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A Vibrational Spectroscopic Study of the Phosphate Mineral Vantasselite

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A Vibrational Spectroscopic Study of the Phosphate Mineral Vantasselite Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 147 (2015) 185–192 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa A vibrational spectroscopic study of the phosphate mineral vantasselite Al4(PO4)3(OH)3Á9H2O ⇑ Ray L. Frost a, , Ricardo Scholz b, Fernanda Maria Belotti c, Andrés López a, Frederick L. Theiss a 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 c Federal University of Itajubá, Campus Itabira, Itabira, MG 35,903-087, Brazil highlights graphical abstract We have studied the phosphate mineral vantasselite Al2(PO4)(OH)3Á3H2O. Using a combination of SEM with EDX and Raman and infrared spectroscopy. Chemical analysis shows a mineral containing Al, Fe and P. A comparison is made with other aluminum containing phosphate minerals. Vibrational spectroscopy offers a mechanism for the study of the molecular structure of vantasselite. article info abstract Article history: We have studied the phosphate mineral vantasselite Al4(PO4)3(OH)3Á9H2O using a combination of SEM Received 28 October 2014 with EDX and Raman and infrared spectroscopy. Qualitative chemical analysis shows Al, Fe and P. Received in revised form 15 January 2015 À1 3À Raman bands at 1013 and 1027 cm are assigned to the PO4 m1 symmetric stretching mode. The obser- Accepted 23 March 2015 vation of two bands suggests the non-equivalence of the phosphate units in the vantasselite structure. Available online 28 March 2015 À1 3À Raman bands at 1051, 1076 and 1090 cm are attributed to the PO4 m3 antisymmetric stretching vibra- tion. A comparison is made with the spectroscopy of wardite. Strong infrared bands at 1044, 1078, 1092, Keywords: 1112, 1133, 1180 and 1210 cmÀ1 are attributed to the PO3À m antisymmetric stretching mode. Some of Vantasselite 4 3 these bands may be due to dAl OH deformation modes. Vibrational spectroscopy offers a mechanism for Phosphate 2 SEM the study of the molecular structure of vantasselite. EDS Ó 2015 Elsevier B.V. All rights reserved. Raman spectroscopy Infrared spectroscopy Introduction b = 16.541(3), c = 20.373(6) and Z = 8. The crystals occur with lamellar habitus, elongated along 100. The crystal aggregates forms Vantasselite is a rare aluminum phosphate mineral with chemi- rosettes up to 5 mm. Vantasselite was first described from Bihain, cal formula given as Al4(PO4)3(OH)3Á9H2O. The mineral crystalizes Vielsalm, Stavelot Massif, Belgium [1]. The mineral occurs on with orthorhombic symmetry, Point Group: 222, unit cell parame- dumps in a quartzite quarry in quartz veinlets or lining schistosity ters are: Space Group: Pmam, Pma2 or P21am; a = 10.528(4), planes. The associated minerals are wavellite, cacoxenite, variscite, turquoise, lithiophorite, cryptomelane, quartz, clinochlore and muscovite. There is no crystal structure determination for vantas- ⇑ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. selite. The only scientific publication about this mineral is the E-mail address: [email protected] (R.L. Frost). http://dx.doi.org/10.1016/j.saa.2015.03.090 1386-1425/Ó 2015 Elsevier B.V. All rights reserved. 186 R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 147 (2015) 185–192 original description [1]. Due to the cleavage and the type of frag- not interpret the spectra of the phosphate because of complexity ment like plates, the unit cell refinement is not useful, or is very and no detailed assignments were given. Breitinger et al. reported complicated to be used for the determination of the crystal struc- the combined vibrational spectra of a natural wardite. Breitinger ture. Thus, it seems appropriate in order to determine aspects of et al. used a full array of techniques including inelastic neutron the molecular structure of vantasselite to use vibrational spectro- scattering, infrared, Raman and near infrared techniques [6]. scopic techniques. These workers used a natural wardite with significant amounts Interestingly Farmer in his book on the infrared spectra of of ferric iron in the structure. In other words, the sample analyzed minerals divided the vibrational spectra of phosphates according was fundamentally a solid solution of wardite and cyrilovite, but at to the presence, or absence of water and hydroxyl units in the the wardite end. The original papers on the infrared spectrum of minerals [2]. In aqueous systems, Raman spectra of phosphate isolated phosphate units was published by Lazarev [9]. Of course, À1 oxyanions show a symmetric stretching mode (m1) at 938 cm , phosphates structures are different. Usually they have rather low À1 the antisymmetric stretching mode (m3) at 1017 cm , the sym- symmetry: orthorhombic, monoclinic, or even triclinic [10]. À1 metric bending mode (m2) at 420 cm and the m4 mode at Farmer based upon the work of Petrov et al. [11–13] made a com- 567 cmÀ1 [3–5]. Farmer reported the infrared spectra of berlinite parison of the results of the vibrational spectrum of a series of (AlPO4) with PO4 stretching modes at 1263, 1171, 1130 and phosphates. 