This may be the author’s version of a work that was submitted/accepted for publication in the following source: Frost, Ray, Xi, Yunfei, Scholz, Ricardo, & Horta, Laura (2013) The phosphate mineral arrojadite-(KFe) and its spectroscopic characteri- zation. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 109, pp. 138-145. This file was downloaded from: https://eprints.qut.edu.au/58843/ c Consult author(s) regarding copyright matters This work is covered by copyright. Unless the document is being made available under a Creative Commons Licence, you must assume that re-use is limited to personal use and that permission from the copyright owner must be obtained for all other uses. If the docu- ment is available under a Creative Commons License (or other specified license) then refer to the Licence for details of permitted re-use. It is a condition of access that users recog- nise and abide by the legal requirements associated with these rights. If you believe that this work infringes copyright please provide details by email to [email protected] License: Creative Commons: Attribution-Noncommercial-No Derivative Works 2.5 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.2013.02.027 1 The phosphate mineral arrojadite-(KFe) and its spectroscopic characterization 2 3 Ray L. Frosta, Yunfei Xia, Ricardo Scholzb, Laura Frota Campos Hortab 4 5 a School of Chemistry, Physics and Mechanical Engineering, Science and Engineering 6 Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, 7 Australia. 8 9 b Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do 10 Cruzeiro, Ouro Preto, MG, 35,400-00, Brazil. 11 12 Abstract: 13 The arrojadite-(KFe) mineral has been analyzed using a combination of scanning electron 14 microscopy and a combination of Raman and infrared spectroscopy. The origin of the mineral 15 is Rapid Creek sedimentary phosphatic iron formation, northern Yukon. The formula of the 16 mineral was determined as 17 K2.06Na2Ca0.89Na3.23(Fe7.82Mg4.40Mn0.78)∑13.00Al1.44(PO4)10.85(PO3OH0.23)(OH)2. 18 The complexity of the mineral formula is reflected in the spectroscopy. Raman bands at 975, -1 -1 3- 19 991 and 1005 cm with shoulder bands at 951 and 1024 cm are assigned to the PO4 ν1 20 symmetric stretching modes. The Raman bands at 1024, 1066, 1092, 1123, 1148 and 1187 -1 3- 21 cm are assigned to the PO4 ν3 antisymmetric stretching modes. A series of Raman bands -1 22 observed at 540, 548, 557, 583, 604, 615 and 638 cm are attributed to the ν4 out of plane 23 bending modes of the PO4 and H2PO4 units. The ν2 PO4 and H2PO4 bending modes are 24 observed at 403, 424, 449, 463, 479 and 513 cm-1. Hydroxyl and water stretching bands are 25 readily observed. Vibrational spectroscopy enables new information about the complex 26 phosphate mineral arrojadite-(KFe) to be obtained. 27 28 Key words: arrojadite, phosphate, Raman spectroscopy, infrared spectroscopy 29 30 Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1 31 Introduction 32 33 The arrojadite mineral group is a complex group of phosphates with general chemical 34 formula given as: A2B2Ca1Na2+xM13R(PO4)11(PO3OH1-x)W2, where the site A is occupied by 35 large and divalent cations (Ba, Sr, Pb) plus vacancy, or monovalent cations (K, Na). The B 36 site is occupied by either small divalent cations (Fe, Mn, Mg) plus vacancy, or monovalent 37 cations (Na). The M site is essentially occupied by Fe2+ or Mn2+ and possibility of 38 substitution by Mg, Zn, Li. R are trivalent cations (Al, Fe3+) and W are OH- and F- [1]. The 39 nomenclature of arrojadite group was established by Chopin et al. (2006) [2]. 40 41 The crystal structure of arrojadite was first described by Krutik et al. [3] and latter refined by 42 Merlino et al. [4], Moore et al. [5], Steele [6] and Cámara et al. [1]. The structure of a 43 synthetic Fe3+ arrojadite was refined by Yakubovich et al. [7]. Arrojadite group minerals 44 crystallizes in the monoclinic crystal system, Space Group Cc [1]. Unit cell parameters are 45 variable between the members of the group, however refined data for arrojadite-(KFe) are 46 still not available and are restricted to the data published by Lindberg [8]. 47 48 In recent years, the application of spectroscopic techniques for the understanding the 49 structure of phosphate minerals is increasing, with special attention to Al phosphates [9-12]. 