Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

The molecular structure of chloritoid: A mid-infrared and near-infrared spectroscopic study Kuo Li a,b, Qinfu Liu a,⇑, Hongfei Cheng a,b,d, Yutao Deng c, Ray L. Frost d,⇑ a

School of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, PR China State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 100083, PR China c China Coal Geology Engineering Corporation, 100073 Beijing, PR China d School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Near-infrared spectroscopy was used

to study the structure of chloritoid.  The crystal structure of chloritoid was

proposed.  There is not exist water in the

chloritoid crystal structure.  There are two different octahedral

layers in the structure of chloritoid.

a r t i c l e

i n f o

Article history: Received 25 August 2014 Received in revised form 4 January 2015 Accepted 19 February 2015 Available online 7 March 2015 Keywords: Chloritoid Molecular structure Infrared spectroscopy

a b s t r a c t The mineral chloritoid collected from the argillite in the bottom of Yaopo Formation of Western Beijing was characterized by mid-infrared (MIR) and near-infrared (NIR) spectroscopy. The MIR spectra showed all fundamental vibrations including the hydroxyl units, basic aluminosilicate framework and the influence of iron on the chloritoid structure. The NIR spectrum of the chloritoid showed combination (m + d)OH bands with the fundamental stretching (m) and bending (d) vibrations. Based on the chemical component data and the analysis result from the MIR and NIR spectra, the crystal structure of chloritoid from western hills of Beijing, China, can be illustrated. Therefore, the application of the technique across the entire infrared region is expected to become more routine and extend its usefulness, and the reproducibility of measurement and richness of qualitative information should be simultaneously considered for proper selection of a spectroscopic method for the unit cell structural analysis. Ó 2015 Elsevier B.V. All rights reserved.

Introduction Chloritoid is a rock-forming mineral found in Al-rich metapelitic rocks. The mineral chloritoid was found in the bottom of Yaopo Formation of western hills of Beijing, China. Its chemical formula is ⇑ Corresponding authors. Fax: +86 10 62331825 (Q. Liu), +61 7 3138 2407 (R.L. Frost). E-mail addresses: [email protected] (Q. Liu), [email protected] (R.L. Frost). http://dx.doi.org/10.1016/j.saa.2015.02.091 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

(Fe2+ with unit cell parameters: 1.93Mn0.33Ti0.06)Al3.92Si2.11O12.18 a = 9.48728 Å, b = 5.43418 Å, c = 18.11236 Å, b = 102.0191, z = 4 (calculated in another paper) [1]. Different stacking polytypes of chloritoid occur in the nature, the two most abundant being are a two layer monoclinic one, 2M2, and a one-layer triclinic one, 1T, (Jefferson and Thomas 1978) [2]. The two polymorphs are related to each other in such a way that the monoclinic cell can essentially be considered to contain two triclinic cells [3]. Moreover, nonrandom stacking disorders in the structure of chloritoid can be

605

K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

clearly observed by High Resolution Electron Microscopic (HREM) [2]. Although a lot of work has been done to gain insight into the structure of chloritoid, there are still many open questions on its structure stability and crystal chemistry and few papers can be found about structure research of the mineral chloritoid. Vibrational spectroscopy is one of the most important tools for investigating the structure of layered materials. Especially, near-infrared (NIR) and mid-infrared (MIR) spectroscopy have been considered as an alternative analytical method because they are fast, non-destructive and non-pollution [4]. Mineral infrared spectrum produced by its molecular vibration can provide large information of chemical composition and structure. MIR had been used for identifying chloritoid, but few reports about using NIR spectroscopy to test this mineral can be found. However, NIR spectroscopy is such a powerful technique and is seriously underutilized in this regard. In the present work, MIR and NIR spectroscopy have been used to analysis for structural characteristics of the mineral chloritoid. The purpose is not only to demonstrate that the NIR region provides information comparable with that obtained by MIR, but also to reveal higher sensitivity of this spectral region to structure of minerals. Experimental methods Materials The sample used in this study was chloritoid collected from the argillite in the bottom of Yaopo Formation of Western Beijing. Chloritoid is in the shape of chrysanthemum-like, radial and bundle under polarized microscope. Its {0 0 1} cleavage and (0 1 0) partings are obvious, and the interference color is the first order gray under the cross-polarizer. The chemical composition of the mineral chloritoid, shown in Table 1, was analyzed by electron probe (Fig. 1 and Table 1). MIR spectroscopy MIR spectra were obtained in reflectance mode using a Nicolet Nexus 6700 Fourier transform infrared spectroscopy (FT-IR) spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000–400 cm 1 range were obtained by the co-addition of 64 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.

