Vibrational Spectroscopy of the Borate Mineral Chambersite Mnb7o13cl - Implications for the Molecular Structure

This may be the author’s version of a work that was submitted/accepted for publication in the following source: Frost, Ray, Lopez, Andres, Scholz, Ricardo, & Xi, Yunfei (2014) Vibrational spectroscopy of the borate mineral chambersite MnB7O13Cl - Implications for the molecular structure. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 120, pp. 270-273. This file was downloaded from: https://eprints.qut.edu.au/65134/ 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 document 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.2013.10.026 1 Vibrational spectroscopy of the borate mineral chambersite MnB7O13Cl 2 – Implications for the molecular structure 3 4 Ray L. Frost1 , Andrés López1, Ricardo Scholz 2 and Yunfei Xi 1 5 6 1. School of Chemistry, Physics and Mechanical Engineering, Science and Engineering 7 Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 8 4001, Australia. 9 10 2. Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro 11 do Cruzeiro, Ouro Preto, MG, 35,400-00, Brazil 12 13 Abstract: 14 Chambersite is a manganese borate mineral with formula: MnB7O13Cl and occurs as colorless 15 crystals in the monoclinic pyramidal crystal system. Raman bands at 902, 920, 942 and 963 -1 16 cm are assigned to the BO stretching vibration of the B7O13 units. Raman bands at 1027, 17 1045, 1056, 1075 and 1091 cm-1 are attributed to the BCl in-plane bending modes. The 18 intense infrared band at 866 cm-1 is assigned to the trigonal borate stretching modes. The 19 Raman band at 660 cm-1 together with bands at 597, 642 679, 705 and 721 cm-1 are assigned 20 to the trigonal and tetrahedral borate bending modes. The molecular structure of a natural 21 chambersite has been assessed using vibrational spectroscopy. 22 23 Key words: borate, chambersite, ferroelectric properties, Raman spectroscopy, infrared, 24 evaporite 25 26 Author to whom correspondence should be addressed ([email protected]) 1 27 Introduction 28 Chambersite is a manganese borate mineral formulaMn3B7O13Cl [1] . The mineral is a 29 member of the borate mineral series that includes ericacite and boracite. Chambersite is the 30 manganese analogue of boracite. It was first discovered at Barber’s Hill salt dome, Chambers 31 County, Texas, USA. The mineral is also known from the Dongshuichang deposits in Jixan, 32 Tianjin, China [2]. The mineral is an evaporite mineral and occurs associated with other 33 evaporite minerals such as halite, anhydrite and gypsum. The mineral is known for a number 34 of significant borate deposits worldwide [3-6]. It occurs in brine residues from extraction 35 wells in salt domes. 36 37 Boracite is a magnesium borate mineral with formula: Mg3B7O13Cl [7] and occurs as blue 38 green, colourless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal 39 system [8, 9]. Boracite also shows pseudo-isometric cubical and octahedral forms [10]. 40 These are thought to be the result of transition from an unstable high temperature isometric 41 form upon cooling. The mineral is related to the homonymous group that includes 2+ 2+ 2+ 42 chambersite – (Mn )3B7O13Cl; congolite – (Fe ,Mg)3B7O13Cl , ericaite – (Fe )3B7O13Cl 2+ 43 and; trembathite - (Mg,Fe )3B7O13Cl. Boracite is typically found in evaporite sequences 44 associated with gypsum, anhydrite, halite, sylvite, carnallite, kainite and hilgardite. 45 46 The mineral chambersite has been synthesised [11-13]. The reason for the synthetic 47 analogues of chambersite is the ferroelectric properties of the mineral. Chambersite is 48 orthorhombic, pseudocubic. Crystals are pseudotetrahedral. The cell data is: Space Group: 49 Pca21 with a = 8.68(1), b = 8.68(1), c = 12.26(1) and Z = 4. Boracite is orthorhombic, 50 pseudocubic of point group: mm2 [14] and space group: Pca21, with unit-cell parameters a = 51 8.577(6), b = 8.553(8), c = 12.09(1) and Z = 4 [9, 14-16]. Boracite is dimorphic with 2+ 52 trigonal trembathite (Mg,Fe)3(B7O13)Cl and forms a series with ericaite Fe3 (B7O13)Cl 53 (named after English heather because of the reddish colour). The boracite group mineral 54 congolite has a trigonal structure [10]; however the higher temperature form is cubic with a 55 phase transition temperature of 268°C [17, 18]. 