Raman and Infrared Spectroscopy of Arsenates of the Roselite and Fairfeldite Mineral Subgroups

This may be the author’s version of a work that was submitted/accepted for publication in the following source: Frost, Ray (2009) Raman and infrared spectroscopy of arsenates of the roselite and fairfeldite mineral subgroups. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 71(5), pp. 1788-1794. This ﬁle was downloaded from: https://eprints.qut.edu.au/17596/ c Copyright 2009 Elsevier Reproduced in accordance with the copyright policy of the publisher. Notice: Please note that this document may not be the Version of Record (i.e. published version) of the work. Author manuscript versions (as Sub- mitted for peer review or as Accepted for publication after peer review) can be identiﬁed by an absence of publisher branding and/or typeset appear- ance. If there is any doubt, please refer to the published source. https://doi.org/10.1016/j.saa.2008.06.039 QUT Digital Repository: http://eprints.qut.edu.au/ Frost, Ray L. (2009) Raman and infrared spectroscopy of arsenates of the roselite and fairfieldite mineral subgroups. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 71(5). pp. 1788-1794. © Copyright 2009 Elsevier Raman and infrared spectroscopy of arsenates of the roselite and fairfieldite mineral subgroups Ray L. Frost• Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. Abstract Raman spectroscopy complimented with infrared spectroscopy has been used to determine the molecular structure of the roselite arsenate minerals of the roselite and 2+ fairfieldite subgroups of formula Ca2B(AsO4)2.2H2O (where B may be Co, Fe , Mg, 2- Mn, Ni, Zn). The Raman arsenate (AsO4) stretching region shows strong differences between the roselite arsenate minerals which is attributed to the cation substitution for calcium in the structure. In the infrared spectra complexity exists with multiple 2- (AsO4) antisymmetric stretching vibrations observed, indicating a reduction of the tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. -1 Strong Raman bands around 450 cm are assigned to ν4 bending modes. Multiple -1 bands in the 300 to 350 cm region assigned to ν2 bending modes provide evidence of symmetry reduction of the arsenate anion. Three broad bands for roselite are found at 3450, 3208 and 3042 cm-1 and are assigned to OH stretching bands. By using a Libowitzky empirical equation hydrogen bond distances of 2.75 and 2.67 Å are estimated. Vibrational spectra enable the molecular structure of the roselite minerals to be determined and whilst similarities exist in the spectral patterns, sufficient differences exist to be able to determine the identification of the minerals. Key words: arsenate, phosphate, roselite, talmessite, Raman spectroscopy Introduction • Author to whom correspondence should be addressed ([email protected]) 1 The fairfieldite mineral group are triclinic arsenates and phosphates of the 2+ general formula Ca2B(XO4)2.2H2O where B is Co, Fe , Mg, Mn, Ni, Zn and X is either As or P [1-6]. The minerals form two subgroups based upon whether the anion is phosphate or arsenate. Minerals in the fairfieldite group include cassidyite 2+ [Ca2(Ni,Mg)(PO4)2.2H2O], collinsite [Ca2(Mg,Fe )(PO4)2.2H2O], fairfieldite 2+ [Ca2(Mn,Fe )(PO4)2.2H2O], gaitite [Ca2Zn(AsO4)2.2H2O], messelite 2+ 2+ 2+ [Ca2(Mn ,Fe )(PO4)2.2H2O], parabrandite [Ca2Mn (AsO4)2.2H2O], roselite-beta [Ca2Co(AsO4)2.2H2O], and talmessite [Ca2Mg(AsO4)2.2H2O]. The minerals form solid solutions for example the collinsite-fairfieldite series. Many of these minerals are found in Australia [7-9]. The structure of the fairfieldite group minerals is dominated by the chains of tetrahedra (XO4) and octahedra [B-O4(H2O)2] which parallel the c axis. The fairfieldite group crystallizes in the triclinic space group P1. The cation octahedra are compressed resulting in disorder of the chains. This affects the hydrogen bonding of the water in the structure. The roselite subgroup is monoclinic and crystallise with space group P21/c [1]. The vibrational modes of oxyanions in aqueous systems are well known. The -1 symmetric stretching vibration of the arsenate anion (ν1) is observed at 810 cm and coincides with the position of the antisymmetric stretching mode (ν3). The symmetric -1 bending mode (ν2) is observed at 342 cm and the antisymmetric bending mode (ν4) at 398 cm-1. The positions of the arsenate vibrations occur at lower wavenumbers than any