess the composition of the kornerupine samples. No bands in this position were found in this work, although the two resolved component bands at 923 and 947 cm-1 may be attributed to the symmetrical stretching vibrations of trigonal boron.

The complexity of the infrared spectrum (Figure 4b) makes the assignment of the infrared bands quite difficult. The series of infrared bands observed at 1035, 1052, 1068, 1084, 1100,

1133, 1163 and 1177 cm-1 are associated with the SiO vibrations of the different siloxane units involved in the structure of kornerupine. The two infrared bands at 972 and 993 cm-1 are attributed to the antisymmetric stretching modes of tetrahedral boron. The infrared band at 893 cm-1 is assigned to a hydroxyl deformation mode. The infrared bands of lower intensity at 674, 726, 740 and 795 cm-1 are attributed to the bending modes of trigonal and tetrahedral boron.

The Raman spectrum of kornerupine in the 350 to 700 cm-1 and 100 to 350 cm-1 spectral ranges are shown in Figure 5. Dowty calculated the band position of these bending modes for different siloxane units [18]. Dowty demonstrated the band position of the bending

-1 modes for SiO3 units at around 650 cm . This calculated value is in harmony with the higher wavenumber band observed at 668 cm-1. Other lower intensity Raman bands at 554, 586, 648 and 668 are also assigned to the bending modes for SiO3 units.

The series of Raman bands at 355, 364, 394 and 403 cm-1 are assigned to metal oxygen stretching vibrational modes (Mg-O,Fe-O, Al-O). Other bands observed at 298, 316, 324 and

336 cm-1 may also be associated with metal-oxygen stretching bands (Figure 5b). An intense

Raman band is found at 180 cm-1. Other bands of lower intensity are found at 123, 138, 150,

219, 224, 236, 254 and 261 cm-1 and are simply assigned to lattice vibrations.

The Raman spectrum of kornerupine over the 3100 to 3700 cm-1 spectral range is illustrated in Figure 6a. Two intense Raman bands are observed at 3547 and 3612 cm-1 and are assigned

8 to the stretching vibrations of OH units. The observation of two bands supports the concept that there are two independent OH units in the structure of kornerupine. Bands of lower intensity are noted at 3521, 3538, 3556, 3599 and 3619 cm-1. These bands are also attributed to the stretching vibrations of the OH units. The broad Raman band at 3275 cm-1 is attributed to water in the structure of kornerupine.

The infrared spectrum of kornerupine over the 2400 to 3800 cm-1 spectral range is reported in

Figure 5b. Two very sharp infrared bands are observed at 3544 and 3610 cm-1; these bands are in a very similar positions to the bands observed in the Raman spectrum. These bands are assigned to the stretching vibration of the OH units. Two broad bands are found at 2880 and

3267 cm-1and are attributed to the water stretching bands. The presence of water is also observed in the infrared spectrum shown in Figure 7 as the strong infrared band at 1653 cm-1.

The two bands at 1496 and 1550 cm-1 are due to the antisymmetric stretching of trigonal boron.

Conclusions

The mineral kornerupine is a rare mineral noted for its colour variation including an emerald green colour. It is famous for its gem like qualities. The mineral is also noted for its pleochroic properties. It is a boro-silicate mineral of formula Mg3Al6(Si,Al,B)5O21(OH) has been analysed by using a combination of scanning electron microscopy with energy dispersive analysis and Raman and infrared spectroscopy. The mineral is fundamentally a silicate mineral of magnesium with some substitution by aluminium and boron.

Raman spectroscopy identifies stretching and bending vibrations of the Si2O5 and Si2O7 units.

Raman bands attributable to trigonal boron are observed. Infrared spectra show much greater

9 complexity making their assignment more difficult. Two Raman bands at 3547 and 3612 cm-

1 are assigned to OH stretching bands. Bands associated with water stretching vibrations are observed in the spectra of kornerupine. Vibrational spectroscopy enables aspects of the molecular structure of kornerupine to be ascertained.

Acknowledgements

The financial and infra-structure support of the Discipline of Nanotechnology and Molecular

Science, Science and Engineering Faculty of the Queensland University of Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. The authors would like to acknowledge the Center of Microscopy at the

Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for experiments involving electron microscopy.

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List of figures

Figure 1 - Backscattered electron image (BSI) of a Kornerupine single crystal up to 0.5 mm in length.

Figure 2 - Energy dispersive spectroscopy analysis of Kornerupine

Figure 3 (a) Raman spectrum of Kornerupine over the 100 to 4000 cm-1 spectral range

(b) Infrared spectrum of Kornerupine over the 500 to 4000 cm-1 spectral range

Figure 4(a) Raman spectrum of Kornerupine over the 850 to 1100 cm-1 spectral range

(b) Infrared spectrum of Kornerupine over the 650 to 1200 cm-1 spectral range

Figure 5 (a) Raman spectrum of Kornerupine over the 300 to 700 cm-1 spectral range

(b) Raman spectrum of Kornerupine over the 100 to 350 cm-1 spectral range

Figure 6 (a) Raman spectrum of Kornerupine over the 3100 to 3700 cm-1 spectral range

(b) Infrared spectrum of Kornerupine over the 2400 to 3800 cm-1 spectral range

Figure 7 Infrared spectrum of Kornerupine over the 1300 to 1900 cm-1 spectral range