Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 106 (2013) 216–223 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Infrared and Raman spectroscopic characterization of the phosphate mineral 2+ 2+ fairfieldite – Ca2(Mn ,Fe )2(PO4)2Á2(H2O) ⇑ Ray L. Frost a, , Yunfei Xi a, Ricardo Scholz b, Fernanda Maria Belotti c, Andres Lopez a 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 c Federal University of Itajubá, Campus Itabira, Itabira, MG 35,903-087, Brazil highlights graphical abstract " We have studied fairfieldite from Cigana mine of the Eastern Brazilian Pegmatite Province in Minas Gerais. " The chemical formula was determined using an electron probe. " Vibrational spectroscopy was used to determine the structure. " Multiple bands in antisymmetric stretching spectral region provide evidence of symmetry reduction of the phosphate anion. article info abstract Article history: Raman spectroscopy complimented with infrared spectroscopy has been used to determine the molecu- Received 28 November 2012 3À lar structure of the phosphate mineral fairfieldite. The Raman phosphate ðPO4Þ stretching region shows Accepted 8 January 2013 strong differences between the fairfieldite phosphate minerals which is attributed to the cation substitu- Available online 17 January 2013 2À tion for calcium in the structure. In the infrared spectra complexity exists with multiple ðPO4Þ antisym- metric stretching vibrations observed, indicating a reduction of the tetrahedral symmetry. This loss of Keywords: degeneracy is also reflected in the bending modes. Strong Raman bands around 600 cmÀ1 are assigned Raman spectroscopy to m phosphate bending modes. Multiple bands in the 400–450 cmÀ1 region assigned to m phosphate Fairfieldite 4 2 bending modes provide further evidence of symmetry reduction of the phosphate anion. Three broad- Infrared spectroscopy À1 Phosphate bands for fairfieldite are found at 3040, 3139 and 3271 cm and are assigned to OH stretching bands. Pegmatite By using a Libowitzky empirical equation hydrogen bond distances of 2.658 and 2.730 Å are estimated. Vibrational spectroscopy enables aspects of the molecular structure of the fairfieldite to be ascertained. Ó 2013 Elsevier B.V. All rights reserved. 2+ Introduction (PO4)2Á2H2O], collinsite [Ca2(Mg,Fe )(PO4)2Á2H2O], fairfieldite 2+ [Ca2(Mn,Fe )(PO4)2Á2H2O], gaitite [Ca2Zn(AsO4)2Á2H2O], messelite 2+ 2+ 2+ The fairfieldite mineral group are triclinic arsenates and phos- [Ca2(Mn ,Fe )(PO4)2Á2H2O], parabrandite [Ca2Mn (AsO4)2Á2H2O], phates of the general formula Ca2B(XO4)2Á2H2O where B is Co, roselite-beta [Ca2Co(AsO4)2Á2H2O], and talmessite [Ca2Mg(AsO4)2Á 2+ Fe , Mg, Mn, Ni, Zn and X is either As or P [1–6]. The minerals form 2H2O]. The minerals form solid solutions for example the two subgroups based upon whether the anion is phosphate or collinsite–fairfieldite series. Many of these minerals are found in arsenate. Minerals in this group include cassidyite [Ca2(Ni, Mg) Australia [7–9]. The structure of the fairfieldite group minerals is dominated by ⇑ the chains of tetrahedra (XO4) and octahedra [B-O4(H2O)2] which Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. parallel the C axis. The fairfieldite group crystallizes in the triclinic 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.2013.01.008 R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 106 (2013) 216–223 217 have not been reported. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (m1)at À1 À1 938 cm , the antisymmetric stretching mode (m3) at 1017 cm , À1 the symmetric bending mode (m2) at 420 cm and the m4 mode at 567 cmÀ1 [12,13,18]. S.D. Ross in Farmer [19] (p404) listed some well-known minerals containing phosphate which were either hy- drated or hydroxylated or both [19]. Farmer (p392) listed the band positions of collinsite, one of the fairfieldite minerals. The m1 3À ðPO4Þ symmetric stretching vibration was listed as 945 and À1 3À 920 cm ; the m3 ðPO4Þ symmetric stretching vibrations as À1 1112 and 1000 cm , the m4 bending modes as 577 and 562 cmÀ1. A band at 770 cmÀ1 was observed but not assigned. In comparison the value for the m1 symmetric stretching vibration of PO4 units as determined by infrared spectroscopy was given as 930 cmÀ1 (augelite), 940 cmÀ1 (wavellite), 970 cmÀ1 (rockbridge- ite), 995 cmÀ1 (dufrenite) and 965 cmÀ1 (beraunite). The position of the symmetric stretching vibration is mineral dependent and a Fig. 1. A backscattered electron image (BSI) of a fairfieldite single crystal up to function of the cation and crystal structure. The fact