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Raman and Infrared Spectroscopic Characterization of the Phosphate 2+ Mineral Paravauxite Fe Al2(PO4)2(OH)28H2O ⇑ Ray L

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Raman and Infrared Spectroscopic Characterization of the Phosphate 2+ Mineral Paravauxite Fe Al2(PO4)2(OH)2�8H2O ⇑ Ray L Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 116 (2013) 491–496 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Raman and infrared spectroscopic characterization of the phosphate 2+ mineral paravauxite Fe Al2(PO4)2(OH)2Á8H2O ⇑ Ray L. Frost a, , Ricardo Scholz b, Andrés Lópes a, Yunfei Xi a,Zˇeljka Zˇigovecˇki Gobac c, Laura Frota Campos Horta b 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 35400-00, Brazil c Institute of Mineralogy and Petrography, Department of Geology, Faculty of Science, University of Zagreb, Horvatovac 95, 10000 Zagreb, Croatia highlights graphical abstract Paravauxite is a rare phosphate mineral of formula 2+ Fe Al2(PO4)2(OH)2Á8H2O. Chemically, it is closely related to its basic namesake vauxite. Another chemically closely related mineral is metavauxite a dimorph of paravauxite. We have analysed this mineral using EDX analyses. The mineral has been studied by vibrational spectroscopy. article info abstract Article history: We have undertaken a vibrational spectroscopic study of paravauxite the Siglo XX mine, Bustillo Prov- Received 17 June 2013 ince, northern of Potosí department, Bolivia. This mine is important source for rare and unusual second- Received in revised form 15 July 2013 ary phosphate minerals and is the type locality for a number of rare phosphates such as vauxite, sigloite, Accepted 22 July 2013 metavauxite and for jeanbandyite. The chemical formula of the studied sample was determined as Available online 2 August 2013 2þ Fe0:95; ðAl0:07ÞR1:02ðAlÞ2:09ðPO4Þ1:97ðOHÞ1:98 Á 7:90ðH2OÞ. À1 3À The Raman spectrum is dominated by an intense Raman band at 1020 cm assigned to the PO m1 Keywords: 4 symmetric stretching mode. Low intensity Raman bands found at 1058, 1115 and 1148 cmÀ1 are assigned Paravauxite to the PO3À antisymmetric stretching vibrations. Raman bands of paravauxite at 537, 570, 609 and Phosphate 4 m3 À1 3À À1 Raman spectroscopy 643 cm are assigned to the m4 PO4 bending modes whilst the Raman bands at 393 and 420 cm 3À Infrared spectroscopy are due to the m2 PO4 bending modes. The Raman spectral profile of paravauxite in the hydroxyl stretch- ing region is broad with component bands resolved at 3086, 3215, 3315, 3421, 3505 and 3648 cmÀ1. Vibrational spectroscopy enables the assessment of the molecular structure of paravauxite to be undertaken. Ó 2013 Elsevier B.V. All rights reserved. Introduction mineral on wavellite crystals at the tin mines of Llallagua, Bolivia [2]. Paravauxite is a triclinic mineral with a = 5.233, b = 10.541, 2+ Paravauxite is a rare phosphate mineral of formula Fe Al2(- c = 6.962 Å, a = 106.9°, b = 110.8°, c = 72.1°, space group P1 and PO4)2(OH)2Á8H2O which occurs in hydrothermal tin veins, and also Z =1 [3]. Chemically, it is closely related to its basic namesake in complex granite pegmatites [1]. It was discovered as a new vauxite [4,5], and appears to be different from vauxite only in the number of water molecules in the structure. Paravauxite has ⇑ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. eight water molecules while vauxite has only six in its formula. E-mail address: [email protected] (R.L. Frost). Another chemically closely related mineral is metavauxite which 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.07.048 492 R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 116 (2013) 491–496 is a dimorph of paravauxite. In this case, the structure of stages of hydrothermal deposition, and were derived mostly from paravauxite is triclinic [3] while the structure of metavauxite [8] decomposition of primary fluorapatite [22]. is monoclinic in symmetry. Other members of the stewartite min- In the Siglo XX mine, colorless to light blue paravauxite aggre- eral group [7], in which paravauxite belongs, are gordonite MgAl2(- gates of crystals up to 3.0 mm in length mainly occur in association 2+ 2+ PO4)2(OH)2Á8H2O [8], kastningite (Mn ,Fe ,Mg)Al2(PO4)2(OH)2 with eosphorite. Other secondary phosphates such as wavellite, 2þ 3þ Á8H2O [9,10], laueite Mn Fe2 ðPO4Þ2ðOHÞ2 Á 8H2O [11,12] man- metavauxite, vauxite, crandallite and childrenite also can be found 2+ 2+ gangordonite (Mg ,Fe ,Mg)Al2(PO4)2(OH)2Á8H2O [8,15], sigloite in association. The sample was incorporated to the collection of the 3+ 3þ Fe Al2(PO4)2(OH)3Á7H2O [16,17], stewartite MnFe2 ðPO4Þ2ðOHÞ2Á Geology Department of the Federal University of Ouro Preto, Minas 3þ 8H2O [13] and ushkovite