Fig. 1. Electron probe image of chloritoid.

package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was carried out using Peakfit software (Jandel Scientific, Postfatch 4107, D-40688 Erkrath, Germany). Lorentz–Gauss cross product functions were used throughout and Peakfit analysis undertaken until squared correlation coefficients with r2 > 0.998 were obtained. Results and discussion MIR spectroscopy Chloritoid has three different layers: the Al–O layer, silicon-oxygen tetrahedral layer and brucite-type layer. And the distance between hydrogen atoms and oxygen in the trioctahedral sheet are very close to form hydrogen bonding [5]. The wavelength and shape of the absorption band associated with (OH) in many substances appear to depend on the degree of interaction between the (OH) ion and neighbor O and (OH) ions [6]. The MIR spectra of chloritoid is shown in Figs. 2 and 3. For convenience, the MIR spectra of the sample are divided into two sections; they are (a) the 3850–2850 cm 1 region attributed to OH stretching vibration modes (Fig. 2) and (b) the 1750–650 cm 1 region due to the basic aluminosilicate framework and water molecule in the structure of these two samples (Fig. 3). 3850–2850 cm

1

region

NIR spectroscopy NIR spectra were collected in reflectance mode using a Nicolet Nexus FT-IR spectrometer with a Nicolet near-IR Fibreport accessory (Nicolet Nexus, Madison, Wisconsin, USA). A white light source was used, with a quartz beam splitter and TEC NIR InGaAs detector. Spectra were obtained from 10,000 to 4000 cm 1 (1000–2500 nm) by the co-addition of 64 scans at a resolution of 8 cm 1. A mirror velocity of 1.2659 m/s was used. The spectral manipulations of baseline adjustment, smoothing and normalization were performed using the Spectra calculation software

The infrared spectrum of chloritoid in 3850–2850 cm 1 region is shown in Fig. 2. Peaks present in this region are related to stretching vibrations of OH groups. The broad band at 3420 cm 1 support the concept of the non-equivalence of the OH units in the chloritoid structure [7]. The broad feature may be ascribed to water stretching modes; however, no water bending mode was observed in the infrared spectrum in the 1500–1800 cm 1 spectral range. Few reports can be found about the bands, 3675 and 3640 cm 1, the former band generated by Al2OH absorption and when a Al in the Al2OH was displaced by Fe3+ (forming

Table 1 Electron probe analysis results of chloritoid (xB/%). Samples

Na2O

MgO

Al2O3

SiO2

P2O5

K2O

CaO

TiO2

MnO

FeO

Total

1 2 3 Average

0.08 0.08 0.12 0.09

0.01 0.00 0.09 0.03

39.57 39.36 40.78 39.90

25.16 25.08 25.32 25.19

0.26 0.05 0.20 0.17

0.05

0.08

0.07 0.06

0.02 0.05

0.11 0.15 0.03 0.10

0.43 0.58 0.33 0.45

28.03 28.66 26.50 27.73

93.78 93.96 93.46 93.73

606

K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

Fig. 2. The 2850–3850 cm

Fig. 3. The 400–1750 cm

1

1

region of MIR spectra of chloritoid.

region of MIR spectra of chloritoid.

