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, Lopez, Andres, Fernandes Lima, Rosa Malena, & Ferreira, Claudiane Moraes (2013) Vibrational spectroscopic characterization of the phosphate mineral series eosphorite-childrenite-(Mn,Fe)Al(PO4)(OH)2.(H2O). Vibrational Spectroscopy, 67, pp. 14-21. This file was downloaded from: https://eprints.qut.edu.au/60932/ 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.vibspec.2013.03.005 Vibrational spectroscopic characterization of the phosphate mineral series eosphorite- childrenite – (Mn,Fe)Al(PO4)(OH)2·(H2O) Ray L. Frosta, Yunfei Xia, Ricardo Scholzb, Claudiane Moraes Ferreirab 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, 35,400-00, Brazil Abstract: The phosphate mineral series eosphorite-childrenite – (Mn,Fe)Al(PO4)(OH)2·(H2O) has been studied using a combination of electron probe analysis and vibrational spectroscopy. Eosphorite is the manganese rich mineral with lower iron content in comparison with the childrenite which has higher iron and lower manganese content. The determined formula of the minerals are (Mn0.71,Fe0.13,Ca0.01)(Al)1.03(PO4)1.07(OH1.85,F0.02)·2(H2O) and (Fe0.49,Mn0.35,Mg0.06,Ca0.04)(Al)1.03(PO4)1.05(OH)1.80·2(H2O). Raman spectroscopy enabled the observation of bands at 969, 978 and 1011 cm-1 assigned to monohydrogen phosphate, phosphate and dihydrogen phosphate units. Differences are observed in the area of the peaks between the two eosphorite minerals. -1 Raman bands at 562, 595, and 608 cm are assigned to the ν4 out of plane bending modes of -1 the PO4, HPO4 and H2PO4 units; Raman bands at 405, 427 and 466 cm are attributed to the ν2 modes of these units. Raman bands of the hydroxyl and water stretching modes are observed. Key words: eosphorite, childrenite, phosphate, pegmatite, Raman spectroscopy, infrared spectroscopy Author to whom correspondence should be addressed ([email protected]) P +61 7 3138 2407 F: +61 7 3138 1804 1 Introduction Eosphorite is a pink manganese mineral with formula MnAl(PO4)(OH)2·(H2O) which crystallises in a monoclinic crystal system [1] and forms prismatic crystals which form radiating clusters. The mineral shows pseudo-orthrhombic morphology due to twinning. The mineral ocurs worldwide in pegamatites [2-7] and is always associated with other phosphate minerals [8]. Eosphorite forms a solid solution series with the mineral childrenite [9-11]. Childrenite's formula is (Fe, Mn)AlPO4(OH)2·H2O and differs from eosphorite by being rich in iron instead of manganese. The structures of the two minerals are the same and therefore it would be expected that their differences in physical properties between the two would be related to the iron/manganese content. Eosphorite is less dense and is generally pinkish to rose-red in color whereas childrenite's colors tends towards various shades of brown. In terms of crystal habits the two also differ. Eosphorite forms prismatic, slender crystals and rosettes. Childrenite forms tabular or bladed individuals or lamellar aggregates. It has been said that the two different habits belie their solid solution relationship. In this work, samples of the mineral series eosphorite and childrenite from different pegmatites from Minas Gerais were studied. Characterization include chemistry via Electron Probe Microanalysis in the WDS mode (EPMA) and spectroscopic characterization of the structure with infrared and Raman. Experimental Samples description and preparation The eosphorite and childrenite samples studied in this work were obtained from the collection of the Geology Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code SAA-090 and SAA-072 respectively. The samples are from two different granitic pegmatites from Minas Gerais, Brazil. Sample SAA-072 was collected from the Ponte do Piauí mine, located in the Piauí valley, municipality of Itinga. The region is well-known as an important source of rare phosphates 2 and gemological minerals. The pegmatite is located in the Araçuaí pegmatite district, one of the subdivisions of the Eastern Brazilian Pegmatite province [12]. The pegmatite is mined for gemstones and samples for the collectors market. It is heterogeneous with well-developed mineralogical and textural zoning. The primary mineral association is represented by quartz, muscovite, microcline, schorl and almandine-spessartine. The secondary association is mainly composed by albite, Li bearing micas, cassiterite, elbaite and hydrothermal rose quartz. In the Ponte do Piauí pegmatite, secondary phosphates, namely childrenite, eosphorite, fluorapatite, zanazziite, occur in miarolitic cavities in association with albite, quartz and muscovite. Childrenite grows usually along the surface of quartz crystals and in albite agregates. Sample SAA-090 was collected from Roberto mine, a granitic pegmatite located in Divino das Laranjeiras east of Minas Gerais. The region is situated 65 km ENE of Governador Valadares. The region is well-known as an important source of rare minerals such as brazilianite. The pegmatite is located in the Conselheiro Pena pegmatite district, also one of the subdivisions of the Eastern Brazilian Pegmatite province (EBP). The pegmatite is mined for rare minerals for the collectors market. It is heterogeneous with well-developed mineralogical and textural zoning. The primary mineral association is represented by quartz, muscovite, microcline, schorl, almandine-spessartine and triphylite. The secondary association is mainly composed by albite, quartz crystals and a number of secondary phosphates, namely eosphorite, fluorapatite, zanazziite and brazilianite. The phosphates occur in miarolitic cavities. The sample was gently crushed and the associated minerals were removed under a stereomicroscope Leica MZ4. The eosphorite and childrenite samples were phase analyzed by X-ray diffraction. Scanning electron microscopy (SEM) was applied to support the mineralogical chemical Scanning electron microscopy (SEM) Experiments and analyses involving electron microscopy were performed in the Center of Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil (http://www.microscopia.ufmg.br). Eosphorite and childrenite single crystals were coated with a 5 nm layer of evaporated Au. Secondary Electron and Backscattering Electron images were obtained using a JEOL JSM- 3 6360LV equipment. Qualitative and semi-quantitative chemical analysis in the EDS mode were performed with a ThermoNORAN spectrometer model Quest and was applied to support the mineral characterization and to determine the major elements to be measured by Electron probe micro-analysis. Electron probe micro-analysis (EPMA) The quantitative chemical analysis of eosphorite and childrenite single crystals was carried via EPMA. The chemical analysis was carried out with a Jeol JXA8900R spectrometer from the Physics Department of the Federal University of Minas Gerais, Belo Horizonte. For each selected element was used the following standards: Fe and Mg – olivin, Mn – rodhonite, P and Ca - Apatite Artimex, Al - Corundum and F - Fluorite. The epoxy embedded eosphorite and childrenite crystals was polished in the sequence of 9μm, 6μm and 1μm diamond paste MetaDI® II Diamond Paste – Buhler, using water as a lubricant, with a semi-automatic MiniMet® 1000 Grinder-Polisher – Buehler. Finally, the epoxy embedded samples was coated with a thin layer of evaporated carbon. The electron probe microanalysis in the WDS (wavelength dispersive spectrometer) mode was obtained at 15 kV accelerating voltage and beam current of 10 nA. Chemical formula was calculated on the basis of seven oxygen atoms (O, F, OH, H2O). Raman microprobe spectroscopy Crystals of eosphorite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and