50 Farmer [13] divided the vibrational spectra of phosphates according to the presence, or 51 absence of water and hydroxyl units. In aqueous systems, Raman spectra of phosphate −1 52 oxyanions show a symmetric stretching mode (ν1) at 938 cm , the antisymmetric stretching −1 −1 53 mode (ν3) at 1017 cm , the symmetric bending mode (ν2) at 420 cm and the ν4 mode at 567 −1 54 cm [14-17]. The value for the ν1 symmetric stretching vibration of PO4 units as determined 55 by infrared spectroscopy was given as 930 cm−1 (augelite), 940 cm−1 (wavellite), 970 cm−1 56 (rockbridgeite), 995 cm−1 (dufrénite) and 965 cm−1 (beraunite). The position of the symmetric 57 stretching vibration is dependent upon the crystal chemistry of the mineral and is a function 58 of the cation and crystal structure. The fact that the symmetric stretching mode is observed in 59 the infrared spectrum affirms a reduction in symmetry of the PO4 units. 60 61 The value for the ν2 symmetric bending vibration of PO4 units as determined by infrared 62 spectroscopy was given as 438 cm−1 (augelite), 452 cm−1 (wavellite), 440 and 415 cm−1 63 (rockbridgeite), 455, 435 and 415 cm−1 (dufrénite) and 470 and 450 cm−1 (beraunite). The 64 observation of multiple bending modes provides an indication of symmetry reduction of the 2 65 PO4 units. This symmetry reduction is also observed through the ν3 antisymmetric stretching 66 vibrations. Augelite shows infrared bands at 1205, 1155, 1079 and 1015 cm−1 [18]; wavellite 67 at 1145, 1102, 1062 and 1025 cm−1; rockbridgeite at 1145, 1060 and 1030 cm−1; dufrénite at 68 1135, 1070 and 1032 cm−1; and beraunite at 1150, 1100, 1076 and 1035 cm−1. 69 70 In this work, spectroscopic investigation of monomineral arrojadite-(KFe) sample from Rapid 71 Creek, Yukon, Canada has been carried out. The analysis includes spectroscopic 72 characterization of the structure with infrared and Raman spectroscopy. Chemical analysis 73 was applied to support the mineral characterization. 74 75 Experimental 76 77 Samples description and preparation 78 The arrojadite-(KFe) sample forms part of the collection of the Geology Department of the 79 Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code SAB065. The 80 sample was gently crushed and the associated minerals were removed under a 81 stereomicroscope Leica MZ4. The arrojadite-(KFe) sample was phase analyzed by X-ray 82 diffraction. 83 84 The Rapid Creek sedimentary phosphatic iron formation comprises the upper and youngest 85 portion of an Aptian-Albian flyschoid sequence which reaches a maximum thickness of four 86 kilometres in the Blow Trough. The phosphate association is composed mainly of rare 87 minerals such as satterlyite, arrojadite group minerals, augelite, lazulite and gormanite, which 88 reflect an original calcium-deficient composition. The deposition of iron and magnesium 89 phosphates as well as apatite is strongly indicated, and this condition is unique for marine 90 phosphorites [19]. 91 92 93 Scanning electron microscopy (SEM) 94 Arrojadite-(KFe) crystals were coated with a 5nm layer of evaporated carbon. Secondary 95 Electron and Backscattering Electron images were obtained using a JEOL JSM-6360LV 96 equipment. Qualitative and semi-quantitative chemical analyses in the EDS mode were 97 performed with a ThermoNORAN spectrometer model Quest and was applied to support the 98 mineral characterization. 3 99 100 Raman microprobe spectroscopy 101 Crystals of arrojadite-(KFe) were placed on a polished metal surface on the stage of an 102 Olympus BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The 103 microscope is part of a Renishaw 1000 Raman microscope system, which also includes a 104 monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were 105 excited by a Spectra-Physics model 127 He-Ne laser producing highly polarized light at 633 106 nm and collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range 107 between 200 and 4000 cm-1. Repeated acquisitions on the crystals using the highest 108 magnification (50x) were accumulated to improve the signal to noise ratio of the spectra. 109 Raman Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. The Raman 110 spectrum of at least 10 crystals was collected to ensure the consistency of the spectra. 111 112 An image of the arrojadite-(KFe) crystals measured is shown in the supplementary 113 information as Figure S1. Clearly the crystals of arrojadite-(KFe) are readily observed, 114 making the Raman spectroscopic measurements readily obtainable. 115 116 Infrared spectroscopy 117 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 118 endurance single bounce diamond ATR cell.