AlFe3+OH) the later band appear [8]. This can approve that part of Al substitute for Si4+ and Fe3+ substitute for Al in the crystal structure of chloritoid. The complex anion [AlO4]5 has more negative charge than [SiO4]4 , this can lead to more powerful hydrogen bond, therefore the mOH shift to low wavenumber. The weak band appear at 2937 cm 1 may be related with O[1A]–H[1B] stretching vibration [7]. 1750–400 cm

1

region

This area contains three kind of vibrational modes, the Si–O stretching vibration, the O–H oscillation and the Si–O bending vibration. The in-plane OH bending mode shift upward on H-bond formation due to an increase in the restoring force tending to keep the O–H bond directed towards the oxygen atom. Therefore, the dOH corresponding to the mOH at 2937 cm 1 occurs at 1122 cm 1 [7]. The spectrum in this region presents the band at 752 cm 1, which is typical of the OH translational vibrations [9]. The bands at 1050, 952 and 908 cm 1 are assigned to the m3–SiO4 antisymmetric stretching vibration. The strong bands at 998 cm 1 is

assigned to the Si–O–Si in-plane vibrations. The two sharp peak 1070 and 1050 cm 1 are due to the E mode, it shows that the E degenerate mode splits [8]. The band appeared at 950 cm 1 is assigned to Al2OH in-plane bending, while the weak peaks 836 and 815 cm 1 are assigned to Al2OH out-plane bending [8]. The low frequency bands at 606, 545 and 485 cm 1 are tentatively assigned to the triply degenerated SiO4 bond m4 and the band at 450 cm 1 to the doubly degenerated SiO4 bond m2. Band sat 420 cm 1 may be ascribed to OH translational vibrations. NIR spectroscopy NIR technique mainly measures overtones and combination bands of the fundamental vibration of O–H, Si–O and M–O bands in the mid-infrared region [10,11]. The analyses results of the MIR and NIR spectra are summarized in Table 2. For convenience, the NIR spectrum of chloritoid is divided into two sections depending upon the type of vibration being analyzed, they are (a) the 5000–4000 cm 1 region attributed to the combination of the stretching and deformation modes of the Al–OH units of chloritoid

K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

(Fig. 4); (b) the 7400–6000 cm 1 region attributed to the first overtone of the hydroxyl stretching modes (Fig. 5).

Table 2 Band component analysis of the MIR and NIR spectra of chloritoid. Frequency value cm 1

Suggested assignments

3420 3675, 3640 2937 1122 752 1050, 952, 908 998 950 836, 815 606, 545, 485 450 420 4180, 4296 4377 4562, 4559

Non-equivalence of the OH units Al2OH absorption O[1A]–H[1B] stretching vibration O[1A]–H[1B] bending vibration OH translational vibrations m3–SiO4 anti-symmetric stretching vibration Si–O–Si in-plane vibrations Al2OH out-plane bending Al2OH out-plane bending Triply degenerated SiO4 bond m4 Doubly degenerated SiO4 bond m2 OH translational vibrations (m + d)FeOH (m + d)MnOH Combination of mOH and m3–SiO4 anti-symmetric stretching vibration (m + d)AlOH Overtone of the OH stretching vibrations

4616 7173, 7104, 6965

607

5000–4000 cm

1

region

The NIR spectrum in the 5000–4000 cm 1 region of chloritoid was shown in Fig. 4. Spectra in this region are attributed to the combination bands of the stretching, bending and translation modes of the AlFe3+OH and FeFeOH units [11]. Here combination bands are observed. The two strong bands appeared at 4180, 4296 cm 1 are attributed to the combination of hydroxyl stretching and the bending modes of the FeFeOH unite. This can approve that another Al in the AlFe3+OH can be displaced by Fe (FeFe3+OH). The weaker band appeared at 4377 cm 1 may be relate to Mg–OH or Mn–OH, usually Mg–OH combination bands appear this area [12], but Mg percentage composition is far below Mn percentage composition (Table 1), thus the authors believe that the weak band is attribute to Mn–OH. The bands appeared at 4562 and 4559 cm 1 are assigned to the combination for the hydroxyls whose coupled

Fig. 4. The 4000–5000 cm

1

region of NIR spectra of chloritoid.

Fig. 5. The 6600–7400 cm

1

region of NIR spectra of chloritoid.