56 2 57 The number of vibrational spectroscopic studies of borate minerals is quite few and far 58 between [19-22]. The number of Raman studies of borate minerals is also very limited [23, 59 24]. There have been a number of infrared studies of some natural borates [25-28]. Most of 60 these references are not new and there have been no recent studies on the vibrational 61 spectroscopy of natural borates. Ross in Farmer’s treatise reported the infrared spectra of 62 several borate minerals [29]. The use of infrared spectroscopy is limited by the spatial 63 resolution of the technique which is around 25 microns. In comparison, the spatial resolution 64 using Raman spectroscopy is 1 micron. Thus, when studying a mineral using spectroscopic 65 techniques it is advantageous to use Raman spectroscopy. The selection of the target mineral 66 is more easily made. With infrared spectroscopy, any impurities will be measured as well as 67 the target mineral. Raman spectroscopy has proven most useful for the study of secondary 68 minerals. 69 70 To the best of the authors’ knowledge, there have been very few infrared spectroscopic 71 studies of chambersite type structure [28, 30] and few Raman studies of this natural mineral 72 have been forthcoming. What Raman studies that have been undertaken are related to the 73 determination of the ferroelectric transition point. Thermodynamic studies of this transition 74 have been forthcoming [31]. In this way the cubic (43m)̅ → orthorhombic (mm2) 75 ferroelectric phase transition is determined. Burns showed the application of powder 76 diffractions to identify boracites [32]. The structure of boracites has been refined [10]. A 77 wide range of boracites have been synthesised and their dielectric properties determined [33]. 78 The effect of the metal cations and halogens on the B-O bonds in the boracite structure was 79 studied by IR spectroscopy [34]. Many of the boracites show ferroelectric properties [35] 80 and some Raman studies have been undertaken to determine the ferroelectric phase 81 transitions [36-38]. Vibrational spectroscopy has focussed on the ferroelectric transitions. 82 Different types of boracites have been synthesised. 83 84 The objective of this paper is to report the vibrational spectroscopic study of a natural 85 chambersite mineral and relate the spectra to the molecular chemistry and the crystal 86 chemistry of this borate mineral. We have characterised chambersite using a combination of 87 Raman and infrared spectroscopy. 3 88 89 Experimental 90 Mineral 91 The sample of chambersite was incorporated into the collection of the Geology Department 92 of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code SAC-114. 93 The sample was gently crushed and the associated minerals were removed under a 94 stereomicroscope Leica MZ4. Scanning electron microscopy (SEM) was applied to support 95 the chemical characterization. Details of the mineral have been published (page 80, Vol. 5) 96 [39]. 97 98 Raman spectroscopy 99 Crystals of chambersite were placed on a polished metal surface on the stage of an Olympus 100 BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is 101 part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a 102 filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a 103 Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and 104 collected at a nominal resolution of 2 cm-1 and a precision of ± 1 cm-1 in the range between 105 200 and 4000 cm-1. Repeated acquisitions on the crystals using the highest magnification 106 (50x) were accumulated to improve the signal to noise ratio of the spectra. The spectra were 107 collected over night. Raman Spectra were calibrated using the 520.5 cm-1 line of a silicon 108 wafer. The Raman spectrum of at least 10 crystals was collected to ensure the consistency of 109 the spectra. 110 111 Infrared spectroscopy 112 Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart 113 endurance single bounce diamond ATR cell. Spectra over the 4000525 cm-1 range were 114 obtained by the co-addition of 128 scans with a resolution of 4 cm-1 and a mirror velocity of 115 0.6329 cm/s. Spectra were co-added to improve the signal to noise ratio.

Load more