of the other naturally occurring oxyanions. Farmer lists a number of infrared spectra of arsenates including roselite, annabergite, erythrite, symplesite and köttigite. [10] The effect of reduced site symmetry in the crystal (compared with the free arsenate ion) will remove the degeneracy and allow splitting of the bands according to factor group analysis. Farmer based upon the work of Moenke reported the infrared spectra of roselite [10]. Farmer listed two bands at 985 and 920 cm-1 and assigned 2- 2- these bands to the ν1 (AsO4) symmetric stretching vibrations [10]. The ν3 (AsO4) symmetric stretching vibrations were listed as 870, 850 and 805 cm-1. The assignment of these bands does not appear to correct. The ν4 bending modes were 2 -1 -1 found at 453 and 435 cm . No ν2 bands were shown. A band at 535 cm was not assigned but may well be attributed to a water libration mode. No OH stretching vibrations were listed. For comparison Farmer listed the ν1 and ν3 bands of -1 -1 annabergite at 832 cm and 795 cm . The ν4 bending modes were found at 510, 460 and 427 cm-1 for annabergite. Two OH stretching vibrations were observed at 3430 and 3160 cm-1 for annabergite. A number of bands were listed which were unassigned. To the best of our knowledge no Raman spectra of the fairfieldite or roselite mineral subgroups has been undertaken. Raman spectroscopy has proven very useful for the study of minerals [11-13]. Indeed Raman spectroscopy has proven most useful for the study of diagentically related minerals as often occurs with carbonate minerals [14-18]. Some previous studies have been undertaken by the authors using Raman spectroscopy to study complex secondary minerals formed by crystallisation from concentrated sulphate solutions [19]. Very few spectroscopic studies of these types of minerals have been forthcoming and what studies that are available are not new. Few comprehensive studies of the fairfieldite and roselite mineral subgroups and related minerals such as divalent cationic arsenates have been undertaken. [10] Most of the infrared data predates the advent of Fourier transform infrared spectroscopy [20-25]. Although some Raman studies of some arsenate minerals have been undertaken [26, 27] no Raman spectroscopic investigation of roselite arsenate minerals has been forthcoming. Griffith (1970) did report the results of the Raman spectrum of a synthetic 2- annabergite. The symmetric stretching mode of the (AsO4) unit was observed at 859 cm-1; the antisymmetric stretching mode at 880 cm-1, the symmetric bending mode at 438 cm-1 and antisymmetric bending mode at 452 cm-1; other bands were located at 797 and 820 cm-1 [28]. The structural investigation of some arsenates and the nature of the hydrogen bond in these structures have been undertaken. It was found that the hydroxyl unit was coordinated directly to the metal ion and formed hydrogen bonds to the arsenate anion. [29] The minerals selected for this study are fundamentally related. As part of a comprehensive study of the molecular structure of minerals containing oxyanions using IR and Raman spectroscopy, we report the Raman and infrared properties of the above named phases. Experimental 3 Minerals The minerals used in this work, their formula and origin are listed in Table 1. The minerals were analysed by powder X-ray diffraction for phase composition and by EDX analysis for chemical composition. It is noted that talmessite is triclinic and belongs to the fairfieldite subgroup [30] where as roselite is monoclinic and belongs to the roselite mineral subgroup. Raman spectroscopy The crystals of roselite were placed and oriented on the stage of an Olympus BHSM microscope, 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 a silicon wafer. In order to ensure that the correct spectra are obtained, the incident excitation radiation was scrambled. Previous studies by the authors provide more details of the experimental technique. Details of the technique have been published by the authors [31-34]. Mid-IR 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 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. Spectral manipulation such as baseline adjustment, smoothing 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 4 accordingly. Band fitting was done using a Lorentz-Gauss cross-product function with the minimum number of component bands used for the fitting process.

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