608

K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

Fig. 6. Schematic representation of chloritoid structure projected on the (0 1 0) PLANE.

stretching and bending modes gave rise to bands at 3695– 3620 cm 1 and near 908 cm 1 [13]. The band at 4616 cm 1 may be related to (m + d)AlOH combination modes, this is the result of the combination of the Al2OH stretching and deformation frequencies, which occur at higher wavenumbers than that of the FeFeOH vibrations. 7400–6600 cm

1

region

The 7400–6600 cm 1 NIR spectral region is shown in Fig. 5. The NIR spectral region between 6000 and 8000 cm 1 is the region where the first fundamental overtone of the OH stretching bands in clay minerals [14]. The bands observed at 7173 and 7104 cm 1 are assigned to the first fundamental overtone of the OH stretching vibrations at 3467, 3640 and 3675 cm 1. The band appeared at 7173 cm 1 is assigned to the Al2OH stretching vibration at 3675 cm 1 combination of O(1B)–H(1B) stretching at 3467 cm 1, while the band at 7173 cm 1 is assigned to the AlFe3+OH stretching vibration at 3640 cm 1 combination of O(1B)–H(1B) stretching at 3467 cm 1. The band observed at 6965 cm 1 is assigned to the first overtone of the FeFeOH stretching vibration. The infrared features in this region are similar with ferruginous smectite, except for the 6840 cm 1 band, which is considered that this band is due to the overtone of the hydroxyl stretching frequencies of water coordinated to the clay mineral. Therefore, it can prove that there is no water in the chloritoid crystal structure. Obviously, the exist of iron reduce the overtone frequency of hydroxyl. The band positions of chlorites vary from chamosite at 7102 cm 1 to ripidolite, then sheridanite to clinochlore and penninite at 7205 cm 1, the wavenumber changes significantly with Fe content [12]. Another mineral is nontronites whose principle band is at 6965 cm 1 whereas the ferruginous smectites are characterised by an intense band at 7070 cm 1 [11]. Given the content of iron (Table 2 and Fig. 5), this kind mineral would be more accurately to be named ferruginous chloritoid. Thus, infrared spectroscopy combined with XRD procedures can be more effective to identify iron-bearing minerals. Structure The structure of natural chloritoid has been investigated by Brindley & Harrison (1952, 1957), Hanscom (1975) and Jefferson & Thomas (1978) et al. [2,5,6]. Chloritoid is one kind of layered silicates, its octahedral sheets parallel (0 0 1) plane connected by isolated tetrahedral [15]. There are two different octahedral layers, layer 1 is a trioctahedral sheet contains two sites of substitution in the structure, M(1A) and M(1B), layer 2 is corundum-type composed by Al6O16 (Brindley and Harrison 1952) [16] in which each octahedron shares four edges with adjacent octahedral, and three out of every four octahedral sites are occupied.

Structural refinement of a monoclinic chloritoid by Hanscom has revealed a more detailed picture of the internal structure of the two different octahedral layers of the mineral [3]. There exist two different octahedral in the brucite-type layer, M(1A) and M(2B), which are coordinated by 4OH and 2O2 ligands in a trans and cis configuration. Based on X-ray diffraction calculations, Fe3+ substitutes for Al in M(1A) and that Mn2+ substitute for Fe2+ in M(1B). The MIR and NIR spectroscopic in this paper also prove the substitution in the chloritoid structure. In the corundum-type layer both cations sites, M(2A) and M(2B), are believed to be fully occupied by aluminum, and having only O2 ligands. Based on the chemical component data and the analysis result from the MIR and NIR spectrum, the crystal structure of chloritoid from western hills of Beijing, China can be illustrated (Fig. 6). Layer 1 is not a typical brucite-type layer; the main cations positioned in the octahedron will be Fe2+ and Mn2+ more than mg2+ from the electron probe data (Fig. 1 and Table 1). Layer 2 is an Al–O corundum-type layer; the aluminum ion is coordination with six oxygens. These two kinds of octahedron are connected by isolated tetrahedron. To equalize the valence, part of Fe3+ and Al3+ may substitute Si4+ in the tetrahedron. Hydrogen atoms are considered to be closely associated with the oxygen in the trioctahedral sheet that is not bonded to silicon [7]. The interlayer distance in monoclinic chloritoid is conducive to the formation of hydrogen bands between these hydroxyl groups and oxygen in adjacent layers. The structure of triclinic polymorph 1Tc is very similar to the monoclinic 2M2 structure. The triclinic structure contains only one of each octahedral layer type in the unit cell, the c period in the triclinic polymorph is half as long as in the monoclinic form. The reduction in symmetry from monoclinic to triclinic is mainly caused by displacements in the layer 2. The M2B site is located on two special positions in the triclinic structure. Conclusions The combination of MIR and NIR spectroscopic investigations has proven to be very informative in the determination of the chloritoid structure and composition. MIR spectra provide greater spectral features and better spectral resolution, and can be effectively used to analyze the OH stretching vibration. The band 3640 cm 1 show that Fe3+ substituted for Al in the Al2OH, and the basic structural vibration in chloritoid suggest there exist E mode split. The trioctahedron sheet is not a typical brucite-type layer; the main cations positioned in the octahedron are Fe2+ and Mn2+ more than mg2+. There is not exist water in the chloritoid crystal structure convinced by NIR spectroscopy date. The NIR spectroscopy of chloritoid offers some bands that used to verification our previous results from MIR spectra and other equipment. Therefore, the full spectra range including MIR and NIR

