Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 7–13 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa SEM–EDX, Raman and infrared spectroscopic characterization of the phosphate 2+ 3+ mineral frondelite (Mn )(Fe )4(PO4)3(OH)5 ⇑ Ray L. Frost a, , Yunfei Xi a, Ricardo Scholz b, Fernanda M. Belotti c, Martina Beganovic 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 Federal University of Itajubá, Campus Itabira, Itabira, MG, Brazil highlights graphical abstract " We have analyzed a frondelite mineral sample from the Cigana mine, located in the municipality of Conselheiro Pena. " The chemical formula was determined as (Mn0.68, 3+ Fe0.32)(Fe )3,72(PO4)3.72(OH)4.99. " The structure of the mineral was assessed using vibrational spectroscopy. " Bands attributed to the stretching 3À and bending modes of PO4 and 3À HOPO3 units were identified. article info abstract Article history: We have analyzed a frondelite mineral sample from the Cigana mine, located in the municipality of Con- Received 3 July 2012 selheiro Pena, a well-known pegmatite in Brazil. In the Cigana pegmatite, secondary phosphates, namely Received in revised form 6 November 2012 eosphorite, fairfieldite, fluorapatite, frondelite, gormanite, hureaulite, lithiophilite, reddingite and vivia- Accepted 11 February 2013 nite are common minerals in miarolitic cavities and in massive blocks after triphylite. The chemical for- Available online 4 March 2013 3+ mula was determined as (Mn0.68,Fe0.32)(Fe )3,72(PO4)3.17(OH)4.99. The structure of the mineral was assessed using vibrational spectroscopy. Bands attributed to the Keywords: stretching and bending modes of PO3À and HOPO3À units were identified. The observation of multiple Frondelite 4 3 bands supports the concept of symmetry reduction of the phosphate anion in the frondelite structure. Rockbridgeite À1 Raman Sharp Raman and infrared bands at 3581 cm is assigned to the OH stretching vibration. Broad Raman À1 Infrared bands at 3063, 3529 and 3365 cm are attributed to water stretching vibrational modes. Phosphate Ó 2013 Elsevier B.V. All rights reserved. Pegmatite Introduction triphylite–lithiophilite and amblygonite–montebrasite, producing complex paragenesis [1]. In recent published articles, the charac- Secondary phosphates are common minerals in lithium bearing terization of the phosphate mineralogy became an important tool pegmatites and most of them are related to the superposition of in the study of the geochemical and petrological evolution of peg- different geological and geochemical process. Different stages of matites [2]. hydrothermalism and the low temperature supergene alteration In addition to the occurrence in granitic pegmatites, minerals of are responsible for the substitution of primary lithium phosphates the frondelite–rockbridgeite series were also identified in sedi- ments of Elk Lake, Canada [3]. Low temperature Fe and Mn phos- phates play important role in the environmental geochemistry. ⇑ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. The crystalization of authigenic minerals in sediments, develop E-mail address: [email protected] (R.L. Frost). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.02.008 8 R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 7–13 an important function in the removal and storage of metals and The Cigana pegmatite is mined out and in the past was mined phosphate pollutants [4]. Despites their importance, some phos- for industrial feldspar and with minor importance gemstones and phate minerals are poorly characterized by spectroscopic methods, samples for the collectors market. In the Cigana pegmatite, second- such as frondelite, rockbridgeite, reddingite and hureaulite. ary phosphates, namely eosphorite, fairfieldite, fluorapatite, frond- Frondelite is a manganese and iron basic phosphate mineral elite, gormanite, hureaulite, lithiophillite, reddingite and vivianite 2+ 3+ with general chemical formula expressed by (Mn )(Fe )4(PO4)3(- are common minerals in miarolitic cavities and in massive blocks OH)5 and belongs to the rockbridgeite group [5]. Frondelite forms a formed after the aggregates of primary triphylite up to 0.5 m 2+ 3+ complex triple series with rockbridgeite-(Mn )(Fe )4(PO4)3(OH)5, length. Frondelite occurs as botrioidal aggregates up to 10.0 cm 2+ 2+ 2+ 3+ where the Mn is replaced by Fe , and plimerite-(Zn )(Fe )4(- in length. The aggregates appear in miarolitic cavities and replaces 2+ 2+ PO4)3(OH)5, where the Mn is replaced by Zn [5,6]. Frondelite triphylite–lithiophilite crystals. Other minerals in association are crystallizes in orthorrombic crystal system, Bbmm space group hureaulite and lithiophilite. with unit-cell parameters a = 13.81 Å, b = 16.96 Å, c = 5.18 Å, Z =4 Deep green frondelite botrioidal aggregates were collected. The and V = 1214.29 Å3. sample was incorporated in the collection of the Geology Depart- Farmer [7] divided the vibrational spectra of phosphates ment of the Federal University of Ouro Preto, Minas Gerais, Brazil, according to the presence, or absence of water and hydroxyl units with sample code SAA-088. The aggregate was hand selected from in the minerals. In aqueous systems, Raman spectra of phosphate a sample in association with hureaulite and lithiophilite. The À1 oxyanions show a symmetric stretching mode (m1) at 938 cm , frondelite fragments were phase analyzed by X-ray diffraction À1 the antisymmetric stretching mode (m3) at 1017 cm , the sym- and Scanning electron microscopy (SEM) for simple À1 metric bending mode (m2) at 420 cm and the m4 mode at characterization. À1 567 cm [8–10]. The value for the m1 symmetric stretching vibra- tion of PO4 units as determined by infrared spectroscopy was given Electron probe micro-analysis (EPMA) À1 À1 À1 as 930 cm (augelite), 940 cm (wavellite), 970 cm (rock- A quantitative chemical analysis was carried via EPMA. Frondel- À1 À1 bridgeite), 995 cm (dufrenite) and 965 cm (beraunite). The po- ite–rockbridgeite botrioidal aggregate selected for this study was sition of the symmetric stretching vibration is mineral dependent analyzed with the performance of five spots. The chemical analysis and a function of the cation and crystal structure. The fact that was carried out with a Jeol JXA8900R spectrometer from the Phys- the symmetric stretching mode is observed in the infrared spec- ics Department of the Federal University of Minas Gerais, Belo Hor- trum affirms a reduction in symmetry of the PO4 units. izonte. For each selected element was used the following The value for the m2 symmetric bending vibration of PO4 units as standards: Fe – magnetite, Mn – rodhonite, P – Ca P O Ca – Apa- À1 2 2 7 determined by infrared spectroscopy was given as 438 cm (aug- tite Astimex. The limit detection is up to 0.01 wt%. The epoxy À1 À1 elite), 452 cm (wavellite), 440 and 415 cm (rockbridgeite), embedded frondelite sample was polished in the sequence of À1 À1 455, 435 and 415 cm (dufrenite) and 470 and 450 cm (bera- 9 lm, 6 lm and 1 lm diamond paste MetaDIÒ II Diamond Paste unite). The observation of multiple bending modes provides an – Buhler, using water as a lubricant, with a semi-automatic Mini- indication of symmetry reduction of the PO4 units. This symmetry MetÒ 1000 Grinder-Polisher – Buehler. Finally, the epoxy embed- reduction is also observed through the m3 antisymmetric stretching ded hureaulite was coated with a thin layer of evaporated vibrations. Augelite shows infrared bands at 1205, 1155, 1079 and carbon. The electron probe microanalysis in the WDS (wavelength À1 À1 1015 cm [11]; wavellite at 1145, 1102, 1062 and 1025 cm ; dispersive spectrometer) mode was obtained at 15 kV accelerating À1 rockbridgeite at 1145, 1060 and 1030 cm ; dufrenite at 1135, voltage and beam current of 10 nA. Chemical formula was calcu- À1 1070 and 1032 cm ; and beraunite at 1150, 1100, 1076 and lated on the basis of seventeen oxygen atoms (O, OH, F). 1035 cmÀ1. In the infrared study of triploidite, a basic manganese phos- Raman microprobe spectroscopy phate, Farmer reports the infrared spectrum with the (m1)at À1 À1 Botrioidal aggregates of frondelite were placed on a polished 957 cm ,(m3) at 1090, 1058, 1030 and 1010 cm ,(m2)at À1 À1 metal surface on the stage of an Olympus BHSM microscope, which 420 cm and the m4 mode at 595, 570, 486 cm [12]. An hydroxyl stretching frequency of 3509 cmÀ1 was given. In the spectroscopic is equipped with 10Â,20Â, and 50Â objectives. The microscope is study of strengite, in the region below to 400 cmÀ1, Frost and We- part of a Renishaw 1000 Raman microscope system, which also in- ier [11] described the metal stretching vibrations for MnO and also cludes a monochromator, a filter system and a CCD detector (1024 the OMnO bending modes. pixels). The Raman spectra were excited by a Spectra-Physics mod- In this work, samples of a pure, monomineral frondelite–rock- el 127 He–Ne laser producing highly polarized light at 633 nm and À1 bridgeite from the Cigana pegmatite, located in the municipality collected at a nominal resolution of 2 cm and a precision of À1 À1 of Conselheiro Pena, Brazil has been carried out. Studies include ±1 cm in the range between 200 and 4000 cm . Repeated acqui- chemistry and backscattering images via SEM in the EDS mode, sitions on the samples using the highest magnification (50Â) were spectroscopic characterization of the structure with infrared and accumulated to improve the signal to noise ratio of the spectra.