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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Queensland University of Technology ePrints Archive This is the author’s version of a work that was submitted/accepted for pub- lication in the following source: Theiss, Frederick L., López, Andrés, Scholz, Ricardo, & Frost, Ray L. (2015) A SEM, EDS and vibrational spectroscopic study of the clay mineral fraipontite. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 147, pp. 230-234. This file was downloaded from: https://eprints.qut.edu.au/84488/ c Copyright 2015 Elsevier B.V. Licensed under the Creative Commons Attribution-NonCommercial- NoDerivatives 4.0 International http://creativecommons.org/licenses/by- nc-nd/4.0/ Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document. For a definitive version of this work, please refer to the published source: https://doi.org/10.1016/j.saa.2015.03.088 1 A SEM, EDS and vibrational spectroscopic study of the clay mineral fraipontite 2 3 Frederick L. Theiss,a Andrés Lópeza, Ricardo Scholz,b Ray L. Frosta• 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 mineral fraipontite has been studied by using a combination of scanning electron 14 microscopy with energy dispersive analysis and vibrational spectroscopy (infrared and 15 Raman). Fraipontite is a member of the 1:1 clay minerals of the kaolinite-serpentine group. 16 The mineral contains Zn and Cu and is of formula (Cu,Zn,Al)3(Si,Al)2O5(OH)4. Qualitative 17 chemical analysis of fraipontite shows an aluminium silicate mineral with amounts of Cu and 18 Zn. This kaolinite type mineral has been characterized by Raman and infrared spectroscopy; 19 in this way aspects about the molecular structure of fraipontite clay are elucidated. 20 21 Key words: fraipontite, phyllosilicate, kaolinite, Raman spectroscopy, infrared spectroscopy 22 • Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1 23 24 Introduction 25 Clays are an important group of minerals which are found in soils, sediments and are also the 26 result of some hydrothermal alteration. Clay minerals form a part of the phyllosilicates or 27 layered silicates, which are characterized by a sheet-like structure [1-3]. Both the order and 28 composition of these sheets contributes to the properties of the clay mineral. A large and 29 important group of clay minerals is the Kaolinite-serpentine which consists of phyllosilicate 30 minerals including: kaolinite, antigorite, chrysotile and dickite [4]. Other minerals in this 31 group that can be part of hydrothermal paragenesis associated to ore deposits are 32 manandonite, pecoraite and fraipontite [5, 6], which is the focus of our current investigation. 33 34 Fraipontite is a 1:1 clay mineral with the formula (Cu,Zn,Al)3(Si,Al)2O5(OH)4. As 35 demonstrated by the above formula, the mineral contains Zn. Takahashi et al. [17] showed 36 that the Al was able to occupy both the octahedral and tetrahedral positions in fraipontite. 37 Fraipontite has been described from Turkmenistan, where it was formed during thermal 38 activity [7, 8]. Perez et al. [9] described intermediate solid solutions between berthierine and 39 fraipontite in Be and Zn mineralisation in the biotite granite and syenite Taghouaji complex 40 (Aiir Mountains, Niger), while Parcalabescu et al. [10, 11] described fraipontite from the 41 northern Bals batholith in Romania. Several other known occurrences have been described in 42 [12-15]. Synthetic analogues of fraipontite have also been produced and characterized [16- 43 18], however, such synthetic compounds are may not necessarily exhibit the same structure 44 and properties as samples of the natural mineral. 45 46 In this article, we expand our knowledge of the 1:1 kaolinite type minerals through the study 47 of a specific member of this group, the mineral fraipontite. The vibrational spectra (infrared 48 and Raman) of fraipontite are investigated and related to the structure of the mineral. 49 Qualitative chemical analysis utilizing scanning electron microscopy with energy-dispersive 50 X-ray spectroscopy has also been obtained. 51 52 EXPERIMENTAL 53 Samples description and preparation 54 The fraipontite sample studied in this work forms part of the collection of the Geology 55 Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code 56 SAC-091. The sample is from the Blue Bell mine, USA. 2 57 58 The sample was gently crushed and the associated minerals were removed under a 59 stereomicroscope Leica MZ4. The fraipontite studied in this work occurs in association with 60 an unidentified clay mineral. Scanning electron microscopy (SEM) in the EDS mode was 61 applied to support the mineral characterization. 