K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 604–609

will provide the best results rather than only the specific spectral regions. Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (51034006) and Beijing Joint Project of Beijing (20121141301) and Fourier Transform Infrared Spectrometer support by the State Key Laboratory for Coal Resources & Safe Mining, China University of Mining & Technology. References [1] Kuo Li, Qin-fu Liu, Mineralogical study on chloritoid from the Jurassic coal measures in western hills of Beijing, J. Hunan Univ. Sci. Technol. (Nat. Sci. Ed.) 29 (2014) 94–98. [2] J.A. Thomas, High resolution electron microscopic and X-ray studies of nonrandom disorder in an unusual layered silicate (chloritoid), Math. Phys. Eng. Sci. A 361 (1978) 399–411. [3] R. Hanscom, The structure of triclinic chloritoid and chloritoid polymorphism, Am. Mineral. 65 (1980) 534–539. [4] H. Chung, M.-S. Ku, J.S. Lee, Comparison of near-infrared and mid-infrared spectroscopy for the determination of distillation property of kerosene, Vib. Spectrosc. 20 (1999) 155–163.

609

[5] R.H. Hanscom, Refinement of the crystal structure of monoclinic chloritoid, Acta Cryst. B31 (1975) 780–784. [6] F.W. Harrison, The crystal structure of chloritoid, Acta Cryst. 10 (1957) 77–82. [7] E. De Grave, R. Vanleerberghe, L. Verdonck, G. De Geyter, Mossbauer and infrared spectroscopic studies of belgian chloritoids, Phys. Chem. Miner. 11 (1984) 85–94. [8] Lu Wen, Liang Wanxue, Zhang Zhenggang, The Infrared Spectroscopy of Minerals, Chongqing University Press, 1988. [9] Hongfei Cheng, Qinfu Liu, Jing Yang, Jinshan Zhang, R.L. Frost, J. Mol. Struct. 990 (2011) 21–25. [10] Q. Zhou, Y. Xi, H. He, R.L. Frost, Application of near infrared spectroscopy for the determination of adsorbed p-nitrophenol on HDTMA organoclay— implications for the removal of organic pollutants from water, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 69 (2008) 835–841. [11] Ray L. Frost, J. Theo Kloprogge, Zhe Ding, Near-infrared spectroscopic study of nontronites and ferruginous smectite, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 58 (2002) 1657–1668. [12] J.L. Post, S.M. Crawford, Uses of near-infared spectra for the identification of clay minerals, Appl. Clay Sci. 95 (2014) 383–387. [13] M. Castellano, A. Turturro, P. Riani, T. Montanari, E. Finocchio, G. Ramis, G. Busca, Bulk and surface properties of commercial kaolins, Appl. Clay Sci. 48 (2010) 446–454. [14] Hongfei Cheng, Jing Yang, Qinfu Liu, Jinshan Zhang, Ray L. Frost, A spectroscopic comparison of selected Chinese kaolinite, coal bearing kaolinite and halloysite—A mid-infrared and near-infrared study, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 77 (2010) 856–861. [15] Ulf Halenius, Hans Annersten, Klaus Langer, Spectroscopic studies on natural chloritoids, Phys. Chem. Miner. 7 (1981) 117–123. [16] G.W. Brindley, F.W. Harrison, The structure of chloritoid, Acta Cryst. 5 (1952) 698.