that the sym- 0.5 mm in length. metric stretching mode is observed in the infrared spectrum af- firms a reduction in symmetry of the PO4 units. Fairfieldite is a common calcium and manganese hydrate phos- space group P1: The cation octahedra are compressed resulting in phate mineral in granitic pegmatites. The general chemical formula 2+ 2+ disorder of the chains. This affects the hydrogen bonding of the of fairfieldite is expressed by Ca(Mn ,Fe )(PO4)2Á2(H2O) [20]. The water in the structure. The amount of published data on the Raman mineral forms a series with collinsite, its Mg2+ analogous, in substi- spectra of mineral phosphates is limited [10–14]. There has been tution to manganese. Fairfieldite belongs to the homonymous limited published data on the fairfieldite mineral group [15–17]. group that includes a number of phosphates and arsenates such The Raman spectra of the hydrated hydroxy phosphate minerals as talmessite, messelite, gaitite and collinsite among others. The Table 1 Chemical composition of fairfieldite from Cigana pegmatite, Minas Gerais. H2O calculated by stoichiometry on the basis of ideal formula. Sample P2O5 CaO MnO FeO MgO Na2OAl2O3 SrO CuO ZrO H2O Total SAA-099 38.64 30.53 10.93 2.19 3.69 0.03 0.01 0.02 0.02 0.02 9.97 96.09 Fairfieldite* 39.32 31.07 9.87 – – – – – – – 9.97 100.00 Collinsite* 42.96 33.95 – – 12.20 – – – – – 10.90 100.00 P Ca Mn Fe Mg Na Al Sr Cu Zr H Total SAA-099 1.99 1.99 0.56 0.11 0.33 0.00 0.00 0.00 0.00 0.00 2.02 7.00 Fairfieldite* 2.00 2.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.00 7.00 Collinsite* 2.00 2.00 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 2.00 7.00 * Calculated by stoichimetry acording to ideal formula of end members. Fig. 2. EDS analysis of fairfieldite. 218 R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 106 (2013) 216–223 mineral was first described by Brush and Dana in 1879 from Fair- via electron microprobe (EMP) in the WDS mode and the spectro- field County, Connecticut, USA [21]. Other occurrences were later scopic characterization of the structure with infrared and Raman. described in the Cigana mine, Brazil [22]; Tanco mine, Canada [23] and Barroso-Alvão pegmatite, Portugal [24]. The crystal struc- Experimental ture of fairfieldite was refined by Fanfani et al. [3]. The mineral crystallizes in the triclinic crystal system, space group P1 with Samples description and preparation unit cell parameter a = 5.78Å, b = 6.57Å, c = 5.48Å, a = 102.08°, b = 108.71° and c = 90.09°. The fairfieldite sample studied in this work was collected from Studies concerning the mineralogy of phosphates of the fairfiel- the Cigana mine (also named as Jocão mine), a lithium-bearing dite group minerals are rare in literature [25] and to the best pegmatite located in the Conselheiro Pena Pegmatite district knowledge of the authors, data about vibrational spectroscopic (CPD), one of the eleven metallogenetic subdivisions of the Eastern characterization are restricted to the database of the University Brazilian Pegmatite Province (EBP) in Minas Gerais [29]. The Cig- of Arizona (rruff.info), however no interpretation is given. No ana mine is an important source for rare and unusual phosphate Raman spectroscopic investigation of these phosphate phases re- and industrial minerals (microcline, triphylite and spodumene) lated to the leucophosphite group has been published. However, In the Cigana mine, white crystals of fairfieldite and aggregates in recent years, the application of spectroscopic techniques to up to 3.0 mm occur in association with pyrite, quartz, albite and understand the structure of phosphates has been increasing [26– muscovite. The collected sample was incorporated to the collection 28]. In this work, samples of fairfieldite from the Cigana pegmatite, of the Geology Department of the Federal University of Ouro Preto, located in the municipality of Conselheiro Pena, Brazil have been Minas Gerais, Brazil, with sample code SAA-099. The sample was selected for study. These studies include chemistry with analysis gently crushed and the associated minerals were removed under Fig. 3. (a) Raman spectrum of fairfieldite over the 100–4000 cmÀ1 spectral range and (b) infrared spectrum of fairfieldite over the 500–4000 cmÀ1 spectral range. R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 106 (2013) 216–223 219 a stereomicroscope Leica MZ4. Scanning electron microscopy onda Eletrônica, Instituto de Geociências, Universidade de Brasília (SEM) was applied to support the chemical characterization and (IG/UnB), Brasília.