62 63 Scanning electron microscopy (SEM) 64 Experiments and analyses involving electron microscopy were performed in the Center of 65 Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, 66 Brazil (http://www.microscopia.ufmg.br). 67 68 Fraipontite crystals were coated with a 5nm layer of evaporated carbon. Secondary Electron 69 and Backscattering Electron images were obtained using a JEOL JSM-6360LV equipment. 70 Qualitative and semi-quantitative chemical analyses in the EDS mode were performed with a 71 ThermoNORAN spectrometer model Quest and was applied to support the mineral 72 characterization. 73 74 Raman spectroscopy 75 The crystals of fraipontite were placed and oriented on the stage of an Olympus BHSM 76 microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000 Raman 77 microscope system, which also includes a monochromator, a filter system and a Charge 78 Coupled Device (CCD). Raman spectra were excited by a HeNe laser (633 nm) at a nominal 79 resolution of 2 cm-1 in the range between 100 and 4000 cm-1. Details of the experimental 80 procedure have been published. The spatial resolution of the instrument is 1 µm. Thus, if 81 crystals are less than this value, a mixture of crystals will be measured. However, the crystals 82 of fraipontite used in this experiment were > 1.1 µm. 83 84 Infrared spectroscopy 85 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 86 endurance single bounce diamond ATR cell. Spectra over the 4000−525 cm-1 range were 87 obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of 88 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio. 89 3 90 Spectral manipulation such as baseline adjustment, smoothing and normalisation were 91 performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, 92 NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software 93 package which enabled the type of fitting function to be selected and allows specific 94 parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz-Gauss 95 cross-product function with the minimum number of component bands used for the fitting 96 process. The Lorentz-Gauss ratio was maintained at values greater than 0.7 and fitting was 97 undertaken until reproducible results were obtained with squared correlations of r2 greater 98 than 0.995. 99 100 RESULTS AND DISCUSSION 101 102 Chemical characterization 103 The SEM/BSI image of the fraipontite crystal fragment studied in this work is shown in 104 Figure 1. The sample occurs in association with unidentified clay mineral. A perfect cleavage 105 can be observed. Qualitative chemical analysis of fraipontite shows an aluminium silicate 106 mineral with amounts of Cu and Zn (Figure 2). It is likely there are low concentrations of rare 107 earth elements in the chemical composition of fraipontite. 108 109 Vibrational spectroscopy of fraipontite (IR and Raman) 110 The Raman spectrum of fraipontite over the 4000 to 100 cm-1 spectral range is shown in 111 Figure 3a. This figure shows the position and relative intensity of the Raman bands of 112 fraipontite. It is noted there are large parts of the spectrum where little or no intensity is 113 observed, consequently the Raman spectrum is therefore subdivided into sections based upon 114 the types of vibration being studied. Additionally it is noted that there is little intensity in the 115 hydroxyl stretching region (2500 to 3800 cm-1 spectral range). The infrared spectrum of 116 fraipontite over the 4000 to 500 cm-1 spectral range is displayed in Figure 3b which shows the 117 position and relative intensities of the infrared bands. The infrared spectrum is also 118 subdivided into sections based upon the type of vibration being analysed. Compared with the 119 Raman, the infrared spectrum is quite broad and there is minimal intensity observed beyond 120 1500 cm-1. 121 4 122 The Raman spectrum of fraipontite over the 1200 to 700 cm-1 spectral range is shown in 123 Figure 4a. Two Raman bands are observed at 731 and 1094 cm-1 with a shoulder band at 124 1089 cm-1. These bands are assigned to the bending and stretching vibrations of the SiO units. 125 In contrast, the infrared spectrum of fraipontite over the 1600 to 900 cm-1 spectral range as 126 shown in Figure 4b displays broad bands with bands observed at 991, 1255, 1357, 1418 and 127 1487 cm-1.