MgFe2 ðPO4Þ2ðOHÞ2 Á 8H2O [19–21]. Gerais, Brazil, with sample code SAB-006B. The sample was gently 2þ 3þ As paravauxite is isostructural with laueite Mn Fe2 ðPO4Þ2 crushed and the associated minerals were removed under a stereo- ðOHÞ2 Á 8H2O [3,12,14], it could be indirectly concluded that the microscope Leica MZ4. structure of paravauxite is based on an infinite chain of vertex- linked oxygen octahedra, with Al occupying the octahedral centres, Scanning electron microscopy (SEM) the chain oriented parallel to the c-axis. Chains are in turn con- Experiments and analyses involving electron microscopy were nected to others by PO4 tetrahedra which also bridge through iso- performed in the Center of Microscopy of the Universidade Federal lated octahedra (with Fe2+ as centres). The laueite structural de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil (http:// 2þ 3þ formula is Mn Fe2 ðOHÞ2ðPO4Þ2ðH2OÞ6 Á 2H2O [12], and according www.microscopia.ufmg.br). 2+ to analogy, the paravauxite structural formula is then Fe Al2(- Paravauxite crystal aggregate was coated with a 5 nm layer of OH)2(PO4)2(H2O)6Á2H2O and the non-octahedrally bonded waters evaporated Au. Secondary electron and backscattering electron appearing in a cavity left in the structure. In detailed description, images were obtained using a JEOL JSM-6360LV equipment. Qual- in analogy with laueite structure [14], the chains of Al-octahedra itative and semi-quantitative chemical analysis in the EDS mode 3À decorated by flanking PO4 groups (which extend in c-direction) were performed with a ThermoNORAN spectrometer model Quest meld in the a-direction by sharing one quarter of the flanking and was applied to support the mineral characterization. PO4 vertices with octahedra of adjacent chains to form an [Al2(- PO4)2(OH)2(H2O)2] sheet. In the resulting sheet, the PO4 tetrahedra Electron probe micro-analysis (EPMA) are three-connected. There are two distinct octahedra in these The quantitative chemical analysis of paravauxite single crys- sheets, one of which is six-connected within the sheet, and the tals was carried out using EPMA, with a Jeol JXA8900R spectrome- other of which is only four-connected and has (H2O) at two verti- ter from the Physics Department of the Federal University of Minas ces. From another view, it could be also described that paravauxite Gerais, Belo Horizonte. The sample was embedded in epoxy resin sheet is composed of [Al(PO4)2O2] chains that are linked by (AlO6) and after polishing, coated with a thin layer of evaporated carbon. octahedra. These sheets stack in the b-direction and are linked by The electron probe microanalysis in the WDS (wavelength disper- 2+ (Fe O2H2O4 octahedra, and by hydrogen bonds involving the sive spectrometer) mode was obtained at 15 kV accelerating volt- 2+ interstitial (H2O) groups bonded to Fe and interstitial (H2O) age and beam current of 10 nA. For each selected element was groups held in the structure solely by hydrogen bonds. used the following standards: Fe – magnetite, Mg – MgO, Mn – Raman spectroscopy has proven most useful for the study of rhodonite, P and Ca - Ca2P2O7, Al – Corundum, Na – Albite100 mineral structures. The objective of this research is to report the and F – Fluorite. Chemical formula was calculated on the basis of Raman and infrared spectra of paravauxite and to relate the spectra 18 oxygen atoms. to the molecular structure of the mineral. The number of paravaux- ite occurrences is limited. This is the first report of a systematic Raman microprobe spectroscopy study of paravauxite from Bolivia. Crystals of paravauxite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped Experimental with 10Â,20Â, and 50Â objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a Samples description and preparation monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 The paravauxite sample studied in this work was collected by He–Ne laser producing highly polarized light at 633 nm and col- one of the authors (Scholz) from the Siglo XX mine (also named lected at a nominal resolution of 2 cmÀ1 and a precision of Siglo Veinte, Catavi or Llallagua), a tin deposit located in the Andes ±1 cmÀ1 in the range between 200 and 4000 cmÀ1. Repeated acqui- Mountain, Bustillo Province, northern of Potosí department, Bolivia. sitions on the crystals using the highest magnification (50Â) were In the middle of the 20th century Siglo XX was the most productive accumulated to improve the signal to noise ratio of the spectra. Ra- tin mine in the world. The mine is also an important source for rare man Spectra were calibrated using the 520.5 cmÀ1 line of a silicon and unusual secondary phosphate minerals and is the type locality wafer.
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