1114 cmÀ1; bending modes at 511, 480, 451, 379 and 605 cmÀ1. Raman spectroscopy has proven most useful for the study of Al–O modes were found at 750, 705, 698 and 648 cmÀ1. On hydra- mineral structures [14–19]. The objective of this research is to tion of the mineral as with variscite (AlPO4Á2H2O), PO4 stretching report the Raman and infrared spectra of vantasselite and to relate modes were found at 1160, 1075, 1050 and 938 cmÀ1; bending the spectra to the molecular structure of the minerals. The mineral À1 modes at 515, 450 and 420 cm ; in addition H2O stretching bands vantasselite may be described as an environmental mineral in that were found at 3588, 3110, 2945 cmÀ1. For the mineral augelite it is formed in waste dumps and mining wastes, as such the min- (AlPO4(OH)3), infrared bands were observed at 930 (m1), 438 (m2), eral is often small in crystal size and difficult to measure by X- À1 1205, 1155, 1079, 1015 (m3) and 615, 556 cm (m4). For augelite, ray diffraction. The number of vantasselite occurrences is limited OH stretching modes were not observed. [20]. This is the first report of a systematic vibrational spectro- There are a couple of minerals the chemistry of which may be scopic study of vantasselite from Belgium. compared with vantasselite. These include wardite and cyrilovite. There have been some studies of the vibrational spectroscopy of Experimental wardite [6,7]. Tarte et al. collected the infrared spectra of five sam- ples of cyrilovite NaFe (PO ) (OH) Á2(H O) [8] and wardite [7]. 3 4 2 4 2 Mineral Cyrilovite is analogous to wardite, with ferric iron replacing the aluminum in the structure. It is likely that solid solutions of the The vantasselite sample studied in this work was collected from two minerals are formed with varying amounts of ferric ion and the Bihain, Vielsalm, Stavelot Massif, Luxembourg Province, aluminum in the structure. The mineral wardite is capable of crys- Belgium. The sample is incorporated into the collection of the tallizing in a similar form to that of cyrilovite because of their clo- Geology Department of the Federal University of Ouro Preto, sely related chemical compositions. Between wardite’s Minas Gerais, Brazil, with sample code SAC-085. The sample was composition, NaAl (PO ) (OH) Á2(H O), and cyrilovite composi- 3 4 2 4 2 gently crushed and the associated minerals were removed under tion, NaFe (PO ) (OH) Á2(H O), these minerals are able to form 3 4 2 4 2 a stereomicroscope Zeiss Stemi DV4 from the Museu de Ciência e end members of a series of solid solutions. Either of the two miner- Técnica of the Federal University of Ouro Preto. Scanning electron als can occur in various proportions in a series of solid solutions in microscopy (SEM) was applied to support the mineral the wardite mineral group. Cyrilovite is a rare accessory mineral in characterization. some oxidizing phosphate-bearing granite pegmatites and iron deposits. The vibrational spectrum is dependent upon the ratio of the Al/Fe. Tarte et al. found that the two minerals wardite and cyri- Scanning electron microscopy (SEM) lovite can be distinguished by the spectral patterns of the OH stretching region in the infrared spectrum [7]. These workers did 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). A vantasselite crystal aggregate up to 0.5 mm was coated with a 5 nm layer of evaporated carbon. Backscattering Electron images was 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 were applied to support the mineral characterization. Raman spectroscopy Crystals of vantasselite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10Â,20Â, and 50Â 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 polarized light at 633 nm and col- À1 Fig. 1. Backscattered electron image (BSI) of a vantasselite fragment up to 0.5 mm lected at a nominal resolution of 2 cm and a precision of 1 1 in length.
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